[Federal Register Volume 61, Number 109 (Wednesday, June 5, 1996)]
[Rules and Regulations]
[Pages 28684-28696]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 96-13947]
[[Page 28683]]
_______________________________________________________________________
Part IV
Department of Transportation
_______________________________________________________________________
Federal Aviation Administration
_______________________________________________________________________
14 CFR Part 25
Standards for Approval for High Altitude Operation of Subsonic
Transport Airplanes; Final Rule
Federal Register / Vol. 61, No. 109, Wednesday, June 5, 1996 / Rules
and Regulations
[[Page 28684]]
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 25
[Docket No. 26070, Amendment No. 25-87]
RIN 2120-AB18
Standards for Approval for High Altitude Operation of Subsonic
Transport Airplanes
Agency: Federal Aviation Administration (FAA), DOT.
Action: Final rule.
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Summary: This amendment to the Federal Aviation Regulations (FAR)
specifies airplane and equipment airworthiness standards for subsonic
transport airplanes to be operated up to an altitude of 51,000 feet.
This action is prompted by an increase in the number of applications
received to raise the maximum certificated operating altitude for
transport category airplanes, and is intended to ensure an acceptable
level of safety for airplanes operated at high altitudes.
Effective Date: July 5, 1996.
For Further Information Contact: Robert C. McCracken, Flight Test and
Systems Branch, ANM-111, Transport Airplane Directorate, Aircraft
Certification Service, 1601 Lind Avenue Southwest, Renton, Washington
98055-4056; telephone (206) 227-2118.
SUPPLEMENTARY INFORMATION:
Background
This amendment is based on Notice of Proposed Rulemaking (NPRM) No.
89-31, which was published in the Federal Register on November 22, 1989
(54 FR 48538). The notice proposed to upgrade airplane and equipment
airworthiness standards for subsonic transport airplanes to be operated
up to an altitude of 51,000 feet, and it was based on special
conditions that have been used for type certification for many years.
Current policy for FAA rulemaking projects is to endeavor to
achieve harmonization with the Joint Airworthiness Authorities (JAA)
and other airworthiness authorities through the Aviation Rulemaking
Advisory Committee (ARAC) and its harmonization working groups.
Although this rulemaking project has not been the subject of a
harmonization working group activity, because it was initiated prior to
the time harmonization became a high priority with the FAA and JAA,
comments received from the JAA members were addressed in this
amendment.
As noted in Notice 89-31, the higher operational altitudes made
feasible by the advent of turbojet transport airplanes introduced
certain risks with respect to crew and passenger breathing that were
not experienced with earlier propeller-driven airplanes. Accordingly,
certification standards were developed in the early 1950s to permit
safe operation of early turbojet transport airplanes up to certain
maximum operating altitudes--typically 41,000 or 42,000 feet.
Subsequent to the type certification of the early turbojet transport
airplanes, applicants requested approval to operate certain later
airplanes at higher altitudes. These were in most cases small
``executive'' transport airplanes, and the requested altitudes ranged
up to 51,000 feet.
The operation of these airplanes at altitudes above 40,000 feet
usually involved a number of novel or unusual design features that were
not addressed by the airworthiness requirements in the current
regulations. In order to ensure a level of safety equivalent to that
established by part 25 of the FAR, Secs. 21.16 and 21.101 of part 21
require that additional standards be developed in the form of special
conditions and that compliance with the special conditions be
demonstrated.
The regulatory changes adopted by this amendment codify and
consolidate the different high-altitude criteria that have been made
applicable by special conditions to previously certificated subsonic
transport airplanes. In addition, the changes acknowledge a human
physiological limit of 34,000 feet (see Glossary), the level above
which persons not using supplementary oxygen are in serious peril. To
assure compatibility or equivalency with other provisions of part 25,
which were amended after many of the special conditions discussed
herein were implemented, these changes are written so that terminology
relating to the probability of certain failures is consistent with
those other provisions. Generally, the intent of those provisions is to
recognize that the degree of hazard of any given failure is inversely
related to the probability of occurrence of that failure. Failures that
are considered to be catastrophic must be shown to be extremely
improbable, and hazardous failures must be shown to be improbable (see
Glossary). Examples of these terms are found in Secs. 25.671, 25.672,
and 25.1309.
It must be noted that widespread operation of transport category
airplanes at altitudes greater than 51,000 feet is not currently
envisioned. A major factor in an approval for operation up to 51,000
feet is an emergency descent during a decompression, which must be
shown to result in a maximum cabin altitude of no more than 40,000
feet. Accordingly, the changes adopted in this amendment have been
developed to provide adequate standards for safe operation of such
airplanes up to 51,000 feet. Should an applicant seek approval to
operate a transport category airplane above that altitude, additional
standards may be needed for safe operation. If so, appropriate special
conditions would be adoptive to require compliance with those
standards.
The changes in this amendment involve ventilation, cabin cooling,
pressurization and pressure vessel integrity, and oxygen equipment. The
following paragraphs describe the changes, and the reasons for the
changes, in the regulations incorporated with the adoption of this
amendment. The comments received in response to Notice 89-31, the
disposition of the comments, and, when applicable, the effect of the
comments on the changes, are discussed immediately following this
section.
1. Ventilation (Airflow and Contamination)
Prior to this amendment, Sec. 25.831(a) required each passenger and
crew compartment to be ventilated and each crew compartment to have
enough fresh air to enable crewmembers to perform their duties without
undue discomfort or fatigue. For the crew compartment, a minimum of 10
cubic feet of fresh air per minute per crewmember was required. Section
25.1309 (specifically Secs. 25.1309(b)(2) and 25.1309(d)(3)) requires
that the effects on occupants of any failures of required systems be
analyzed, but Sec. 25.1309 is a general rule and does not specifically
address minimum airflow requirements.
The executive transport special conditions that have been applied
in the past supplemented Sec. 25.831(a) by specifying that the minimum
fresh airflow of 10 cubic feet per minute (cfm) per crewmember was to
be provided to each occupant during normal operation. The special
conditions also required that each occupant be furnished with enough
uncontaminated air to provide reasonable comfort during normal
operating conditions and also after any probable failure of any system
that would adversely affect the cabin ventilation air. This rule amends
Sec. 25.831 to include the additional airflow requirements contained in
previous special conditions, stipulating that the ventilation system
must be designed to provide 10 cfm (converted to pounds of air) for
each occupant.
[[Page 28685]]
Some airplanes now incorporate ventilation systems in which fresh
air is augmented with conditioned and recirculated air. Section
25.831(a) as amended permits a ventilation system that uses a mixture
of the minimum amount of fresh air and any desired quantity of
recirculated air that is shown to be uncontaminated by odors,
particulates, or gases. In this regard, the minimum amount of fresh air
is specified by weight rather than by volume in order to provide a
parameter independent of altitude. Ten cubic feet of standard air at a
typical cabin altitude of 8,000 feet and typical cabin temperature of
75 deg.F. weighs approximately 0.55 pounds. This rule amends
Sec. 25.831 to include the additional airflow requirements as noted
above. This standard is equivalent to the present requirement for
crewmembers.
2. Cabin Cooling
During the Supersonic Transport (SST) review in the 1960s, it was
noted that certain pressurization system failures, whether considered
by themselves or in combination with the use of hot ram air for
emergency pressurization, could lead to cabin temperatures exceeding
human tolerance. The FAA therefore concluded that any failure or
combination of failures that could lead to temperature exposures that
would cause undue discomfort must be shown to be improbable (see
Glossary). Minor corrective actions (e.g., selection of alternate
equipment or procedures) would be allowed if necessary for probable
failures. The FAA also concluded that any failure or combination of
failures that could lead to intolerable temperature exposures must be
extremely improbable. Major corrective actions (e.g., emergency
descent, configuration changes) would be allowed for an improbable
failure condition. Temperature limits were incorporated into the
special conditions imposed on executive transport airplanes when
approved for high altitude operation. The SST and executive transport
special conditions contained two graphs which explained the
requirements for the probable and improbable cases. In formulating this
amendment, the FAA has determined that the public interest is served by
adopting the time-temperature limits associated with improbable failure
conditions, and they are adopted as a new Sec. 25.831(g). This
amendment does not allow the time of exposure at any given temperature
to exceed the values given in the associated graph.
3. Pressurization and Pressure Vessel Integrity
Section 25.365(d), increases the fuselage pressure relief valve
safety factor of 1.33 by 25 percent to 1.67, codifying the standard
that was originally contained in the SST special conditions. This
increased structural safety factor was also included in the executive
transport special conditions to reduce the likelihood of structural
failure and to limit the size of the opening if a failure occurs. It is
included in this amendment for this reason.
The FAA had considered proposing both pressurization standards
similar to those previously required by the special conditions for
executive transport and separate standards similar to those required
for large transport airplanes. The separate standards were thought to
be necessary because of the inherent differences in pressurized volume
of the two types of transports, and the belief that a larger airplane
may decompress more slowly than a smaller airplane. Upon further
review, this approach was deemed impractical because certain larger
transport airplanes have decompression characteristics more analogous
to smaller transport airplanes and vice versa. Therefore, this
amendment applies the same standard to all transport airplanes.
It should be noted that the special conditions required
consideration of specific failures, which are addressed later in this
discussion. Subsequent to the issuance of the special conditions,
reliability, probability, and damage tolerance concepts addressing
other failures and methods of analysis were incorporated into part 25.
This amendment allows the use of these additional methods of analysis
and failure considerations.
The earlier executive transport special conditions required a
pressure demand mask (see Glossary). Later special conditions included,
pursuant to the recommendations of the FAA Civil Aeromedical Institute
(CAMI), a requirement for a pressure demand mask with a mask-mounted
regulator (see Glossary). The requirement for the use of the same type
of equipment is adopted by this amendment.
The objective of the amended Sec. 25.841(a) (pressurization) when
applied in conjunction with amended Sec. 25.1447(c) (oxygen equipment)
is to provide airworthiness standards that allow subsonic airplanes to
operate at their maximum achievable altitudes. This is the highest
altitude for which an applicant chooses to demonstrate that, after
decompression caused by a single failure or combination of failures
that are not shown to be extremely improbable: (1) the flightcrew will
remain alert and be able to fly the airplane; (2) the cabin occupants
will be protected from the effects of hypoxia; and (3) in the event
that some occupants do not receive supplemental oxygen, they
nevertheless will be protected against permanent physiological damage.
Section 25.841(a)(1) as amended is equivalent to the existing
Sec. 25.841(a) with the exception of editorial changes and elimination
of the words ``reasonably'' and ``or malfunctions.'' The ``probable''
failure criteria are the same as those contained in Sec. 25.1309. The
term ``failure conditions'' has been added to this section to clarify
that failure combinations that lead to a probable depressurization
event must also be considered.
Section 25.841(a)(2) as amended limits exposure of the airplane
occupants, after decompression, to a cabin altitude no greater than
40,000 feet. This requirement is unchanged from that previously
established in part 25 for certification of transport category
airplanes using diluter demand (flightcrew) and continuous flow
(passenger) oxygen equipment (see Glossary).
Section 25.841(a)(2) as amended is a combination of the later
executive transport high altitude special conditions and Sec. 25.1309,
i.e., the degree of the hazard must be inversely related to the
probability of the failure condition. The amended Sec. 25.841(a)(2) was
developed from the recommendations of CAMI and is based on the concept
of ``Time of Safe Unconsciousness'' documented by James G. Gaume (see
Reference 1). The use of continuous-flow oxygen masks by passengers
following rapid decompression to cabin altitudes above 34,000 feet may
fail to provide protection from hypoxia, as noted in the discussion
under Paragraph 4. ``OXYGEN EQUIPMENT,'' below. Additionally, some
passengers might be exposed to high cabin altitudes following
decompression without the use of oxygen. A few passengers may lose
consciousness at 34,000 feet cabin altitude, and more may lose
consciousness at greater altitudes even with the use of continuous-flow
oxygen equipment. Exposure to cabin altitudes in excess of 25,000 feet
for more than 2 minutes without supplemental oxygen may cause permanent
physiological (brain) damage. Therefore, in order to demonstrate
compliance with this rule, approved emergency descent procedures and a
cabin altitude analysis must be prepared to ensure that these
[[Page 28686]]
altitude limits are not exceeded following a decompression failure that
is not shown to be extremely improbable.
Section 25.841(a)(3) as amended describes the failure conditions
that must be considered in evaluating cabin decompression. Possible
modes of failure to be evaluated include malfunctions and damage from
external sources such as tire burst, wheel failure, uncontained engine
failure, engine fan, compressor or turbine multi-blade failure, and
loss of antennas. Sections 25.1309 and 25.571, and associated advisory
material, provide guidance in determining the sources of failure.
System failures (both latent and active), combinations of system
failures, system failures combined with pressure vessel leaks, system
failures causing engine shutdown, uncontained engine failures causing
structural and system damage, and structural failures without system
failures must all be evaluated. Typical systems include engine bleed
air systems, air conditioning systems, power sources, outflow valves
and control systems. Failures which expose the occupants to cabin
altitudes in excess of either 25,000 feet for more than 2 minutes or
40,000 feet for any amount of time must be shown to be extremely
improbable.
The executive transport airplane special conditions required
evaluation of uncontained engine failure (including fan, compressor and
turbine blades, and rotor disc) and complete loss of thrust from all
engines. The FAA policy has been to presume that these failures will
occur and permit the use of analytical methods to assess the damage.
Multiple engine failures have occurred because of secondary effects
from uncontained engine failure and from operational errors. Multiple
fan blade, rotor, and other uncontained engine failures have occurred
during cruise conditions and have caused cabin decompression. The
service history of airplane decompressions resulting from uncontained
engine failure has been acceptable. Flight levels for most transport
airplanes have been at an altitude where oxygen equipment is capable of
providing adequate protection. Uncontained engine failure is most
likely to occur during takeoff and climb; however, approximately 20
percent of the known bursts have occurred in cruise mode, not including
those caused by bird strikes. The possibility of an uncontained engine
failure in cruise mode cannot be ignored, and the damage resulting in
depressurization must be assessed.
Structural failures in large transport airplanes which would result
in decompression are generally considered to include a loss of a
typical skin panel bound by a crack stopper pattern, a door seal,
window, or windshield, unless the design is such that loss of the
windshield is shown to be extremely improbable when operating at the
higher altitudes. Structural failures in executive transport airplanes
leading to decompression, discussed in the various special conditions,
included the following:
1. Any single failure in the pressurization system combined with
the occurrence of a leak produced by the complete loss of a door seal
element, or a fuselage leak through an opening having an area 2.0 times
the area which produces the maximum permissible fuselage leak rate
approved for normal operation in accordance with Sec. 25.841(a).
2. The maximum pressure vessel opening resulting from an initially
detectable crack propagating for a period encompassing four normal
inspection intervals. Mid-panel cracks and cracks through skin-stringer
and skin-frame combinations must be evaluated.
3. Pressure vessel openings resulting from tire burst, uncontained
engine failure, loss of antennas, or stall warning vanes, or any
probable equipment failure. The effects of such damage while operating
under maximum cabin pressure differential must be evaluated.
Subsequent to the initial development and issuance of high altitude
special conditions, Sec. 25.571 was amended by Amendments 25-45 (1978)
and 25-52 (1980) to require damage-tolerance and fatigue evaluation of
airplane primary structure. Section 25.571 requires showing that a
catastrophic failure due to fatigue, corrosion, or accidental damage
will not occur throughout the operational life of the airplane
(Sec. 25.571 (a)). The effects that are required to be considered under
Sec. 25.571 are not limited to depressurization. Compliance with
Sec. 25.571 requires the development of inspection intervals and
procedures for the detection of crack lengths associated with the
decompression of critical vent areas. Any event that would expose the
occupants to cabin pressure altitudes in excess of the limits
established under this amendment must be shown to be extremely
improbable.
In demonstrating compliance with proposed Sec. 25.841, the crew
would presumably perform an emergency descent in accordance with an
approval emergency procedure. The time required for the crew to
recognize a decompression emergency and don their oxygen masks has been
established by tests to be 17 seconds. This 17-second delay is imposed
between the cabin altitude warning and the beginning of action for
descent. The critical failure case (probable system failure) must be
demonstrated by system failure tests at the maximum airplanes altitude.
For improbable failure, the cabin altitude can be established by
analysis, and verified, if necessary, by tests at a much lower
altitude, with the results extrapolated to the higher altitude.
4. Oxygen Equipment
Both diluter demand and pressure demand oxygen equipment have
proven satisfactory for cabin pressure altitudes of 40,000 feet or less
when the person using the oxygen equipment is exposed gradually to
increased altitudes. However, the FAA was concerned that rapid
decompression to cabin pressure altitudes that exceed 34,000 feet could
temporarily negate the protective qualities of such equipment, unless
the mask and oxygen are being used prior to the decompression, leading
to moderate to severe decreases in flightcrew performance. To prevent
such performance decrements, Notice 89-31 proposed that the use of 100
percent oxygen be required by this amendment for flightcrews operating
at airplane altitudes which may expose them to cabin altitudes
exceeding 34,000 feet following a pressurization failure. As discussed
below, in response to public comment, this requirement has been removed
pending further study by the FAA.
Prior to this amendment, Sec. 25,1447(c)(3) required that each
washroom be equipped with two oxygen outlets and two units of
dispensing equipment. The term washroom has been replaced in other
sections of part 25. This reference is deleted for consistency, and the
existing provisions of Sec. 25,1447(c)(3) are incorporated into a
revised Sec. 25.1447(c)(1). The amended regulation does not specify
demand equipment under Sec. 25.1447(c)(2), because
Sec. 25.1447(c)(3)(i) as amended allows the option of using either
diluter demand or pressure demand equipment for airplanes to be
operated above an altitude of 25,000 feet, and Sec. 25,1447(c)(3)(ii)
as amended requires pressure demand equipment for airplanes where
decompression may expose the flightcrew to cabin altitudes in excess of
34,000 feet.
Discussion of Comments
Comments were received from foreign and domestic airplane
manufacturers, foreign government agencies, various trade organizations
representing employee groups, and individuals. The majority of the
commenters support the
[[Page 28687]]
proposals but many suggest changes. Many commenters recommend
editorial, organizational, and clarifying comments which would result
in clearer language.
Several commenters recommend removing the proposed change to
Sec. 25.365(d) that would require a safety factor of 1.67 times the
structural design pressure differential loads corresponding to the
maximum relief valve setting for airplanes to be approved for operation
above 45,000 feet. One commenter notes that the pressure vessel
structural design is based on fatigue loads and their effect on crack
propagation. Another commenter expresses the opinion that, as the
justification for the margin increase is concerned with damage
tolerance rather than static strength, the FAA should attack the
problem through damage tolerance requirements rather than static
strength. This commenter also states that the damage tolerance
requirements, even at altitudes below 40,000 feet, lead to stress
levels sufficiently low so that the 1.67 requirement is ``likely to be
complied with.'' A third commenter recommends changing the wording to
remove the 1.67 factor, substituting a requirement that thermal effects
on structural components and materials must be accounted for. The FAA
does not concur that the higher factor is not necessary for airplanes
operating at altitudes above 45,000 feet. A rapid decompression at
altitudes above 45,000 feet could be catastrophic to the passengers.
Therefore, this event must be extremely improbable; i.e., it is not
expected to occur during the lifetime of an entire fleet of airplanes.
Service history, however, shows that decompressions at higher altitudes
are not extremely remote events even for airplanes assessed to the
damage tolerance criteria. Loss of cabin pressure at lower altitudes
has not been catastrophic to the passengers from environmental effects
due to the higher ambient pressures and relatively short time for
emergency descent. Although application of damage tolerance techniques
will reduce the incidence of pressure vessel failures in service, there
is no reason to expect that current methodology will preclude all
future failures. To address these concerns, the FAA has determined that
requiring the higher safety factor of 1.67 will reduce the probability
of structural failures which could result in depressurization. The
static factor of 1.67 is not appropriate to account for thermal effects
because not all parts are subjected to the same temperature and also
materials may not be affected to the same degree. The current
Sec. 25.603(c) already requires that the effects of temperature be
accounted for in determining material properties. Section 25.365 is,
therefore, amended as proposed.
Two commenters note that the probability terminology regarding
proposed Secs. 25.831 (c), (d), and (g) is not consistent with that
found in regulatory and advisory material associated with Sec. 25.1309.
The FAA concurs with these comments. The terminology in the amendment
is changed to address failure conditions rather than failures or
failure combinations as proposed.
One commenter recommends allowing the fresh air requirements
proposed to be required under Sec. 25.831(a) to remain a crewmember
requirement only. The FAA does not concur with this recommendation. It
has been determined that this level of airflow is required for several
reasons. Members of the flightcrew performing their functions in the
passenger cabin are not sedentary and must perform their duties without
undue discomfort or fatigue. In addition, fresh airflow has been
determined to be necessary to provide adequate smoke clearance in the
event of smoke accumulation due to a system failure or fire. However,
it is clear that the additional airflow is not required at all times
and under all operating conditions. Therefore, the wording in the final
rule has been changed to state that the ventilation system must be
designed to provide the fresh airflow. This also addresses concerns
regarding the low fresh airflow capability that occurs during descent
at low power levels.
Two commenters note that the fresh air requirement should be 0.55
pounds of fresh air per minute per occupant rather than the 0.6 pounds
proposed in the notice. The FAA ``rounded off'' the value for mass flow
from 0.55 to 0.6 pounds of fresh air per second when proposing the
rule. Recognizing that this constitutes an increase in the level of
safety not originally intended by the FAA, and noting that the added
fresh air must be supplied at some specific cost, the final rule is
changed to require that the airplane ventilation system be designed to
provide 0.55 pounds of fresh air per minute per occupant. Another
commenter recommends that the FAA use 0.5 pounds per minute per
occupant rather than 0.6, noting that the Civil Aviation Authorities
(CAA) and other airworthiness authorities use 0.5 pounds per minute.
The FAA has determined that the 10 cubic feet per minute, converted to
0.55 pounds per minute as noted above, provides an acceptable minimum
airflow. The commenter provides no data to support the recommendation.
The rule is issued with the change noted above.
The same commenter notes that the notice does not contain clear
requirements for airflow following failures. The commenter further
notes that the JAA provides guidance in ACJ 25.831(e) regarding this
matter. The FAA has not determined that a need exists to define the
ventilation requirements following failures. The ventilation rates
following various failures conditions were not addressed either in
previously issued special conditions or Notice 89-31. In addition, the
commenter did not provide any data in support of his proposal other
than that it exists in advisory material in other airworthiness
standards.
One commenter states that 0.6 pounds of fresh air per occupant is
impractical and unjustified for commuter airplanes because available
engines do not provide sufficient bleed flow to meet the new
requirement. The FAA does not concur that this proposal is impractical
or unjustified. This rule will not apply to existing airplanes. When
new airplanes are designed and certificated, propulsion systems are
available that can provide adequate bleed air to meet these
requirements. The FAA has determined that health and safety
considerations justify the new requirements for airplanes operating at
all altitudes.
Further, the commenter states that the changes proposed for
Secs. 25.831 (c) and (d) will require an increase in reliability
requirements that is not justifiable for airplanes certificated for
altitudes below 40,000 feet. This commenter believes that the existing
wording, ``reasonably probable,'' is not equivalent to the proposed
wording, ``not extremely improbable.'' The FAA concurs with the
commenter, and has determined that these changes are not needed.
Therefore, because these were the only proposed changes to Secs. 25.831
(c) and (d), the final rule has been revised to remove the changes to
these sections.
Two commenters recommend either removing or defining the word
``uncontaminated'' as used in the proposed Sec. 25.831(a), noting that
the term is too vague, and might well be impossible to meet in, for
instance, the case where the airplane is operating in an environment
which itself contains contaminants, as might be the case near some
airports in congested areas, the FAA does not concur with the comment.
Descriptive wording is often used when the desire is to present
objective design standards. The intent in this case is to ensure that
the system
[[Page 28688]]
designer will consider the need to provide an environment conducive to
crew and passenger comfort. The FAA has prepared and plans to release
advisory material to provide more detailed guidance for use in finding
compliance with this rule.
One commenter recommends removing both the proposed and the
existing Secs. 25.831 (c) and (d), stating that the sections are
ambiguous and that the requirement that the systems perform their
intended functions under all foreseeable (normal and failure)
conditions is addressed in Sec. 25.1309. The FAA does not concur. As
noted above, descriptive terminology is used to present design
standards when specific requirements would be too inflexible and
restrictive. Further, Sec. 25.1309 is not intended to be the sole
regulation for use in determining acceptability of system design when
failure conditions exist. The FAA has found that individual rules are
desirable when addressing specific functions, such as those governing
ventilation requirements, in order to ensure adequate consideration of
the specific issues identified.
One commenter suggest changing the wording of the proposed
Sec. 25.831(d) from ``If the accumulation of hazardous quantities of
smoke * * *,'' noting that in-service experience has shown that
accumulation of smoke is reasonably likely. The FAA concurs that the
accumulation of smoke in cockpits has occurred on numerous occasions,
and is not an extremely improbable event. However, future designs may
embody features that render smoke accumulation extremely improbable.
Should a manufacturer be able to show such reliability, smoke
evacuation should not be required to be demonstrated.
Two commenters note that protection from smoke in the cockpit
cannot be ensured, even while wearing and using the crewmember oxygen
equipment stipulated in the proposed Sec. 25.1447(c)(3), unless an
``emergency pressure (1 to 3 inches of water) is provided to ensure
positive mask pressure and flow into goggles.'' The FAA recognized that
a positive pressure differential between the inside of the mask and
ambient is desirable. Many existing regulators have a ``test'' or
``emergency'' position to provide the pressure differential noted
above. However, the FAA does not concur that this approach needs to be
required by regulation, and has not proposed such a change. For the
purposes of this rulemaking, the preamble of Notice 89-31 merely notes
that one of the advantages of the pressure demand mask is that, if
either the 100 percent or the full positive pressure (sometimes called
``test'') setting is selected, protection from smoke within the cockpit
would be provided. While the degree of protection is not identified,
selection of either of these settings does eliminate the ambient air
which is inspired with diluter demand masks, thus reducing the risk of
smoke or fumes being inhaled by the wearer.
Three parties offer comments on the proposed new Sec. 25.831(g).
One commenter recommends continuing the time/temperature curve proposed
for this section beyond 90 minutes, and recommends referring to the
curve in the FAA SST ``white book,'' TENTATIVE AIRWORTHINESS STANDARDS
FOR SUPERSONIC TRANSPORTS. Copies of the appropriate pages from that
document have been added to the docket for this rulemaking action. The
FAA infers that the commenter believes the curve should be extended to
200 plus minutes because that is the extent of the graph in the white
book. The FAA does not concur with this comment. The curve in the white
book actually ends at 90 minutes for a temperature of 90 degrees
Fahrenheit (90 deg.F), although the actual graph grid extends to over
200 minutes. The FAA, in responding to comments on previously issued
special conditions for high altitude operations, modified the SST time/
temperature curve by increasing the allowable maximum temperature from
90 degrees to 100 degrees Fahrenheit to accommodate aircraft while
operating in high ambient temperature conditions. It was noted that it
would be difficult to meet the temperature maximums while operating on
the ground with outside temperatures above 100 degrees. The end point
on the proposed curve indicates that the exposure time to a temperature
of 100 degrees Fahrenheit (100 deg.F) shall not exceed 90 minutes. The
FAA has determined that the limits established by this curve are
appropriate for improbable failure conditions. In addition, there were
no other comments addressing the proposed time/temperature limits.
Considering the above, the curve in the final rule is retained as
proposed.
A second commenter states that this amendment is not justified for
airplanes operating below 40,000 feet. The FAA infers that the
commenter is recommending removing this proposal. The FAA does not
concur that this change is unjustified. Excessive temperatures in the
crew and passenger compartments can present a hazard to continued safe
flight and landing for any airplane. Therefore, although this hazard is
not regarded as sufficient to warrant retroactive application of these
requirements to existing designs, these improvements in design
standards are appropriate and cost effective for future designs. While
this change was proposed primarily to codify existing special
conditions for high altitude operation, it is also appropriate for
airplanes certificated for operation at lower maximum altitudes. A
third commenter recommends changing the proposed rule to clarify that
the amended rule is directed at airplanes which utilize high
temperature air to maintain pressurization following failure
conditions. While the FAA concurs that the requirement, which
originated in existing special conditions, was directed primarily at
such airplanes, the amended rule is intended to apply to any failure
condition that can result in excessively high temperatures. For the
above reasons, Sec. 25.831(g) is added as proposed.
One commenter recommends leaving the phrase ``Pressurized cabins
and compartments to be occupied * * *'' in Sec. 25.841(a) rather than
changing it to ``Pressurized cabins and any other occupied compartments
* * *'' as proposed. The commenter notes that this change is not
addressed in the preamble to the proposal, and expresses concern that
the change in wording might result in a change in interpretation. The
FAA does not concur with this comment. This change in wording does not
change the meaning of the Section, and, in the opinion of the FAA, is
clearer.
One commenter recommends adding a section to the proposed
Sec. 25.841(a)(3) to note that ``Turbine engine installations failures
must be assessed according to the specific requirements of
Sec. 25.903(d) * * *'' The FAA does not concur with this
recommendation. It is not clear how adding this detail would clarify
the requirements for assessing the damage resulting from an contained
engine failure. Further clarification is considered to be appropriate
for advisory material, and the FAA addresses uncontained engine failure
in the advisory circular which was proposed concurrent with Notice 89-
31.
One commenter states that the proposed Sec. 25.841(a)(1) calls for
``an unjustified reliability increase relating to the pressurization
system.'' The FAA infers that the commenter is requesting that the rule
continue to address only those failures which are ``reasonably
probable.'' The FAA does not concur. As noted earlier, reasonably
probable has been interpreted by the FAA to include both the probable
and
[[Page 28689]]
improbable categories. For this reason, the new wording does not
constitute an increase in the required reliability.
The same commenter states that the proposed Sec. 25.841(a)(2) will
be in conflict with the proposed Sec. 25.841(a)(1). The FAA does not
agree. Section 25.841(a)(1) addresses acceptable cabin pressure
altitudes following probable failure conditions, while
Sec. 25.841(a)(2) addresses cabin altitudes following failure
conditions not shown to be extremely improbable, i.e., probable and
improbable failure conditions.
One commeter expresses the concern that the adoption of the
proposed Sec. 25.841(a)(2)(i), which limits exposure to cabin pressure
altitudes exceeding 25,000 feet to a maximum of 2 minutes for failure
conditions not shown to be extremely improbable, will result in
``severe restrictions on flight routes as well as maximum certification
altitude.'' The commeter states that the proposed Secs. 25.841(a)(2)
and (a)(3) are proposed to address concerns regarding ``extremely rapid
decompressions which may occur with small volume, high altitude (to
51,000 feet) executive transport airplanes,'' and recommends that the
FAA remove these sections from the final rule. The FAA does not concur.
While it is true that one of the reasons for formulating this rule
change was to codify the certification requirements previously issued
as special conditions for small volume transport category airplanes
requesting approval for high altitude operation, the FAA has reviewed
the service history of rapid depressurizations on all transport
category airplanes including those with large pressurized volumes. Such
events, while rare, do occur in service. The effects of exposure to
altitudes above 25,000 feet for more than 2 minutes, or to an altitude
above 40,000 feet for any period of time, are discussed in the preamble
of the notice. If an applicant can show that failure conditions leading
to excellence of these cabin altitudes are extremely improbable, there
is no impact on operating altitude. As to having a significant effect
on operating altitudes, this requirement does not affect airplanes
already certificated, so there would be no ``more extensive
requirements on the current commercial fleet.'' This commenter also
recommends changing ``any probable failure or failure combinations'' to
``any probable failure or probable failure combination.'' As noted
earlier, the FAA is changing the wording for both Secs. 25.831 and
25.841 to ``failure conditions,'' which covers failures and
combinations of failures, and more closely parallels Sec. 25.1309
terminology.
One commenter recommends revising Sec. 25.841(a)(1) to show that
``In case of dispatch with equipment inoperative per an approved
Minimum Equipment List (MEL), only reasonably probable failures or
reasonably probable failure malfunctions need be considered,'' when
addressing the 15,000 feet maximum cabin altitude requirement of this
section. The commenter notes that dispatch under an approved MEL with
one of two air conditioning packs inoperative has been a safe practice.
The FAA does not concur with this recommendation. The certification
rules in part 25 do not address MEL dispatch. In the case of dispatch
with one pack inoperative, the practice followed in recent
certification projects has been to limit the operating altitude of an
airplane dispatching under these conditions to that which has been
demonstrated in that configuration considering the effect of potential
failures. The FAA intends that this practice be continued under this
rule.
One commenter suggests adding a new Sec. 25.841(a)(2)(iii) reading
``Compliance with paragraph (i) is not required for cabin altitude
versus time profiles where exposure above ten thousand feet does not
exceed 10 minutes.'' The commenter notes that operating rules
(Sec. 121.333(a)) assume that the airplane descends from the maximum
altitude to 10,000 feet in ten minutes, and that permanent ill effects
from hypoxia under present operating rules have been rare. Further,
recent special conditions for the Beech Model 400A and British
Aerospace Model BAe Model 125-1000A airplane contains cabin altitude
versus time curves which support the ``ten minutes above 10,000 feet''
criteria. The FAA does not concur with the commenter's suggestion. The
cabin altitude limitations stipulated in the special conditions were
interim standards applicable to those airplanes only. Physiological
data from CAMI have resulted in the FAA establishing the requirements
for cabin altitudes as they are stated in the proposal. Adopting the
commenter's proposal could result in an applicant being allowed to
demonstrate compliance while showing exposures to cabin altitudes up to
40,000 feet for extended periods while still meeting the standards,
which would be unacceptable. The FAA has determined that preventing the
occupants from being exposed to cabin altitudes greater than 25,000
feet for more than 2 minutes or 40,000 feet for any duration will
provide an acceptable level of safety at an acceptable cost.
This commenter also suggests adding a new Sec. 25.841(a)(2)(iv) to
allow the occupants to be exposed to cabin altitudes greater than
25,000 feet or 10,000 feet (if (iii) were adopted) when minimum flight
altitudes make literal compliance with these sections impractical. The
commenter is concerned that literal compliance with Sec. 25.841(b)
would result in prohibition of flight over the Himalayas or Andes, or
in certain areas where minimum altitudes are stipulated. The FAA does
not share this concern. The proposed rule requires design features to
prevent the exposure of occupants to the high cabin altitudes in the
presence of failure conditions. The ability to operate in areas where
operational constraints dictate minimum flight altitudes is a function
of operating rules and appropriate flight planning in terms of
supplemental oxygen, etc. The certification rules do not address these
considerations.
The same commenter recommends changing Sec. 25.841(a)(3) to more
precisely define the manner in which various causes of a decompression
are treated, and suggests subparagraphs treating uncontained engine
failure, fuselage structural failure, discrete source failure, and
system failure separately. The FAA does not agree that these details
are appropriate for inclusion in the certification rule. The FAA plans
to provide guidance material regarding the manner in which the various
failure cases may be addressed.
One commenter supports the rulemaking but states that ``Existing
crew and passenger emergency oxygen systems in civil aircraft do not
have sufficient pressure breathing capability to protect the individual
for the required length of time for controlled descent to below 33,000
feet where, I believe, existing oxygen systems may function adequately
for life support.'' The FAA infers from this comment that the commenter
desires that this proposal contain new requirements for oxygen systems.
The FAA does not agree with this commenter concerning equipment used by
the flightcrew. The FAA has determined that the oxygen dispensing
equipment required by this rule will provide adequate protection when
the exposure envelopes are observed. The FAA shares the commenter's
concern with respect to the passenger oxygen equipment. While the
passenger equipment is certificated to operate to a pressure altitude
of 40,000 feet, the physiological effects of decompression on the
passengers may prevent the equipment from being effective in all cases.
The alternatives would be to require the passengers to breathe 100
percent oxygen at the altitudes of concern or to prohibit operation at
the
[[Page 28690]]
higher altitudes. Breathing 100 percent oxygen by all passengers is
considered to be an unacceptable solution from an operational
standpoint, and the exposure envelopes adopted for this rule have been
selected to mitigate the limitations of the passenger oxygen system. It
is considered that developing new oxygen equipment standards to be
included with this rule is unwarranted. The FAA has determined that
operation at the altitudes addressed in this rule can be accomplished
with an acceptable level of safety, and this rule has established cost
effective means of attaining that goal.
One commenter suggests that the requirement in Sec. 25.1447(c)(1)
for automatic presentation of oxygen dispensing units if certification
for operation above 30,000 feet is requested refer to 31,000 feet, as
30,000 feet (FL300) is not an authorized cruising altitude. The FAA
agrees that this is not a cruising altitude. However, the FAA does not
concur that it is inappropriate to stipulate a requirement for
operation above 30,000 feet. Further, this requirement is unchanged
from the existing rule.
A second commenter recommends amending Sec. 25.1447(c)(1) by
removing the requirement for supplemental oxygen for passengers if the
cabin altitude limits in Notice 89-31 are adopted. The commenter states
that it is not realistic to expect all passengers to utilize the oxygen
system, and infers that if the limits proposed are adopted, the risk to
healthy passengers is minimal. The FAA does not concur with this
comment. If the FAA were to follow the commenter's logic, i.e., not to
require passenger oxygen systems, the exposure envelope would limit the
cabin altitude to 15,000 feet. Historical events and decompression
tests indicate that supplemental oxygen is needed even when the cabin
pressure altitudes required by this rule are observed. Further, this
requirement is unchanged from the existing rule. No other comments were
received on the proposed Secs. 25.1447 (c)(1) and (c)(2) and they are
adopted as proposed.
One commenter states that Sec. 25.1447(c)(3) requires pressure
demand masks for operation above 25,000 feet but the justification in
the preamble of the notice states that diluter demand masks are
acceptable up to 34,000 feet. The FAA does not agree with this comment.
Section 25.1447(c)(3)(i) requires a diluter demand or pressure demand
(pressure demand mask with a diluter demand pressure breathing
regulator) type mask for airplanes to be operated above 25,000 feet.
The pressure demand (pressure demand mask with a diluter demand
pressure breathing regulator) type with a mask-mounted regulator is
required for airplanes operated at altitudes where decompressions that
are not extremely improbable may expose the flightcrew to cabin
pressure altitudes above 34,000 feet.
One commenter recommends that the pressure breathing requirements
of Secs. 25.1447(c)(3)(i) and (ii) be detailed in the form of mask
pressure versus cabin altitude curves. The commenter suggests that the
current pressure breathing equipment specified under Technical Standard
Order TSO-C89 may not be acceptable for cabin altitudes up to 45,000
feet. The commenter provides no rationale in support of his
recommendation. The FAA does not concur. The type of data recommended
by the commenter is appropriate to TSO requirements, and the revision
to those documents is beyond the scope of this notice. Further, one of
the purposes of this rulemaking is to provide protection by preventing
exposure of the occupants to cabin altitudes above 40,000 feet. Masks
and regulators are currently in use that meet the requirements in the
curves submitted by the commenter for conditions up to that altitude.
One commenter notes that a pressure demand mask with a mask-mounted
regulator may have different oxygen delivery percentage requirements
under TSO-C89 depending on the altitude for which it is certificated.
The commenter suggests that the rule clarify the mask and regulator
requirements by stipulating the altitude to which the mask and
regulator are approved under the TSO. The FAA does not concur with this
suggestion. By specifying the type of oxygen equipment for the crew,
and the manner of its use, the FAA has determined that the flightcrew
will retain the ability to safely operate the airplane during a
decompression.
One commenter suggests withdrawing the proposed
Sec. 25.1447(c)(3)(ii) because the equipment standards defined in TSO-
C89 ``provide the necessary oxygen up to 40,000 feet, and are
considered safe.'' The FAA does not concur. There is no requirement
that the equipment used in transport category airplanes be approved
under a TSO. As discussed in the notice, operation at altitudes which
can, in the event of a rapid decompression, result in incapacitation or
a physiological hazard to the occupants requires oxygen equipment to
meet the specific environments that may be encountered. It is
recognized that equipment with TSO authorization is available that will
provide the required protection at a reasonable cost. The intent of
this rulemaking is to identify a minimum equipment standard that is
known to provide this protection, and that equipment is called out in
the amended sections.
Another commenter suggests amending Sec. 25.1443 by addition of a
curve of ``cabin pressure altitude versus minimum required oxygen mass
flow'' for cabin altitudes from 0 to 51,000 feet which would replace
the generic mass flow requirement which appears in Sec. 25.1441. The
FAA does not concur with this comment. A revision to Sec. 25.1443 as
suggested by the commenter would not increase the level of safety.
Existing rules related to oxygen mass flow provide an adequate level of
safety. If such material were to be added, this level of detail would
be more appropriate in a Technical Standard Order or the advisory
material that has been proposed to accompany this rulemaking action.
One commenter recommends deleting Sec. 25.1447(c)(3)(ii) both as it
now exists and as proposed. The existing section is deleted for the
reasons noted in the preamble to Notice 89-31. The commenter believes
that the section as proposed, which stipulates the use of ``a pressure
demand (pressure demand mask with a diluter demand pressure breathing
regulator) type with a mask-mounted regulator,'' is unduly restrictive
by requiring a mask-mounted regulator, and dictates a design solution.
Additionally, the commenter states that Secs. 25.1441(d) and 25.1443(b)
and Technical Standard Order TSO-C89 address oxygen equipment, thereby
obviating the need for the proposed section. Another commenter
recommends that the FAA define the required oxygen equipment (diluter
demand and pressure demand masks) in terms of performance rather than
by stipulating a specific equipment type. The FAA does not concur with
these comments. The specific descriptions for the oxygen equipment that
is proposed in these amendments has been determined by the FAA to be
necessary to provide protection for the flightcrew in cases where the
cabin altitude will exceed the specified levels. Neither of the FAR
sections nor the TSO data provide adequate assurance of that
protection. The FAA believes that this detailed stipulation is
necessary to ensure the protection and to provide standardization in
interpretation of the new requirements. However, the FAA intends to
allow sufficient latitude for system designers to develop safer and/or
less expensive approaches to specific requirements. For this reason,
Sec. 25.1447(c)(3)(ii) is changed to allow other means of protection
for flight
[[Page 28691]]
crewmembers if the proposed equipment affords the same protection.
One commenter states that existing panel-mounted diluter-demand
regulators have proven satisfactory. This party suggests that the
pressure-demand mask with a mask-mounted regulator be mandatory for
newly certificated airplanes only. The FAA agrees that panel mounted
regulators have proven satisfactory, but the FAA has determined that in
a high altitude rapid decompression, the protection afforded by a mask
mounted regulator is superior to that found in panel mounted
regulators. As noted in the preamble of the notice, the time delay in
providing 100 percent oxygen to the flight crewmember, which results
from the air in the hoses of the oxygen equipment, can significantly
negate the hypoxic protection of such equipment. Further, this
amendment constitutes a revision to part 25, and is not applicable to
the existing fleet. It is, however, the FAA's position that every
effort be made to provide a level of safety equal to the latest
certification standards for existing airplanes that are updated by
amended or supplemental type certification. The FAA's policy regarding
establishment of the type certification basis for derivative airplanes
is described in Action Notice A 8110.23, dated September 26, 1990. A
copy of this document has been placed in the Rules Docket. Following
issuance of these amendments, the concepts contained herein would be
applicable to airplanes which incorporate changes in the oxygen systems
or increases in approved operating altitudes, in accordance with
Sec. 21.101. For high altitude approvals, this has been accomplished in
the past through special conditions which contain provisions
essentially the same as those embodied in these amendments.
Several comments express concerns regarding long term use of 100
percent oxygen by fightcrews. One of these parties suggests that the
crew member use normally diluted oxygen with the regulator set at the
``normal'' position. Another states that 100 percent oxygen should not
be permitted unless adequate safeguards have been established. A third
party states that 100 percent oxygen should be used only for short
periods as an emergency measure due to a health hazard. One commenter
recommends deleting the proposed Sec. 25.1447(c)(4) and retaining
Sec. 121.333(c)(2), which requires at least one pilot to wear and use
an oxygen mask at altitudes of 41,000 feet and greater. Another
commenter believes that wearing an oxygen mask at lower altitudes ``is
not necessary nor is it useful.'' One commenter notes that breathing
100 percent oxygen will dry out the lungs, can lead to narcosis, and
states that the long term effects are not clearly understood. Another
commenter recommends deleting the proposal to require the wearing of
masks and revert to the requirements in the operating rules. Another
commenter states that large volume transports decompress slowly giving
crews more time to don oxygen masks, and current large transports are
certificated to 45,000 feet without requiring the flightcrew to be
using oxygen. The FAA infers that the commenter believes that this
proposal should not apply to ``large'' transport airplanes. The FAA
does not concur with this viewpoint. The physical size of the airplane
is not germane; the important parameter is the post-decompression cabin
altitude and its effect on occupants. One commenter notes that the
requirement for prebreathing 100 percent oxygen would necessitate
additional oxygen supplies at added cost. Finally, one commenter
questions whether breathing 100 percent rather than 40 percent oxygen
provides better protection in terms of blood oxygen saturation level.
This commenter provides data showing that prebreathing 30 to 40 percent
oxygen provides adequate protection against the effects of hypoxia
following rapid decompression. The data show that the blood oxygen
saturation level following the decompression is not significantly
depressed even if the crew member is breathing 30 percent oxygen, as
long as the oxygen supplied to the crew member goes to 100%
immediately. After considering all the negative comments received and
reviewing existing data regarding high altitude decompressions, the FAA
has determined that it is appropriate to withdraw this proposal. The
proposed Sec. 25.1447(c)(4), requiring that one flight crewmember be
wearing an oxygen mask and breathing 100 percent oxygen when operating
at altitudes where the cabin altitude can reach 34,000 feet in the
event of a decompression, has been withdrawn.
One commenter states that, regarding the proposed
Sec. 25.1447(c)(5), portable oxygen equipment would only be ``at hand''
if the crew members were sitting by the oxygen equipment or were
actually using it, and recommends striking the work ``immediately''
from the proposal. The FAA does not believe this change is necessary or
warranted. This requirement is retained from the existing
Sec. 25.1447(c)(4), and is considered met in existing airplanes by
having portable oxygen equipment located adjacent to the crew member
seat with additional units located at specific locations in the
passenger cabin. The FAA anticipates that industry will continue to
provide this protection in the same manner as it has done in existing
airplanes, with no change in the rule or in FAA policy regarding
showing compliance.
Two commenters point out that the nomenclature used in the glossary
of the notice misidentified the type of passenger oxygen equipment used
in airplanes with altitudes above 35,000 feet. One commenter recommends
changing the definition in the Glossary for ``Continuous Flow Oxygen
Systems'' to note that the type of equipment used is a mask with a
``reservoir'' bag rather than a ``rebreather'' bag. The FAA concurs
with these comments, and the glossary is changed to reflect the
terminology used in current descriptive literature.
One commenter notes that, while special conditions have been issued
covering various airplanes requesting approval for high altitude
operations, this proposal impacts all airplanes seeking certification
under part 25 of the FAR, including those with maximum flight altitudes
less than 41,000 feet. These proposals constitute increased standards
for those airplanes. The FAA concurs with this statement. This
rulemaking addresses the physiological limitations of occupants of
transport category airplanes which can experience depressurization to
cabin altitudes greater than 34,000 feet. However, the commenter does
not recommend any specific changes in the proposals.
The JAA notes that future rulemaking relative to the Joint
Airworthiness Regulations (JAR) will require retroactive application
for each new amendment, and asks if the FAA is considering similar
action. As noted earlier, application of new amendments to the FAR are
made applicable to type certification programs in accordance with
Sec. 21.101 of the FAR. There are no plans to require retroactive
application of new amendments to the existing fleet, as suggested by
the JAA. The JAA also suggests considering a number of added concerns
regarding operations at high altitudes, such as the effects of icing on
airspeed and pressure probes, changes in static stability criteria for
high mach/high altitude operation, and health hazards related to cosmic
radiation during high altitude cruise. A second commenter recommends
that the proposal be revised to address standards related to the
exposure of crewmembers to cosmic radiation when operating at altitudes
up to 51,000 feet. The effects of icing (ice crystals) on airspeed and
pressure probes and stability criteria
[[Page 28692]]
were not considered in the special conditions issued prior to this
rulemaking, and no data was submitted by the commenter to support its
position. No action is contemplated by the FAA regarding these
comments. The effects of cosmic radiation are not addressed in this
proposal, and no data were submitted by either commenter in support of
their suggestions. The FAA is aware of the concerns expressed by the
commenters and may consider further rulemaking to address those
concerns.
One commenter suggests requiring initial and periodic training
including altitude chamber and pressure breathing instruction for
pilots of airplanes affected by this rulemaking. As the certification
rules in part 25 do not address specific training requirements, this
proposal is outside the scope of this rulemaking. However, this
proposal will be discussed with the FAA organization responsible for
crew training.
One commenter notes that the FAA should require improvements in
pressure demand masks to improve comfort, and suggests that research
and development in comfort and human factors is needed. The FAA
believes that there is oxygen equipment available that meets the
requirements of this rule and also provides an acceptable level of
comfort. The small executive jet airplanes approved under existing
special conditions are so equipped. If further improvements are needed,
the marketplace will drive the development and availability of these
products.
One commenter suggests that the FAA has failed to consider the
relatively small transport category airplanes intended for commuter
airline operation. The example noted is a 16,000 pound airplane
intended to carry 25 passengers, operating at altitudes of 25,000 to
30,000 feet. The commenter states that the manufacturer will apply for
certification to the highest expected operating altitude and the
amendments of this proposal will apply. The specific comments related
to these concerns are addressed elsewhere in this document, but the
commenter apparently believes that these applicants should not have
these requirements imposed on their airplanes. The position adopted by
the FAA with this rulemaking action is that any airplane operating at
flight altitudes where decompression can result in a hazard to the
occupants must be designed to provide protection.
One commenter recommends leaving the regulations as they now exist
for large airplanes operating up to 45,000 feet and directing the
proposed rules to the smaller airplanes operating at higher altitudes.
This party states that large airplanes certified under the existing
rules provide an acceptable level of safety, and the proposed rules
will result in ``undue restrictions or unvalidated costly additional
effort.'' Another commenter expresses a similar opinion, and comments
that adoption of these standards will have a significant economic
impact due to requiring retrofit of many existing airplanes. The FAA
does not share these views. The protection afforded the occupants
should be the same for any transport category airplane, regardless of
volume. Larger airplanes have shown decompression characteristics
similar to the small airplanes. If the applicant can demonstrate that
the cabin altitude does not exceed prescribed limits, many of the
provisions of this amendment do not apply. In any case, these rules are
not retroactive to existing airplanes as a result of this rulemaking,
and only new or modified airplanes are required to meet the new
requirements. Another commenter makes the point that there have been
recent decompression events involving large airplanes wherein the
decompression ``is surely as explosive as any to be realized on a
smaller Lear Jet . . .,'' and agrees with the proposals.
Another commenter believes that existing supplemental oxygen
systems are acceptable, and if the requirements in Notice 89-31 are
adopted, there are strong arguments for elimination of the passenger
oxygen system. The FAA does not concur with these statements. While it
is recognized that not all passengers will be able to don their oxygen
equipment, the protection afforded by the systems currently installed
provides acceptable protection from the effects of hypoxia at an
acceptable cost for the majority of the occupants from the effects of
hypoxia. Even when the decompression event is slower or the cabin
altitude is limited, and the oxygen masks are not absolutely essential
for survival, some protection is afforded to all the passengers when
the cabin altitude exceeds safe limits. The operating rules also
require the installation of this equipment.
One commenter states that the economic analysis reflects an
operating cost increase of $19 million per year, implying that the rule
would have to save 19 lives per year to be reasonable. The same
commenter recommends revising the Regulatory Flexibility Determination
because small entities may operate affected airplanes and may incur
increased operating costs. In each case, the commenter appears to be
referring to FAA's economic analysis of proposed Sec. 25.1447(c)(4). As
noted earlier, Notice 89-31 proposed that Sec. 25.1447(c)(4) require
that one flight crewmember wear an oxygen mask and breathe 100 percent
oxygen when operating at altitudes where the cabin altitude can reach
34,000 feet in the event of a decompression. In response to public
comments and cost considerations, the FAA has withdrawn this proposal
and will subject it to further study. In regard to the commenter's
recommendation regarding small entities, the magnitude of the costs and
the number of affected small entities, rather than simply the incidence
of costs, are the criteria by which a rule is judged to have a
significant economic impact on small entities. A regulatory flexibility
determination of the final rule is presented in the next section of
this document.
The same commenter also states that the Regulatory Evaluation does
not take into consideration evolving FAA policy of applying the latest
FAR amendments when determining the certification basis for amended
type certifications. The FAA agrees and has added this policy to this
final regulatory evaluation, without affecting the justification of the
rule. It is FAA's policy that every effort be made to provide a level
of safety equal to the latest certification standards for existing
airplanes that are updated by amended or supplemental type
certificates. Amendments to the FAR may be made applicable to
derivative airplanes in accordance with Sec. 21.101 if it is determined
that the new or redesigned system is not adequately addressed in the
regulations incorporated by reference to the type design.
The commenter also identifies a statement in the NPRM Regulatory
Evaluation that incorrectly assumes that new airplanes will not have
engines mounted in positions which could damage the fuselage. The
commenter appears to be misinterpreting FAA's language. The statement
being referred to by the commenter is one pertaining only to small
volume transport airplanes. The FAA agrees that most other transport
category airplanes will have wing-mounted engines located such that
fragments from an engine burst could affect the fuselage and pressure
vessel.
References
Reference 1. ``Factors Influencing the Time of Safe
Unconsciousness (TSU) for Commercial Jet Passengers Following Cabin
Decompression'' by James G. Gaume, Aerospace Medicine, April 1970.
Reference 2. Aerospace Information Report (AIR) No. 822 and 825B
(Physiology Section); SAE Committee A-10.
Copies of pertinent portions of these documents have been placed in
the
[[Page 28693]]
Rules Docket and are available for public inspection.
Glossary
Physiology Altitude Limits. The response of human beings to
increased altitude varies with the individual. People that smoke or are
in poor health will be affected at a much lower altitude than people
who are young and in good physical condition. Without supplementary
oxygen, most people will begin to experience a reduction in night
vision or general visual acuity at approximately 5,000 feet altitude.
At an altitude of approximately 10,000 feet, a person will begin to
display measurable deterioration in mental abilities and physical
dexterity after a period of several hours. At 18,000 feet, the mental
deterioration may result in unconsciousness, and the time of useful
consciousness (TUC) is generally about 15 minutes. At 25,000 feet, the
TUC for most people is about 3-10 minutes. At altitudes above 25,000
feet, the TUC decreases very rapidly, becoming only a few seconds at
40,000 feet. If a person is breathing 100 percent oxygen, however, the
partial pressure of oxygen in the lungs at 34,000 feet altitude is the
same as that for a person breathing air at sea level. At 40,000 feet, a
person breathing 100 percent oxygen will have the same partial pressure
of oxygen in the lungs as a person breathing air at 10,000 feet.
Therefore, 34,000 feet is the highest altitude at which a person would
be provided complete protection from the effects of hypoxia, and 40,000
feet is the highest altitude at which 100 percent oxygen will provide
reasonable protection for the time period needed to descend to a safe
altitude.
Hypoxia. Hypoxia is a condition caused by insufficient oxygen. It
results from the reduced oxygen partial pressure in the inspired air
caused by the decrease in barometric pressure with increasing altitude.
Diluter Demand Oxygen System. A flightcrew oxygen system consisting
of a close-fitting mask with a regulator that supplies a flow of oxygen
proportional to cabin altitude. Regulators are usually designed to
provide zero percent oxygen and 100 percent cabin air at cabin
altitudes of 8,000 feet or less, with the ratio changing to 100 percent
oxygen and zero percent cabin air at approximately 34,000 feet cabin
altitude. Oxygen is supplied only when the user inhales, reducing, the
amount of oxygen that is required.
Pressure Demand Oxygen System. Similar to diluter demand equipment,
except that oxygen is automatically supplied to the mask under pressure
at cabin altitudes above approxmately 34,000 feet. This pressurized
supply of oxygen provides some additional protection against hypoxia at
altitudes up to 39,000 feet.
Pressure Demand Mask With Mask-Mounted Regulator. A pressure demand
mask with the regulator attached directly to the mask, rather than
mounted on the instrument panel or other area within the flight deck.
The mask-mounted regulator eliminates the problem of a long hose which
must be purged of air before oxygen is delivered to the mask.
Continuous Flow Oxygen System. The oxygen system typically provided
to passengers. The passenger mask most commonly used in transport
category airplanes is equipped with a reservoir bag, which is
replenished by a continuous flow of oxygen. This design incorporates a
check valve between the reservoir bag and the face mask to prevent
introduction of exhaled gasses into the bag and assure 100% oxygen in
the reservoir. Dilution is accomplished at the later phases in
inspiration by a loaded ambient air valve which introduces ambient air
following depletion of the oxygen in the reservoir bag.
Probable Failures. Probable failures may be expected to occur
several times during the operational life of each airplane. The
probability of occurrence is on the order of 1 x 10-5 or greater
(Advisory Circular 25.1309-1A). The consequences of the failure or the
required corrective action may not significantly impact the safety of
the airplane or the ability of the crew to cope with adverse operating
conditions. Systems that operate within this category are referred to
as nonessential systems.
Improbable Failures. Improbable failures are not expected to occur
during the total operational life of a random single airplane of a
particular type, but may occur during the total operational life of all
airplanes of a particular type. The probability of occurrence is on the
order of 1 x 10-5 or less. The consequences of the failure or
the required corrective action must not prevent the continued safe
flight and landing of the airplane. Systems that operate within this
category are referred to as essential systems.
Extremely Improbable Failures. Extremely improbable failures are so
unlikely that they need not be considered to ever occur, unless
engineering judgement would require their consideration. The
probability of occurrence is on the order of 1 x 10-9 or less.
This category includes failures or combinations of failures that would
prevent the continued safe flight and landing of the airplane. Systems
that operate within this category are referred to as critical systems.
Regulatory Evaluation Summary
Proposed changes to Federal regulations must undergo economic
analyses. First, Executive Order 12866 directs that each Federal agency
shall propose or adopt a regulation only upon a reasoned determination
that the benefits of the intended regulation justify its costs.
Section, the Regulatory Flexibility Act of 1980 requires agencies to
analyze the economic effect of regulatory changes on small entities.
Third, the Office of Management and Budget directs agencies to assess
the effects of regulatory changes on international trade. In conducting
these analyses, the FAA has determined that this rule: (1) will
generate benefits that justify its costs; (2) is not a ``significant
regulatory action'' as defined in the Executive Order and is not
``significant'' as defined in DOT's Regulatory Policies and Procedures;
(3) will not have a significant economic impact on a substantial number
of small entities; and (4) will not constitute a barrier to
international trade. These analyses, available in the docket, are
summarized below.
Regulatory Evaluation Summary
The rule may impose relatively incremental costs in that applicant
manufacturers will be required to demonstrate compliance and operators
may experience increased operating costs. The FAA has determined that
these potential incremental costs will be exceeded by the safety and
efficiency benefits of the rule.
A. Ventilation and Cabin Cooling--Sec. 25.831 (a), (c), (d), and (g)
The FAA has determined that health and safety considerations
justify the airflow design requirements of Sec. 28.831(a) for all
transport category airplanes. First, cabin crewmembers must be able to
perform their duties without undue discomfort or fatigue. Secondly,
benefits may be realized from the assured availability of the
additional airflow when it is required. Third, fresh airflow is
necessary to provide adequate smoke clearance in the event of smoke
accumulation in the passenger cabin, an event which has occurred on
several occasions. Fourth, administrative benefits will be realized
because codified regulations are more efficient than special
conditions. Finally, it is noted that other airworthiness authorities
have comparable ventilation standards.
[[Page 28694]]
The airflow design requirements in revised Sec. 25.831(a) are not
expected to result in significant cost changes. Incremental design and
manufacturing costs will be negligible because most current airplane
models were designed with the additional airflow capability and, even
in the absence of this rule, future airplane models would likely
continue to be so designed. Incremental operating costs are expected to
be nominal because the rule isn't an operating requirement and because
the additional airflow is not required at all times and under all
operating conditions. Furthermore, to the extent that the amendment
codifies special conditions that would have continued to be applied to
future high altitude airplane certifications, it will not cause changes
in costs.
The new Sec. 25.831(g) supplements the requirements found in
Sec. 25.1309 by limiting exposure times to excessive temperatures in
the crew and passenger compartments which can present a hazard to
continued safe flight and landing, and the limits are appropriate for
all transport category airplanes, regardless of certificated maximum
flight altitude.
B. Pressurization and Pressure Vessel Integrity--Secs. 25.365(d) and
25.841(a)
The higher structural safety factor in revised Sec. 25.365(d) is
necessary for airplanes operating above 45,000 feet because a rapid
decompression could be catastrophic to occupants. Therefore, the FAA
finds that this event should be extremely improbable; i.e., not
expected to occur during the lifetime of an entire fleet of airplanes.
Service history shows that decompressions at high altitudes are not
extremely remote events even for airplanes assessed to damage tolerance
criteria. Loss of cabin pressure at lower altitudes has not been
catastrophic due to higher ambient pressures and relatively short
emergency descent time. The higher structural safety factor was
included in the SST and executive transport category airplane special
conditions to reduce the likelihood of structural failure and to limit
the size of the opening if a failure occurs. The amendment will have a
negligible cost.
Revised Sec. 25.841(a) will provide airworthiness standards that
allow subsonic airplanes to operate at the highest altitude for which
the applicant manufacturer chooses to demonstrate that, after
decompression caused by a single failure or combination of failures
that are not shown to be extremely improbable: (1) the flightcrew will
remain alert and be able to fly the airplane; (2) the cabin occupants
will be protected from the effects of hypoxia; and (3) in the event
that some occupants do not receive supplemental oxygen, they
nevertheless will be protected against physiological injury.
Revised Sec. 25.841(a)(1) is equivalent to existing Sec. 25.841(a)
except for editorial changes, elimination of the words ``reasonably''
and ``or malfunctions,'' and addition of the term ``failure
conditions.'' Revised Sec. 25.841(a)(2), which limits exposure of
occupants after decompression to a cabin altitude not greater than
40,000, is unchanged from previously established standards for
airplanes using diluter demand (flightcrew) and continuous flow
(passenger) oxygen equipment. It combines the executive transport
category high altitude special conditions and Sec. 25.1309, i.e., the
degree of the hazard must be inversely related to the probability of
the failure condition.
The FAA has determined that the amendment will provide an
acceptable level of safety at an acceptable cost. To demonstrate
compliance with revised Sec. 25.841, an approved emergency descent
procedure and a cabin altitude analysis must be prepared and the crew
would perform an emergency descent in accordance with the approved
procedure. For probable system failures, the critical failure case
(probable system failure) system failure tests must be conducted at the
maximum airplane altitude. For improbable failures, the cabin altitude
could be established by analysis and verified by tests at a lower
altitude with the results extrapolated to the higher altitude. To the
extent that the rule codifies special conditions that would have
continued to be applied to future high altitude airplane type
certifications, it will have no incremental economic effects. There
will also be administrative benefits in that codified regulations are
more efficient than special conditions.
C. Oxygen Equipment--Sec. 25.1447(c)
The FAA has determined that operation in accordance with the
revised oxygen equipment standards will provide an acceptable level of
safety. By specifying the type of oxygen equipment for the crew and the
manner of its use, there will be assurance that the flightcrew will
retain its ability to safely operate the airplane during a
decompression. Panel-mounted regulators have proven satisfactory, but
the FAA has determined that in a high altitude rapid decompression, the
protection afforded by a mask-mounted regulator is superior to that of
panel-mounted regulators. The FAA intends to allow sufficient latitude
for system designers to develop safer and/or less expensive approaches
to specific requirements. For this reason, Sec. 25.1447(c)(3)(ii) will
allow other means of protection for flight crewmembers if they afford
the same protection.
To the extent that the changes codify special conditions that would
have continued to be applied to future high altitude airplane type
certifications, the amendments will have no incremental economic effect
other than the administrative benefits of codified regulations relative
to special conditions.
Regulatory Flexibility Determination
The Regulatory Flexibility Act of 1980 (RFA) was enacted by
Congress to ensure that small entities are not unnecessarily or
disproportionately burdened by Government regulations. The RFA requires
a Regulatory Flexibility Analysis, in which alternatives are considered
and evaluated if a rule is expected to have ``a significant economic
impact on a substantial number of small entities.'' FAA Order 2100.14A,
Regulatory Flexibility Criteria and Guidance, prescribes standards for
complying with RFA review requirements in FAA rulemaking actions. The
Order defines ``small entities'' in terms of size thresholds,
``significant economic impact'' in terms of annualized cost thresholds,
and ``substantial number'' as a number which is not less than eleven
and which is more than one-third of the small subject to the proposed
or final rule.
The rule will affect manufacturers and operators of transport
category airplanes produced under future new, and some amended and
supplemental, airplane type certifications. For manufacturers, Order
2100.14A specifies a size threshold for classification as a small
entity as 75 or fewer employees. Since no part 25 airplane manufacturer
has 75 or fewer employees, the rule will not have a significant
economic impact on a substantial number of small airplane
manufacturers. The size threshold for classification as a small
operator is the ownership (but not necessarily the operation) of nine
or fewer aircraft. The annualized cost thresholds constituting
``significant economic impact'' for operators of aircraft-for-hire,
when expressed in 1994 dollars, are $120,000 for scheduled operators
whose fleets consist entirely of aircraft with seating capacities of
over 60, $69,000 for other scheduled operators, and $4,900 for
unscheduled operators. The annualized incremental costs of this rule
amortized over a maximum nine-airplane fleet are expected to be less
than these annualized cost thresholds. The FAA
[[Page 28695]]
has therefore determined that the rule will not have a significant
economic impact on a substantial number of small operators.
International Trade Impact Assessment
The rule will have little or no effect on the sale of U.S.
airplanes in foreign markets and the sale of foreign airplanes into the
U.S.
Federalism Implications
The regulations adopted herein will not have substantial direct
effects on the states, on the relationship between the national
government and the states, or on the distribution of power and
responsibilities among the various levels of government. Therefore, in
accordance with Executive Order 12612, it is determined that this final
rule will not have sufficient federalism implications to warrant the
preparation of a Federalism Assessment.
International Compatibility
The FAA has reviewed corresponding International Civil Aviation
Organization regulations and Joint Airworthiness Authorities
regulations, where they exist, and has identified no differences in
these amendments and the foreign regulations.
Paperwork Reduction Act
In accordance with the Paperwork Reduction Act of 1980 (Pub. L. 96-
511), there are no requirements for information collection associated
with this rule.
Conclusion
Because amending the airplane and equipment airworthiness standards
for subsonic transport airplanes for operation to an altitude of 51,000
feet is not expected to result in substantial costs, the FAA has
determined that this final rule is not major as defined in Executive
Order 12866. For the same reason and because this is an issue which has
not prompted a great deal of public concern, this final rule is not
considered to be significant as defined in Department of Transportation
Regulatory Policies and Procedures (44 FR 11034; February 26, 1979). In
addition, since there are no small entities affected by this
rulemaking, it is certified, under the criteria of the Regulatory
Flexibility Act, that this final rule, a promulgation, will not have a
significant economic impact, positive or negative, on a substantial
number of small entities. A copy of the final regulatory evaluation
prepared for this project may be examined in the public docket or
obtained from the person identified under the caption FOR FURTHER
INFORMATION CONTACT.
List of Subjects in 14 CFR Part 25
Air transportation, Aircraft, Aviation safety, Safety.
The Amendment
Accordingly, the FAA amends part 25 of the Federal Aviation
Regulations (FAR) (14 CFR part 25) as follows:
PART 25--AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES
1. The authority citation for part 25 continues to read as follows:
Authority: 49 U.S.C. 106(g), 40113, 44701-44702, 44704.
2. By amending Sec. 25.365, by revising paragraph (d), to read as
follows:
Sec. 25.365 Pressurized compartment loads.
* * * * *
(d) The airplane structure must be designed to be able to withstand
the pressure differential loads corresponding to the maximum relief
valve setting multiplied by a factor of 1.33 for airplanes to be
approved for operation to 45,000 feet or by a factor of 1.67 for
airplanes to be approved for operation above 45,000 feet, omitting
other loads.
* * * * *
3. By amending Sec. 25.831 by revising paragraph (a) and by adding
a new paragraph (g) to read as follows:
Sec. 25.831 Ventilation.
(a) Under normal operating conditions and in the event of any
probable failure conditions of any system which would adversely affect
the ventilating air, the ventilation system must be designed to provide
a sufficient amount of uncontaminated air to enable the crewmembers to
perform their duties without undue discomfort or fatigue and to provide
reasonable passenger comfort. For normal operating conditions, the
ventilation system must be designed to provide each occupant with an
airflow containing at least 0.55 pounds of fresh air per minute.
* * * * *
(g) The exposure time at any given temperature must not exceed the
values shown in the following graph after any improbable failure
condition.
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4. By amending Sec. 25.841 by revising paragraph (a) to read as
follows:
Sec. 25.841 Pressurized cabins.
(a) Pressurized cabins and compartments to be occupied must be
equipped to provide a cabin pressure altitude of not more than 8,000
feet at the maximum operating altitude of the airplane under normal
operating conditions.
(1) If certification for operation above 25,000 feet is requested,
the airplane must be designed so that occupants will not be exposed to
cabin pressure altitudes in excess of 15,000 feet after any probable
failure condition in the pressurization system.
(2) The airplane must be designed so that occupants will not be
exposed to a cabin pressure altitude that exceeds the following after
decompression from any failure condition not shown to be extremely
improbable:
(i) Twenty-five thousand (25,000) feet for more than 2 minutes; or
(ii) Forty thousand (40,000) feet for any duration.
(3) Fuselage structure, engine and system failures are to be
considered in evaluating the cabin decompression.
* * * * *
5. By amending Sec. 25.1447, by revising paragraphs (c) (1) through
(4), to read as follows:
Sec. 25.1447 Equipment standards for oxygen dispensing units.
* * * * *
(c) * * *
(1) There must be an oxygen dispensing unit connected to oxygen
supply terminals immediately available to each occupant, wherever
seated, and at least two oxygen dispensing units connected to oxygen
terminals in each lavatory. The total number of dispensing units and
outlets in the cabin must exceed the number of seats by at least 10
percent. The extra units must be as uniformly distributed throughout
the cabin as practicable. If certification for operation above 30,000
feet is requested, the dispensing units providing the required oxygen
flow must be automatically presented to the occupants before the cabin
pressure altitude exceeds 15,000 feet. The crew must be provided with a
manual means of making the dispensing units immediately available in
the event of failure of the automatic system.
(2) Each flight crewmember on flight deck duty must be provided
with a quick-donning type oxygen dispensing unit connected to an oxygen
supply terminal. This dispensing unit must be immediately available to
the flight crewmember when seated at his station, and installed so that
it:
(i) Can be placed on the face from its ready position, properly
secured, sealed, and supplying oxygen upon demand, with one hand,
within five seconds and without disturbing eyeglasses or causing delay
in proceeding with emergency duties; and
(ii) Allows, while in place, the performance of normal
communication functions.
(3) The oxygen dispensing equipment for the flight crewmembers must
be:
(i) The diluter demand or pressure demand (pressure demand mask
with a diluter demand pressure breathing regulator) type, or other
approved oxygen equipment shown to provide the same degree of
protection, for airplanes to be operated above 25,000 feet.
(ii) The pressure demand (pressure demand mask with a diluter
demand pressure breathing regulator) type with mask-mounted regulator,
or other approved oxygen equipment shown to provide the same degree of
protection, for airplanes operated at altitudes where decompressions
that are not extremely improbable may expose the flightcrew to cabin
pressure altitudes in excess of 34,000 feet.
(4) Portable oxygen equipment must be immediately available for
each cabin attendant.
Issued in Washington, DC, on May 29, 1996.
David R. Hinson,
Administrator.
[FR Doc. 96-13947 Filed 6-4-96; 8:45 am]
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