[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]]


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Part IV





Department of Transportation





_______________________________________________________________________



Federal Aviation Administration



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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.

-----------------------------------------------------------------------

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<SUP>-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<SUP>-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<SUP>-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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