Special Conditions: Learjet, Inc., Model 45 Airplane, High Altitude Operation

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. NM-92; Final Special Conditions No. 25-ANM-81]

Special Conditions: Learjet, Inc., Model 45 Airplane, High

Altitude Operation

AGENCY: Federal Aviation Administration, DOT.

ACTION: Final special conditions.

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SUMMARY: These special conditions are issued to Learjet Inc. of

Wichita, KS, for the Learjet Inc., Model 45 airplane. This new airplane

has an unusual design feature associated with an unusually high

operating altitude (51,000 feet), for which the applicable

airworthiness regulations do not contain adequate or appropriate safety

standards. These special conditions contain the additional safety

standards that the Administrator considers necessary to establish a

level of safety equivalent to that established by the existing

airworthiness standards.

EFFECTIVE DATE: April 25, 1994.

FOR FURTHER INFORMATION CONTACT:

Mark Quam, FAA, Standardization Branch, ANM-113, Transport Airplane

Directorate, Aircraft Certification Service, 1601 Lind Avenue SW.,

Renton, Washington, 98055-4056; telephone (206) 227-2145.

SUPPLEMENTARY INFORMATION:

Background

On January 27, 1992, Learjet Inc., (Learjet), 8220 West Harry

Street, Wichita, KS 67209-2942, or P.O. Box 7707, Wichita, KS 67277-

7707, applied for a new type certificate in the transport airplane

category for the Model 45 airplane. The Learjet Model 45 is a T-tail,

low wing, medium sized business jet powered by two Garrett TFE 731-20

turbofan engines mounted on pylons extending from the aft fuselage.

Each engine will be capable of delivering 3,500 pounds thrust, with

auto performance reserve 3,650 pounds thrust. The airplane will be

capable of operating with two flight crewmembers and a maximum of ten

passengers (standard is eight passengers). The type design of the

Learjet Model 45 series airplanes contains a number of novel and

unusual design features for an airplane type certificated under the

applicable provisions of part 25 of the Federal Aviation Regulations

(FAR). Those features include the relatively small passenger cabin

volume and a high operating altitude. The applicable airworthiness

requirements do not contain adequate or appropriate safety standards

for the Learjet 45 series airplanes; therefore, special conditions are

necessary to establish a level of safety equivalent to that established

in the regulations.

Type Certification Basis

Under the provisions of Sec. 21.17 of the FAR, Learjet must show,

except as provided in Sec. 25.2, that the Model 45 meets the applicable

provisions of part 25, effective February 1, 1965, as amended by

Amendments 25-1 through 25-75. In addition, the proposed certification

basis for the Model 45 includes part 34, effective September 10, 1990,

plus any amendments in effect at the time of certification; and part

36, effective December 1, 1969, as amended by Amendments 36-1 through

the amendment in effect at the time of certification. These special

conditions will form an additional part of the type certification

basis. In addition, the certification basis may include other special

conditions that are not relevant to these special conditions.

If the Administrator finds that the applicable airworthiness

regulations (i.e., part 25, as amended) do not contain adequate or

appropriate safety standards for the Learjet Model 45 because of a

novel or unusual design feature, special conditions are prescribed

under the provisions of Sec. 21.16 to establish a level of safety

equivalent to that established in the regulations.

Special conditions, as appropriate, are issued in accordance with

Sec. 11.49 of the FAR after public notice, as required by Secs. 11.28

and 11.29, and become part of the type certification basis in

accordance with Sec. 21.17(a)(2).

Novel or Unusual Design Feature

The Learjet Model 45 will incorporate an unusual design feature in

that it will be certified to operate up to an altitude of 51,000 feet.

The FAA considers certification of transport category airplanes for

operation at altitudes greater than 41,000 feet to be a novel or

unusual feature because current part 25 does not contain standards to

ensure the same level of safety as that provided during operation at

lower altitudes. Special conditions have therefore been adopted to

provide adequate standards for transport category airplanes previously

approved for operation at these high altitudes, including certain

Learjet models, the Boeing Model 747, Dassault-Breguet Falcon 900,

Canadair Model 600, Cessna Model 650, Israel Aircraft Industries Model

1125, and Cessna Model 560. The special conditions for the Cessna Model

650 or previously certified Learjet models are considered the most

applicable to the Model 45 and its proposed operation and are therefore

used as the basis for the special conditions described below.

Damage tolerance methods are proposed to be used to assure pressure

vessel integrity while operating at the higher altitudes, in lieu of

the \1/2\-bay crack criterion used in some previous special conditions.

Crack growth data are used to prescribe an inspection program that

should detect cracks before an opening in the pressure vessel would

allow rapid depressurization. Initial crack sizes for detection are

determined under Sec. 25.571, as amended by Amendment 25-72. The

maximum extent of failure and pressure vessel opening determined from

the above analysis must be demonstrated to comply with the

pressurization section of the proposed special conditions, which state

that the cabin altitude after failure must not exceed the cabin

altitude/time curve limits shown in Figures 3 and 4.

In order to ensure that there is adequate fresh air for crewmembers

to perform their duties, to provide reasonable passenger comfort, and

to enable occupants to better withstand the effects of decompression at

high altitudes, the ventilation system must be designed to provide 10

cubic feet of fresh air per minute per person during normal operations.

Therefore, these special conditions require that crewmembers and

passengers be provided with 10 cubic feet of fresh air per minute per

person. In addition, during the development of the supersonic transport

special conditions, it was noted that certain pressurization failures

resulted in hot ram or bleed air being used to maintain pressurization.

Such a measure can lead to cabin temperatures that exceed human

tolerance. Therefore, these special conditions require airplane

interior temperature limits following probable and improbable failures.

Continuous flow passenger oxygen equipment is certificated for use

up to 40,000 feet; however, for rapid decompressions above 34,000 feet,

reverse diffusion leads to low oxygen partial pressures in the lungs,

to the extent that a small percentage of passengers may lose useful

consciousness at 35,000 feet. The percentage increases to an estimated

60 percent at 40,000 feet, even with the use of the continuous flow

system. Therefore, to prevent permanent physiological damage, the cabin

altitude must not exceed 25,000 feet for more than 2 minutes, or 40,000

feet for any time period. The maximum peak cabin altitude of 40,000

feet is consistent with the standards established for previous

certification programs. In addition, at high altitudes the other

aspects of decompression sickness have a significant, detrimental

effect on pilot performance (for example, a pilot can be incapacitated

by internal expanding gases).

Decompression resulting in cabin altitudes above the 37,000-foot

limit depicted in Figure 4 approaches the physiological limits of the

average person; therefore, every effort must be made to provide the

pilots with adequate oxygen equipment to withstand these severe

decompressions. Reducing the time interval between pressurization

failure and the time the pilots receive oxygen will provide a safety

margin against being incapacitated and can be accomplished by the use

of mask-mounted regulators. These special conditions therefore require

pressure demand masks with mask-mounted regulators for the flightcrew.

This combination of equipment will provide the best practical

protection for the failures covered by the special conditions and for

improbably failures not covered by the special conditions, provided the

cabin altitude is limited.

Discussion of Comments

Notice of proposed special conditions No. SC-93-8-NM was published

in the Federal Register on December 22, 1993 (58 FR 67716). No comments

were received, and the special conditions are adopted as proposed.

Conclusion

This action affects only certain unusual or novel design features

on one model of airplane. It is not a rule of general applicability and

affects only the manufacturer who applied to the FAA for approval of

these features on the airplane.

List of Subjects in 14 CFR Part 25

Aircraft, Aviation safety, Reporting and recordkeeping

requirements.

The authority citation for these special conditions is as follows:

Authority: 49 U.S.C. app. 1344, 1348(c), 1352, 1354(a), 1355, 1421

through 1431, 1502, 1651(b)(2), 42 U.S.C. 1857f-10, 4321 et seq.; E.O.

11514; and 49 U.S.C. 106(g).

The Special Conditions

Accordingly, the following special conditions are issued as part of

the type certification basis for the Learjet Model 45 series airplanes:

Operation to 51,000 Feet

1. Pressure vessel integrity. (a) The maximum extent of failure and

pressure vessel opening that can be demonstrated to comply with

paragraph 4 (Pressurization) of this special condition must be

determined. It must be demonstrated by crack propagation and damage

tolerance analysis supported by testing that a larger opening or a more

severe failure than demonstrated will not occur in normal operations.

(b) Inspection schedules and procedures must be established to

assure that cracks and normal fuselage leak rates will not deteriorate

to the extent that an unsafe condition could exist during normal

operation.

(c) With regard to the fuselage structural design for cabin

pressure capability above 45,000 feet altitude, the pressure vessel

structure, including doors and windows, must comply with

Sec. 25.365(d), using a factor of 1.67 instead of the 1.33 factor

described.

2. Ventilation. In lieu of the requirements of Sec. 25.831(a), 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 during normal operating conditions and also in the

event of any probable failure to any system that could adversely affect

the cabin ventilating air. For normal operations, crewmembers and

passengers must be provided with at least 10 cubic feet of fresh air

per minute per person, or the equivalent in filtered, recirculated air

based on the volume and composition at the corresponding cabin pressure

altitude of not more than 8,000 feet.

3. Air conditioning. In addition to the requirements of

Sec. 25.831, paragraphs (b) through (e), the cabin cooling system must

be designed to meet the following conditions during flight above 15,000

feet mean sea level (MSL): (a) After any probably failure, the cabin

temperature-time history may not exceed the values shown in Figure 1.

(b) After any improbable failure, the cabin temperature-time

history may not exceed the values shown in Figure 2.

4. Pressurization. In addition to the requirements of Sec. 25.841,

the following apply: (a) The pressurization system, which includes for

this purpose bleed air, air conditioning, and pressure control systems,

must prevent the cabin altitude from exceeding the cabin altitude-time

history shown in Figure 3 after each of the following: (1) Any probable

malfunction or failure of the pressurization system. The existence of

undetected, latent malfunctions or failures in conjunction with

probable failures must be considered.

(2) Any single failure in the pressurization system, combined with

the occurrence of a leak produced by a complete loss of a door seal

element, or a fuselage leak through an opening having an effective area

2.0 times the effective area that produces the maximum permissible

fuselage leak rate approved for normal operation, whichever produces a

more severe leak.

(b) The cabin altitude-time history may not exceed that shown in

Figure 4 after each of the following: (1) 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

considered.

(2) The pressure vessel opening or duct failure resulting from

probable damage (failure effect) while under maximum operating cabin

pressure differential due to a tire burst, engine rotor burst, loss of

antennas or stall warning vanes, or any probable equipment failure

(bleed air, pressure control, air conditioning, electrical source(s),

etc.) that affects pressurization.

(3) Complete loss of thrust from all engines.

(c) In showing compliance with paragraphs 4(a) and 4(b) of these

special conditions (Pressurization), it may be assumed that an

emergency descent is made by approved emergency procedure. A 17-second

crew recognition and reaction time must be applied between cabin

altitude warning and the initiation of an emergency descent.

Note: For the flight evaluation of the rapid descent, the test

article must have the cabin volume representative of what is

expected to be normal, such that Cessna must reduce the total cabin

volume by that which would be occupied by the furnishings and total

number of people.

5. Oxygen equipment and supply. (a) A continuous flow oxygen system

must be provided for the passengers.

(b) A quick-donning pressure demand mask with mask-mounted

regulator must be provided for each pilot. Quick-donning from the

stowed position must be demonstrated to show that the mask can be

withdrawn from stowage and donned within 5 seconds.

BILLING CODE 4910-13-M

TR24MR94.000

TR24MR94.001

TR24MR94.002

TR24MR94.003

BILLING CODE 4910-13-C

Issued in Renton, Washington, on March 9, 1994.

Darrell M. Pederson,

Acting Manager, Transport Airplane Directorate, Aircraft Certification

Service, ANM-100.

[FR Doc. 94-6961 Filed 3-23-94; 8:45 am]

BILLING CODE 4910-13-M