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Proposed Rule

Airworthiness Directives; Learjet Model 35, 35A, 36, and 36A Series Airplanes

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Federal Aviation Administration, DOT.


Notice of proposed rulemaking (NPRM).


This document proposes the adoption of a new airworthiness directive (AD) that is applicable to all Learjet Model 35, 35A, 36, and 36A series airplanes. This proposal would require revision of the Airplane Flight Manual (AFM) to add procedures for donning the flightcrew oxygen masks when the cabin altitude warning horn is activated. This action is intended to prevent incapacitation of the flightcrew due to lack of oxygen and consequent loss of control of the airplane due to absence of AFM procedures for donning the flightcrew oxygen masks when the cabin altitude warning horn is activated.


Comments must be received by July 24, 2000.


Submit comments in triplicate to the Federal Aviation Administration (FAA), Transport Airplane Directorate, ANM-114, Attention: Rules Docket No. 2000-NM-127-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056. Comments may be inspected at this location between 9:00 a.m. and 3:00 p.m., Monday through Friday, except Federal holidays. Comments may be submitted via fax to (425) 227-1232. Comments may also be sent via the Internet using the following address: Comments sent via fax or the Internet must contain “Docket No. 2000-NM-127-AD” in the subject line and need not be submitted in triplicate.

Information regarding this docket may be examined at the FAA, Transport Airplane Directorate, 1601 Lind Avenue, SW., Renton, Washington.

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Ben Sorensen, Flight Test Pilot, ACE-117W, FAA, Small Airplane Directorate, Wichita Aircraft Certification Office, 1801 Airport Road, Room 100, Mid-Continent Airport, Wichita, Kansas 67209; telephone (316) 946-4165; fax (316) 946-4407.

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Comments Invited

Interested persons are invited to participate in the making of the proposed rule by submitting such written data, views, or arguments as they may desire. Communications shall identify the Rules Docket number and be submitted in triplicate to the address specified above. All communications received on or before the closing date for comments, specified above, will be considered before taking action on the proposed rule. The proposals contained in this notice may be changed in light of the comments received.

Submit comments using the following format:

  • Organize comments issue-by-issue. For example, discuss a request to change the compliance time and a request to change the service bulletin reference as two separate issues.
  • For each issue, state what specific change to the proposed AD is being requested.
  • Include justification (e.g., reasons or data) for each request.

Comments are specifically invited on the overall regulatory, economic, environmental, and energy aspects of the proposed rule. All comments submitted will be available, both before and after the closing date for comments, in the Rules Docket for examination by interested persons. A report summarizing each FAA-public contact concerned with the substance of this proposal will be filed in the Rules Docket.

Commenters wishing the FAA to acknowledge receipt of their comments submitted in response to this notice must submit a self-addressed, stamped postcard on which the following statement is made: “Comments to Docket Number 2000-NM-127-AD.” The postcard will be date stamped and returned to the commenter.

Availability of NPRMs

Any person may obtain a copy of this NPRM by submitting a request to the FAA, Transport Airplane Directorate, ANM-114, Attention: Rules Docket No. 2000-NM-127-AD, 1601 Lind Avenue, SW., Renton, Washington 98055-4056.


On October 25, 1999, a Learjet Model 35 series airplane operating under part 135 of the Federal Aviation Regulations (14 CFR 135) departed Orlando International Airport enroute to Dallas, Texas. Air traffic control lost communication with the airplane near Gainsville, Florida. Air Force and National Guard airplanes intercepted the airplane, but the flightcrews of the chase airplanes indicated that the windows of the Model 35 series airplane were apparently frosted over and prevented the chase airplane flightcrews from observing the interior of the Model 35 series airplane. The flightcrews of the chase airplanes reported that they did not observe any damage to the airplane. Subsequently, the Model 35 series airplane ran out of fuel and crashed in South Dakota. To date, causal factors of the accident have not been determined. However, lack of the Learjet flightcrew's response to air traffic control poses the possibility of flightcrew incapacitation and raises concerns with the pressurization and oxygen systems.

Recognizing these concerns, the FAA initiated a special certification review (SCR) to determine if pressurization and oxygen systems on Model 35 series airplanes were certificated properly, and to determine if any unsafe design features exist in the pressurization and oxygen systems.

The SCR team found that there have been several accidents and incidents that may have involved incapacitation of the flightcrews during flight. In one case, the airplane flightcrew did not activate the pressurization system or don their oxygen masks and the airplane flew in excess of 35,000 feet altitude. In another case, the airplane flightcrews did not don their oxygen masks when the cabin aural warning was activated. Further review by the SCR team indicates that the Airplane Flight Manual (AFM) of Learjet Model 35/36 series airplanes do not have an emergency procedure that requires donning the flightcrew oxygen masks when the cabin altitude aural warning is Start Printed Page 36392activated. Additional review has found that the AFM's of Model 35A and 36A series airplanes also do not contain appropriate flightcrew actions when the cabin altitude aural warning is activated. However, the AFM's do contain an abnormal procedure that allows the flightcrew to troubleshoot the pressurization system prior to donning the oxygen masks after the cabin altitude warning sounds. Troubleshooting may delay donning of the oxygen masks to the point that flightcrews may become incapable of donning their oxygen masks.

The SCR findings indicated that the most likely cause for incapacitation was hypoxia (lack of oxygen). The only other plausible cause of incapacitation is exposure to toxic substances. However, no evidence was found to support the existence of toxic substances.

Delayed response of the flightcrew in donning oxygen masks when the activation of the cabin altitude warning horn could lead to incapacitation of the flightcrew and loss of control of the airplane.

Explanation of Requirements of Proposed Rule

Since an unsafe condition has been identified that is likely to exist or develop on other products of this same type design, the proposed AD would require revising Emergency Procedures Section of the AFM to provide the flightcrew with appropriate and timely actions in response to activation of the cabin altitude warning horn.

Cost Impact

There are approximately 739 Learjet Model 35, 35A, 36, and 36A series airplanes of the affected design in the worldwide fleet. The FAA estimates that 500 airplanes of U.S. registry would be affected by this proposed AD, that it would take approximately 1 work hour per airplane to accomplish the proposed actions, and that the average labor rate is $60 per work hour. Based on these figures, the cost impact of the proposed AD on U.S. operators is estimated to be $30,000, or $60 per airplane.

The cost impact figure discussed above is based on assumptions that no operator has yet accomplished any of the proposed requirements of this AD action, and that no operator would accomplish those actions in the future if this proposed AD were not adopted. The cost impact figures discussed in AD rulemaking actions represent only the time necessary to perform the specific actions actually required by the AD. These figures typically do not include incidental costs, such as the time required to gain access and close up, planning time, or time necessitated by other administrative actions.

Regulatory Impact

The regulations proposed herein would not have a substantial direct effect 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, it is determined that this proposal would not have federalism implications under Executive Order 13132.

For the reasons discussed above, I certify that this proposed regulation (1) is not a “significant regulatory action” under Executive Order 12866; (2) is not a “significant rule” under the DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (3) if promulgated, will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act. A copy of the draft regulatory evaluation prepared for this action is contained in the Rules Docket. A copy of it may be obtained by contacting the Rules Docket at the location provided under the caption ADDRESSES.

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List of Subjects in 14 CFR Part 39

  • Air transportation
  • Aircraft
  • Aviation safety
  • Safety
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The Proposed Amendment

Accordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration proposes to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows:

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1. The authority citation for part 39 continues to read as follows:

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Authority: 49 U.S.C. 106(g), 40113, 44701.

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2. Section 39.13 is amended by adding the following new airworthiness directive:

Learjet: Docket 2000-NM-127-AD.

Applicability: Model 35, 35A, 36, and 36A series airplanes, certificated in any category.

Compliance: Required as indicated, unless accomplished previously.

To prevent incapacitation of the flightcrew and consequent loss of control of the airplane due to delays in donning oxygen masks in response to the activation of the cabin altitude warning horn; accomplish the following:

(a) Within 10 days after the effective date of this AD, revise the Emergency Procedure Section of the FAA-Approved Airplane Flight Manual (AFM) to include the following. This may be accomplished by inserting a copy of this AD into the AFM.


1. Oxygen Masks—Don. Select 100% oxygen.

2. Thrust Levers—IDLE.

3. Autopilot—Disengage.

4. SPOILER switch—EXT.

5. Landing gear switch—DN below MMO VLE as appropriate for altitude. Keep sideslip angles to a minimum (ball centered) when extending gear.

6. Descend at MMO or VLE as appropriate, but NOT below minimum safe altitude.



WARNING: If pressurization loss is due to structural failure, limit speeds and maneuvering loads as much as possible in descent.


Descent from 45,000 feet to 15,000 feet requires approximately 2 minutes 45 seconds.

Hats and “ear-muff” type headsets must be removed prior to donning crew oxygen masks.

Communication between crewmembers can be accomplished by using the INPH function of the AUDIO CONTROL PANEL and adjusting the MASTER VOL.

Communication with passengers can be accomplished by using PASS SPKR function of the AUDIO CONTROL panel and adjusting the PASS SPKR VOL level.

The HORN SILENCE switch will mute the cabin altitude warning horn for 60 seconds.

If Time and Conditions Permit:

9. Transponder—Emergency 7700.

10. Pilot and Copilot OXY—MIC switches—ON.

11. Notify controlling agency.

12. Check condition of passengers and provide assistance if conditions permit.”

Alternative Methods of Compliance

(b) An alternative method of compliance or adjustment of the compliance time that provides an acceptable level of safety may be used if approved by the Manager, Wichita Aircraft Certification Office (ACO), FAA, Small Airplane Directorate. Operators shall submit their requests through an appropriate FAA Principal Maintenance Inspector, who may add comments and then send it to the Manager, Wichita ACO.

Note 1:

Information concerning the existence of approved alternative methods of compliance with this AD, if any, may be obtained from the Wichita ACO.

Special Flight Permit

(c) Special flight permits may be issued in accordance with sections 21.197 and 21.199 of the Federal Aviation Regulations (14 CFR 21.197 and 21.199) to operate the airplane to a location where the requirements of this AD can be accomplished.

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Issued in Renton, Washington, on June 2, 2000.

Donald L. Riggin,

Acting Manager, Transport Airplane Directorate, Aircraft Certification Service.

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[FR Doc. 00-14438 Filed 6-7-00; 8:45 am]