Special Certification Review - Learjet 35/36 Pressurization and Oxygen Systems

Air Safety Week, Dec 4, 2000

Just ten days after the Oct. 25, 1999, fatal crash of the Lear 35 carrying golf professional Payne Stewart and five others at Aberdeen, S.D., the Federal Aviation Administration launched a team to conduct a special certification review of the accident airplane's pressurization and emergency oxygen system and procedures. "The lack of crew response" as the airplane flew from Ocala, Fla., until it ran out of fuel some four hours later and crashed in South Dakota "has raised concerns regarding the pressurization and oxygen systems," according to that review team's report.

The Nov. 1999 through Jan. 2000 review produced eight recommendations; they apply not only to smaller on-demand jets like the Lear, but to the larger jets in regular scheduled service.

Highlights of the Special Certification Review team's 37-page report follow:

"The team did not find any unsafe conditions associated with the oxygen and pressurization system modifications installed on the accident airplane. The team found a number of Learjet accidents that appear to be caused by crew incapacitation due to hypoxia (ASW editor's note: oxygen starvation). The team also found it is probable that crews are not donning their oxygen masks when necessary to preclude hypoxia."

"The team reviewed Learjet accident and incident reports and found one accident (Dec. 16, 1988, Mexico) where the crew did not activate the pressurization system or crew oxygen. For this accident, it appears the crew became incapacitated below 30,000 feet. For two other Learjet accidents (Jan. 18, 1990, Detroit-Dayton, and the 1999 Payne Stewart crash) it also appears the crews were incapacitated below 30,000 feet. There was one accident (May 18, 1983, Vienna-North Atlantic) where it appears the crew was incapacitated at 41,000 feet. There is at least one other case (Aug. 11, 1979, Athens-Katab) that is less conclusive but crew incapacitation may be involved."

"All the accident airplanes had a pressurization system that is manually activated, a manually activated alternate source of inflow air for pressurization, an aural warning of high cabin altitude, and no annunciation light for high cabin altitude."

"The Learjet Model 35/36 pressurization system can be inadvertently, or intentionally, left in the 'off' position at takeoff. If the system is not turned on, the aircraft will not pressurize, and the cabin altitude will rise as the airplane climbs..."

"Review of the accident and incident database indicates that this occurs. There is incentive to leave the pressurization system off during taxi and takeoff in warm weather because inflow air can be hotter than cabin ambient air. The procedures for this airplane are to turn the pressurization system 'on' prior to takeoff. Note: Other aircraft are allowed to depart with the pressurization system off. Consequently, these other aircraft are of interest because it may be easy to inadvertently leave the pressurization system off."

"There is no annunciation light in the Learjet Model 35/36 to alert the flight crew that the pressurization system is not activated. Therefore, the flight crew will not be actively alerted to the situation until a 10,000-foot cabin altitude aural warning is activated. Later designs of the Learjet Models 35A and 36A incorporated system improvements. These included the addition of a visual annunciation of high cabin altitude (8,750 ft.) and an automatically activated emergency source of pressurization inflow air."

"Review of the Learjet Model 35/36 AFM (Airplane Flight Manual) reveals that the AFM does not have an emergency procedure that requires donning of the crew

oxygen masks when the cabin altitude aural warning is activated. The AFM contains an abnormal procedure that allows the crew 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 where the crew becomes incapable of donning the masks."

"There were six ASRS (Aviation Safety Reporting System) reports of crews having difficulty donning the oxygen masks and establishing communications on unidentified corporate airplanes (ASW note: ASRS reports are de-identified to assure confidentiality). This would indicate that the masks should be donned early, prior to troubleshooting pressurization problems."

The Big 8: Review Team Recommendations

1. Revise the...AFM to include an emergency procedure associated with the cabin altitude warning. The team recommends that the first crew actions are to don the oxygen masks.

2. Learjet should develop a kit to provide an annunciator light or equivalent to advise the crew if the cabin air switch is in the off position...for those Learjet airplanes not having an automatic emergency pressurization system.

3. The Transport Airplane Directorate should...review the AFM's of all transport category pressurized airplanes certificated for flight above 25,000 feet and ensure there is an emergency procedure (or equivalent) then the cabin altitude warning is activated. The team recommends that the first crew action after a cabin altitude warning should be to don the oxygen mask.