The Payne Stuart Learjet Crash

[MIflyer]

I assume everyone has heard of the Learjet crash that killed the famous pro golfer Payne Stuart, along with everyone else aboard the aircraft.
As a former depot maintenance engineer working in aircraft pressurization, air conditioning, and pneumatics, at the time the mishap made no sense at all to me. Months later, following an investigation, the NTSB concluded it made no sense to them, either.

An episode of the Smithsonian series "Air Disasters" covered the mishap and spurred my interest again. I looked at the pilot's manual for that model Learjet.

I had assumed that the aircraft had suffered an air conditioning failure soon after takeoff and the flight crew had shut down the system but inexplicably not chosen to quickly descend to a lower altitude and land. Typical jet aircraft air conditioning and pressurization systems take hot bleed air out of the jet engine compressor, run it through a heat exchanger that reduces the bleed air temperature by using ram air from outside, and then run it through a cooling turbine that allows the air to expand to a lower pressure and much lower temperature to both cool and pressurize the cabin. Cabin temperature control is handled by letting in enough hot bleed air (from the air duct ahead of the cooling turbine) until people are comfortable.

I theorized that the cooling turbine had failed, allowing nothing but hot air to enter the cabin and the crew had turned the system off, leaving no source of pressurization and then for some reason decided to continue the flight.

The Learjet manual says there is no cooling turbine. The bleed air goes through a heat exchanger and that is it; then it goes into the cabin. That means that on the ground with the pressurization system activated it is going to get hot in that cabin; there is no cooling. In Central Florida in October, you can be sure it will get very uncomfortable. There is an optional vapor cycle refrigeration package for the airplane that would at least help that situation but not all of the airplanes are so equipped.

So, it appears that the flight crew started the engines and taxied out without turning on the pressurization system. Presumably they planned to turn it on soon after takeoff, once they got to an altitude where the ram air was cold enough to significantly cool the bleed air (at 8000 ft the standard temperature outside is 30F) but never took that action. The intercepting F-16's reported the windows frosted over and the post-crash investigation showed the bleed air valves were closed, both of which confirmed that the pressurization system was not turned on.

The flight crew failed to recognize what was occurring and don their oxygen masks.

A flight instructor I know told me that he had heard that the pilot was male and rather arrogant while the co-pilot was female and actually had considerably more Learjet time than the pilot. If that is true, it is not hard to believe that the co-pilot decided to let the stupid a-hole in the Left seat figure everything out on his own

I have been though USAF altitude chamber training, and I think I could recognize the onset of hypoxia, but I guess that kind of training is not the norm for air taxi operators.

Note that there was a Greek airline Boeing 737 that took off from Crete and the flight crew apparently failed to switch the pressurization control from "Manual" where the maintenance crew had set it, to "Automatic." Climbing to altitude on autopilot, the crew's first inkling that something was wrong was when they got an avionics "Overheat" warning. Their first assumption was that the circuit breaker for the cooling fans had popped and presumably were troubleshooting that problem when they passed out. .

 

[MiTasol]

You could well be right but in every pressurization system I have worked on, and that does NOT include the Learjet, all the valves are electrically activated, pneumatically operated and spring loaded closed. As such the valves will always be closed when there is no electric and/or pneumatic power to open them so any post-crash investigation will always show the bleed air valves were closed.

As you say that the Learjet does not have the usual air cycle machine to cool the bleed air they may also have different valves but those valves should always default to the safe, that is closed, position in the event of any failures. Defaulting open will cause a rapid decompression if the bleed air supply fails. Defaulting closed causes maximum pressure retention though that is still only minutes at best.

You are also most likely to be correct in the assumption that Captain Macho was ignoring the more experienced right seater while some pressurization or other fault developed that was outside his abilities. These type of idiots are far too common unfortunately and nearly always insist that they are god in an emergency.

 

[MiTasol]

Note that there was a Greek airline Boeing 737 that took off from Crete and the flight crew apparently failed to switch the pressurization control from "Manual" where the maintenance crew had set it, to "Automatic." Climbing to altitude on autopilot, the crew's first inkling that something was wrong was when they got an avionics "Overheat" warning. Their first assumption was that the circuit breaker for the cooling fans had popped and presumably were troubleshooting that problem when they passed out. .

This unfortunately is not an unusual situation in King Air through to Gulfstream operations. Airline pilots are supposed to follow check lists and in their check rides and sim checks adherence to the checklists is a major compliance item. The NTSB and ATSB and similar investigation organisations find multiple cases every year where corporate and charter accidents are triggered by failure to follow the check lists yet there are far too many Captain Ego's out there that think the checklists are for amateurs. They also take off with the control locks in, fly below minimum safe altitude, etc, etc.

 

[RW Mk. III]

It's surprising to me that there is no alarm to indicate low cabin pressure or oxygen content?

 

[DerAdlerIstGelandet]

It's surprising to me that there is no alarm to indicate low cabin pressure or oxygen content?

Every aircraft I have worked on does...

 

[RW Mk. III]

Every aircraft I have worked on does...

Yeah. So how did it not alert the pilots in these two instances would be my big question. If you get a pressure alarm isn't it fly down now and try to fix it later?

 

[MIflyer]

Aside from any cockpit alarms, in a commercial airliner I would expect the oxygen masks to drop down soon after you hit 10,000 ft if you never pressurized the cabin. And that should give you a clue if nothing else did. In fact, in the case of the Greek airliner one of the flight attendants, who was also a student pilot trying to work toward becoming an airline pilot, went forward to the cockpit, and got into the pilot's seat, but was unable to do anything but wave at the F-16 pilots. Perhaps the masks did drop down and he was able to remain alert until he had to get off the mask and go to the cockpit.

There was one famous hilarious episode where a B-52 lost pressurization and the crew chief had hitched a ride and was asleep in the corridor. One of the crewmen went to put a mask on the crew chief but then passed out himself. Then another crewman took off his mask to help that crewman but passed out and fell from the upper corridor to the lower level. The two crewmen on the lower level went to help that guy and passed out, too. Just about everyone on the airplane but the pilot and copilot passed out at least once. And unlike airliners and Learjets, USAF airplanes have walk-around bottles you can plug your mask into, but no one thought to do that.

 

[DerAdlerIstGelandet]

Yeah. So how did it not alert the pilots in these two instances would be my big question. If you get a pressure alarm isn't it fly down now and try to fix it later?

It's not just that, the masks in the cabin should have dropped automatically, unless the system was not armed or malfunctioned.

I have troubleshot, inspected and repaired a lot of pressurization and emergency oxygen systems throughout my career. When working correctly there are bells and whistles, and mask drops. The system has to be armed though.

 

[MiTasol]

And unlike airliners and Learjets, USAF airplanes have walk-around bottles you can plug your mask into, but no one thought to do that.

Actually in most countries airliners do have walk around bottles for use in medical emergencies but the cabin crew probably do not think of them in pressurization failures.

 

[Joe Broady]

The NTSB noted the disincentive to turning on cabin air when on the ground in warm weather. However, they felt that was not a factor in the Payne Stewart crash because 1) all aboard would have sensed a rapid and continuing rise in cabin altitude, 2) the cabin altitude aural warning would have gone off at 10,000 feet, 3) passenger oxygen masks would have dropped at 14,000 feet, 4) the FO probably would have sounded impaired when she talked to ATC while passing 23,000.

The CVR captured the final 30 minutes of flight. On the recording the cabin pressure warning was sounding, though that doesn't tell us the altitude where it activated. However, it did terminate properly during the final dive.

Both bleed air modulation valves (one per engine) were found almost closed. Since they are spring loaded open and pushed closed by pressure in the bleed system, that implies the system had normal pressure but there was little demand. That part of the NTSB report didn't make sense at first. How do you maintain bleed pressure after both engines flame out? Well, they didn't. And I'm not the only one who had that misconception. The Wikipedia article twice mentions the "engines" rolling back, but the NTSB report only says "the sound of an engine winding down". Inspection of the wreckage showed the right engine was windmilling but the left was operating at impact.

There's a check valve after each modulation valve, then left and right bleed air combine into a common manifold. So if the modulation valve sense lines are connected downstream of the check valves, that would explain why the modulation valve on a flamed out engine was nearly closed. The other engine was ample to supply the demand. But I'm guessing. There's no diagram of the bleed system in the NTSB docket.

The flow control valve (which admits conditioned bleed air to the cabin) was found closed. That would explain the low demand for bleed air and was probably central to the accident. The NTSB had theories but wasn't able to zero in on why it was closed.

The TV show makes much of the checklist procedure for loss of cabin pressure, suggesting the accident probably wouldn't have happened if "Oxygen Masks — Don" had been right on top. The checklist is reproduced in the NTSB report, and indeed I think it could have been written a lot better.

NTSB accident report
NTSB docket