MIflyer
1st Lieutenant
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. .
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. .