Unresponsive Plane Circling Gulf of Mexico

[FLYBOYJ]

As an engineer who has worked in aircraft pressurization, I found the Paine Stewart Learjet crash inexplicable. Given the data, you can only assume a degree of stupidity by the flight crew that sounds impossible.

The NTSB investigation agreed with my own assessment. They found that the pressurization system was not even turned on! The only assumption you can reach is that both the pilot and co-pilot were senseless after engine start.

Just like a car with an air cond system that will not work, they should have realized that before they got to the runway. It gets hot and stuffy really really quick in a sealed box in Florida on the ground in May.

"Investigators also considered the possibility that the pilots failed to select the CABIN AIR switch to NORM, which activates the air conditioning system (and pressurizes the airplane), before takeoff.

Even though the Taxi and Before Takeoff checklist specifies, in item 19, "CABIN AIR SWITCH - NORM," the



FAA Special Certification Review (SCR) team observed that "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."50 However, without the cabin air conditioning system, the occupants of the airplane likely would have perceived a high cabin climb rate after takeoff, possibly causing discomfort. At about 10,000 feet cabin altitude, the cabin altitude aural warning should have begun to sound, further alerting the flight crew to the lack of pressurization. Although the pilots could have manually silenced the warning, they would have had to repeat this action every 60 seconds. At about 14,000 feet cabin altitude, deployment of the passengers' oxygen masks would have provided an additional cue that the cabin was not properly pressurized.51 It is unlikely that the flight crew would have continued to climb despite this clear information that the airplane was unpressurized."

I don't see this crew doing anything wrong...

[MIflyer]

I understand that the NTSB found that the bleed air valves were closed. No bleed air, no air conditioning and no pressurization. The cabin outflow valve would close completely when the aircraft reached 7500 ft. It is inexplicable that the aircrew would not notice this. It would get real stuffy in there real fast.

The fact that the ECS was not on was also confirmed by the observation by the intercepting fighters that the windows were frosted over, but there was no hole in the aircraft. Even if there was a window blown out the ECS air passing over the windshield would defrost it.

I have been through simulated explosive decompression in an alititue chamber. That is not what occurred, but even at a simulated altitude of 20,000 ft you do not pass out instantly - people differ in that regard but I did not. I was told by an A&P that the aircraft's oxygen system had been used perviously and not replinished. Okay so if they had to turn off the ECS (maybe due to cooling turbine failure and the resultant introduction of very hot air into the cabin), then you go on oxygen - and you run out. But even so you should have enough time to get down below 10,000 ft.

I still think that flight crew incompetance to that degree is inexplicable.

[FLYBOYJ]

I understand that the NTSB found that the bleed air valves were closed. No bleed air, no air conditioning and no pressurization. The cabin outflow valve would close completely when the aircraft reached 7500 ft. It is inexplicable that the aircrew would not notice this. It would get real stuffy in there real fast.

The fact that the ECS was not on was also confirmed by the observation by the intercepting fighters that the windows were frosted over, but there was no hole in the aircraft. Even if there was a window blown out the ECS air passing over the windshield would defrost it.

I have been through simulated explosive decompression in an alititue chamber. That is not what occurred, but even at a simulated altitude of 20,000 ft you do not pass out instantly - people differ in that regard but I did not. I was told by an A&P that the aircraft's oxygen system had been used perviously and not replinished. Okay so if they had to turn off the ECS (maybe due to cooling turbine failure and the resultant introduction of very hot air into the cabin), then you go on oxygen - and you run out. But even so you should have enough time to get down below 10,000 ft.

I still think that flight crew incompetance to that degree is inexplicable.

You're entitled to your oppinion but I think it's a lot more complicated than that. This is from the NTSB report;

"Lack of Bleed Air Supply to the Cabin

The flow control valve regulates the flow rate of conditioned bleed air entering the cabin for pressurization and heating. If there is no inlet bleed air, the valve main spring will close the flow control valve completely. Although, as previously discussed, bleed air was available to open the flow control valve, the condition of the flow control valve indicated that it was in its fully closed position at impact. The valve requires several seconds to move from its fully open to fully closed position in normal operation, indicating that the valve was in its closed position before impact. This closed valve would have prevented bleed air from entering the cabin, thereby preventing normal pressurization.47

Closure of the flow control valve on a Learjet Model 35 and the resulting loss of bleed air supply to the cabin would cause the airplane to quickly lose cabin pressure (depressurize) at a rate dependent upon the cabin leakage rate. Computer simulations by Honeywell indicated that if a loss of normal bleed air supply to the cabin occurred at flight altitudes above 25,000 feet, the cabin altitude could ascend to 10,000 feet in about 30 seconds and reach 25,000 feet in about 2 1/2 minutes.

The military pilots who observed the accident airplane in flight before its final descent reported that the accident airplane's windshield was obscured by condensation or frost. Condensation or frost would be consistent with a loss of bleed air supply to the cabin. When bleed air is supplied to the cabin, the cockpit windshield receives a constant flow of warm air that prevents or removes condensation, regardless of the ambient temperature or pressure in the cabin.48 Thus, the windshield would be relatively clear following depressurization from a breach or other undesired outflow from the cabin with continued bleed air supply to the cabin, whereas condensation could form and remain on the windshield following a depressurization caused by a loss of bleed air inflow to the cabin. Therefore, the accident airplane most likely did not have an inflow of bleed air to the cabin.

Possible Explanations for the Closed Flow Control Valve

Investigators considered several possible explanations for the closed flow control valve on the accident airplane. First, Safety Board investigators considered whether the flow control valve might have malfunctioned and closed uncommanded. Investigators identified several mechanical failure modes that might have caused the flow control valve to close, including the loss of the venturi throat pressure sense line, damage to the actuator diaphragm, blockage at the actuator opening chamber inlet orifice, and blockage at the shutoff solenoid bleed port orifice. Because the condition of the wreckage did not allow investigators to determine whether any of these failures occurred on the accident airplane, the Board cannot exclude the possibility that the flow control valve closed uncommanded because of a mechanical malfunction.

Investigators also considered the possibility that the pilots failed to select the CABIN AIR switch to NORM, which activates the air conditioning system (and pressurizes the airplane), before takeoff.49 Even though the Taxi and Before Takeoff checklist specifies, in item 19, "CABIN AIR SWITCH - NORM," the FAA Special Certification Review (SCR) team observed that "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."50 However, without the cabin air conditioning system, the occupants of the airplane likely would have perceived a high cabin climb rate after takeoff, possibly causing discomfort. At about 10,000 feet cabin altitude, the cabin altitude aural warning should have begun to sound, further alerting the flight crew to the lack of pressurization. Although the pilots could have manually silenced the warning, they would have had to repeat this action every 60 seconds. At about 14,000 feet cabin altitude, deployment of the passengers' oxygen masks would have provided an additional cue that the cabin was not properly pressurized.51 It is unlikely that the flight crew would have continued to climb despite this clear information that the airplane was unpressurized.

In addition, the first officer showed no signs of hypoxia52 in her radio transmission at 0927:18, when the airplane was climbing through 23,200 feet.53 Safety Board tests indicated that with the CABIN AIR switch off at this altitude, the cabin altitude would have been increasing to above 20,000 feet. With a cabin altitude of 20,000 feet, flight crewmembers would very likely have been impaired by hypoxia. Further, the cabin altitude warning was not heard in the background of these radio transmissions. While it is possible that the frequency of the pilot's headset, the airplane's radios, or the ATC recording system may have masked the sound of the cabin altitude warning, the lack of such a sound suggests that the airplane had not depressurized to a cabin altitude greater than 10,000 feet by that time. Therefore, although the Board acknowledges that flight crew failure to activate the cabin air conditioning system before takeoff may be a valid safety concern for the Learjet Model 35,54 it considered this unlikely to have occurred on the accident flight.

Investigators also considered the possibility that the flight crew selected the CABIN AIR switch to OFF (closing the flow control valve) during flight. Step 4 of the Learjet Model 35/36 Aircraft Flight Manual (AFM) Abnormal Procedures checklist for a pressurization loss at altitude instructs pilots to select the WSHLD (windshield) HEAT AUTO/MAN switch to AUTO, thus initiating the emergency bleed air supply to the cabin.55 (The wreckage indicated that the windshield anti-ice [defog] shutoff valve was closed at impact,56 strongly suggesting that the emergency bleed air was not activated.) Step 5 in the Abnormal Procedures checklist instructs pilots to select the CABIN AIR switch to OFF, thereby closing the flow control valve. The accident airplane was not equipped with automatic emergency pressurization;57 consequently, if it had experienced a loss of cabin pressurization, the pilots should have executed this procedure to initiate the alternate, emergency source of bleed air.

There is no evidence that an earlier pressurization problem (such as an outflow valve malfunction or a break in the fuselage) preceded the closing of the flow control valve. However, investigators considered the possibility that the flight crew might have experienced (or thought that they had experienced) such a problem and responded by attempting to execute the abnormal procedure for a loss of pressurization at altitude but omitted step 4 (selecting the WSHLD HEAT AUTO/MAN switch to AUTO) before accomplishing step 5 (selecting the CABIN AIR switch to OFF). Therefore, the closed position of the flow control valve could have been a consequence of the flight crew's attempt to address a pressurization malfunction or failure (cause unknown), rather than its cause.

In summary, as previously discussed, an uncommanded closure of the flow control valve would have been sufficient to depressurize the airplane. However, there was insufficient evidence to determine whether the depressurization was initiated by a loss of bleed air inflow (caused by a malfunction of the flow control valve or by inappropriate or incomplete flight crew action) or by some other event."

The next paragraph goes into the maintenance issues this aircraft had and inadequate maintenance recordkeeping. Don't know if fines were levied or tickes pulled...

[FLYBOYJ]

My 2 cents...

I worked around Lears for a short time; I'm an A&P/IA. We did hush kit testing and actually did turn off pressurization during takeoff during some of the testing and the aircraft climbed like a rocket as stated in the report. It was also turned back on about 10K for the reasons indicated. The USAF has a requirement for one crew member to go to O2 over 20K on C-21s. After this accident I believe the FAA put out an AD requiring one crewmember to be on O2 above 40K, this on all Lears.



Regardless of what happened it seems NTSB placed no direct blame on this flight crew (not to say they are entirely blameless but I don't know if they were "incompetent") and this aircraft did have a history of pressurization problems to the point where the company that owned it even had some of the maintenance activity not fully documented. I don't know if you ever flown a jet above 20K (I have) things happen real quickly and it could get ugly if you have multiple system failures, especially if those failures can't be remedied with normal procedures due to mistakes during maintenance (just my oppinion on that one). If the chain of events happened as you say no one will ever know why this crew didn't get down to 10K before they went hypoxic, but again the NTSB did not lay blame directly on the crew (something very rare) so that in itself tells me something was there to cast reasonable doubt not to place blame on the crew as there is a possibility that this crew never had time to put their O2 masks on, possibly because all of the maintenance activity that occurred before this incident. Personally if there's any incompetency to go around, read the NTSB report about the maintenance history on this, it makes me cringe.