Figure NTSB-N47BA Slide 0046, from NTSB Slide Show

Eddie Sez:

When you have a high profile passenger killed with lots of press interest, one of two things are sure to happen in the investigation. If it was a military mishap, the cause will be found and ignored if necessary to point blame at the military. If it was a civilian mishap and the cause points to a company with powerful friends in Washington, D.C., the cause will be ignored if necessary and the blame will be undetermined. This is a case of the latter.

The aircraft failed to pressurize or lost pressurization for unknown reasons. The crew failed to receive supplemental oxygen for unknown reasons. When I was an Air Force safety officer, we were told that while the criminal and legal investigators needed to find evidence "beyond a reasonable doubt," our criteria was, simply, it had to be reasonable. We were in the business of preventing future mishaps, after all. Reading the accident report and examining the accident docket, I believe the following are reasonable conclusions:

  1. Sunjet Aviation failed to service the oxygen system for nearly two months. A company official stated the captain reported the oxygen pressure was in the green zone the day of the accident. I find it highly unlikely that the captain would report this fact to anyone. The captain should have checked it. And if he did, he would more likely simply note the fact and move on. I think it much more likely that the company failed to service the oxygen all that time and the captain failed to check it.

  2. The aircraft out-climbed the pressurization system. The pressurization system had a history of inflow problems but no history of pressure vessel leaks. The NTSB looked at the nearly closed bleed air valves and assumed this meant the inflow was keeping up with the leak rate and was providing sufficient air. This faulty logic ignores the fact the aircraft wasn't adequately pressurized so either the air input was inadequate or the output was excessive. Given the history of the airplane's input system, they should not have dismissed this so quickly.

  3. The crew suffered from slow onset hypoxia. The most dangerous form of hypoxia comes from a gradual increase, studies indicate that about 1,500 fpm being the most insidious. It could very well be that they became partially hypoxic during the climb. The last communication was as the aircraft passed 23,200 feet. Perhaps the cabin had not yet exceeded 10,000 feet when the aural warning would have sounded. When that happened they may have donned their masks but didn't receive any oxygen because the tank was empty. They were mentally impaired and rather than descend they attempted to troubleshoot.

Yes, this is speculation. So regardless if you choose to believe this scenario or not, there are several lessons we can take away from this mishap:

  • Fuselage pressure leaks should be addressed immediately and repaired.

  • Oxygen supply and delivery equipment must be pre-flighted.

  • During every climb, part of your 10,000 foot check must be a check of cabin altitude. Most airplanes have a maximum cabin altitude between 6,000 and 8,000 feet. You certainly should not be above that at 10,000 feet aircraft altitude. You should know your normal cabin climb rate, typically around 300 fpm. Even if it takes you ten minutes to get to 10,000 feet aircraft altitude, you should not see more than 3,000 feet cabin altitude at that point. In any case, if it isn't where it should be, level off and investigate.

  • When troubleshooting a pressurization problem, donning oxygen quickly will not only improve your mental capabilities, it can keep you in the game if things go south quickly or insidiously.

  • When in doubt, descend.

What follows are quotes from the sources listed below, as well as my comments in blue.


Accident Report


Narrative

[NTSB Aircraft Accident Brief, AAB-00/01, page 2]


Analysis

[NTSB Aircraft Accident Brief, AAB-00/01, page 5]

[NTSB Aircraft Accident Brief, AAB-00/01, page 27]

Figure: LR-35 Crew Oxygen System Schematic, from NTSB DCA00MA005, Airworthiness 13 Attach 2 - Maintenance Manual Extracts.

[NTSB Aircraft Accident Brief, AAB-00/01, page 32] Following the depressurization, the pilots did not receive supplemental oxygen in sufficient time and/or adequate concentration to avoid hypoxia and incapacitation. The wreckage indicated that the oxygen bottle pressure regulator/shutoff valve was open on the accident flight. Further, although one flight crew mask hose connector was found in the wreckage disconnected from its valve receptacle (the other connector was not recovered), damage to the recovered connector and both receptacles was consistent with both flight crew masks having been connected to the airplane’s oxygen supply lines at the time of impact. In addition, both flight crew mask microphones were found plugged in to their respective crew microphone jacks. Therefore, assuming the oxygen bottle contained an adequate supply of oxygen, supplemental oxygen should have been available to both pilots’ oxygen masks.

There was a lot of speculation in the Learjet world about the position of the oxygen system pressure regulator and shutoff valve, reasoning that many operators shut the valve off between flights to prevent system leakage. The valve was found in the open position and this operator did not, as a practice, close the valve.

[NTSB Aircraft Accident Brief, AAB-00/01, page 31]

[NTSB Aircraft Accident Brief, AAB-00/01, page 35]


Probable Cause

[NTSB Aircraft Accident Brief, AAB-00/01, page 35]


See Also:

Abnormal Procedures & Techniques / Hypoxia

Abnormal Procedures & Techniques / Slow Onset Hypoxia

Technical / Oxygen


References

NTSB Aircraft Accident Brief, AAB-00/01, DCA00MA005, Sunjet Aviation, Learjet Model 35, N47BA, Aberdeen, South Dakota, October 25, 1999

NTSB Slide Show Presentation