Servicing an oxygen system with the incorrect type of oxygen can have deadly consequences. On April 1, 1997 a pilot succumbed to hypoxia and was killed during an uncontrolled descent and subsequent breakup of the aircraft from 27,700 feet. The passenger onboard sustained only minor injuries.
According to the passenger, the portable oxygen bottle was filled by the FBO the day of the accident. After takeoff, the pilot was cleared to 25,000 feet by ATC. Passing 10,000 feet, both occupants put on oxygen masks. At that point, the pilot asked his passenger if she was okay, and she said yes. They continued to climb, and the last altitude the passenger remembers hearing the pilot call out was 20,000 feet.
The passenger indicated that she knew the oxygen had been turned on because she could feel the cool air, and the flow meter was green. The flow meter turns green when oxygen is flowing.
After passing through 20,000 feet, the passenger reported feeling dizzy and her eyes would not focus. She told the pilot, but thought he was talking to the tower because he didn't respond. ATC then observed the aircraft go through its assigned altitude and level off at 27,700 feet. Shortly thereafter, the aircraft entered an uncontrolled descent.
During the investigation, the NTSB discovered that the tanks used to service oxygen bottles at the airport contained Breathing Air Grade D. Compressed breathing air only contains 21% oxygen, which does not supply sufficient oxygen to the users at higher altitudes. (Aviator or medical grade oxygen contains 99.5% oxygen.) The following chart shows the effects of hypoxia in terms of time of useful consciousness (TUC). This is a measure of the pilot's ability to function.
As you can see by the chart, the pilot in this accident had between 1 and 2 minutes of useful consciousness.
The NTSB determined the cause of the accident to be improper servicing of the portable oxygen system with compressed air, which resulted in the pilot's incapacitation due to hypoxia.
When getting an oxygen system serviced, pilots need to ensure that the correct type of oxygen is being used. The bottles should be marked, and for flying purposes, must contain aviator or medical grade oxygen. Pilots having their oxygen system serviced at an unfamiliar airport may want to visually inspect the cylinders being used.
For those that would like to experience the effects of hypoxia in a controlled environment, the FAA Civil Aerospace Medical Institute offers physiological training classes at locations throughout the United States.
This accident report as well as others can be found in ASF's Online Database.
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