SIFTING THROUGH THE EVIDENCE
Pilot incapacitation near Tindal, 05 June 1991, (AF/A-18 A21-41)
2 page PDF attached
DDAAFS Defence Aviation Air Force Safety
http://www.pigzbum.com/accidents/sif...he-evidenc.pdf
"...If the incident pilot had in fact not realised the cabin pressure switch was incorrectly set to DUMP, the cabin would have been unpressurised and the pilot, on removing his mask at altitude, would have quickly suffered hypoxic hypoxia due to the lack of cabin oxygen....
...The most likely causal factor was deemed to be that the pilot suffered hypoxic hypoxia, as a result of
removing his oxygen mask in an unpressurised cockpit at altitudes greater than 28,000 ft....
...
Footnotes: The
incident pilot had a history of flying with his oxygen mask removed. As recently as two days before the accident, the incident pilot had indicated to another squadron pilot that he considered it unnecessary to wear his oxygen mask at altitudes around 30,000 ft since the cabin altitude was only about 12,000 ft under those circumstances. (With the cabin pressure switch set to NORM, the AF/A-18 cabin pressure schedule maintains a cockpit altitude of approximately 8000 ft until 23,000 ft aircraft altitude. Above 23,000 ft aircraft altitude, the cockpit altitude increases slowly to approximately 14,500 ft at 35,000 ft aircraft altitude, and 20,000 ft at 50,000 ft aircraft altitude.) The Board concluded that since a radio call had been made passing FL220, the incident pilot must have taken his oxygen mask off some time later, probably when he unexplainably levelled off at FL280 for a short period during the climb. (The time of useful consciousness at FL280 is less than two minutes and time to unconsciousness is less than four minutes. These times decrease with increasing altitude.)...
...It was thought, that since the incident pilot had only completed AVMED training once (during his initial pilot training and four years prior to the accident) that he may have had insufficient experience to be fully aware of his individual hypoxia symptoms. At the time of the accident he was 11 months overdue for his AVMED refresher training."