Oxygen Leak
Experienced a very tight situation in an RAAF Canberra enroute Cocos Island to Butterworth Malaysia in the 60s.
Detected unusual low oxy contents beyond PNR. It took a while to figure out the rate of depletion and what to do next. The Navigator was skillfull at attempting to work out a trade off between fuel burn at a survival lower altitude when Oxy depleted. The rate of Oxy leak became the significant factor and was such that it looked close to having to abandon the aircraft before being able to reach a closest airfield.
When Oxy ran out we were forced to a much lower altitude where rate of fuel useage increased considerably. Made it to Butterworth having just enough left to taxi.
I have run short of oxygen on two other occasions with one at night not being recognised until near unconsciousness. The emergency oxy system did not work. In an annoxic state I managed to stay alive by taking deep breaths and pressurising my lungs to a maximum whilst diving steeply at a Mach number close to entering an uncontrollable nose down tuck.
The body's recovery was not like that which you experience in a decompression chamber. It was a supreme effort to continue the flight to a landing and even taxying was a huge task. The next morning I awoke to a bad headache and extreme lethergy. Perhaps there was some brain damage.
Ever since I have tried to determine the minimum blood oxygen percentage which will just keep an average human conscious. It seems to be about 65 to 70 %. Does anyone out there have a more accurate figure? Climbers of Everest should know.