Hypoxia has what is called an insidious onset, meaning the signs and symptoms occur so gradually that they are fully established by the time you become aware of them. When the cabin pressurization system does not work and the pilot is unaware, due to the insidious nature of hypoxia they will also likely be unaware of the signs and symptoms before it is too late. McSpadden said it is possible that the pilot was unaware of a cabin pressurization issue until the hypoxia had taken over and he was impaired or incapacitated. In similar instances, this occurred about 20 minutes into the flight. Based on what is known, there is an indication of an insidious hypoxic event.
The second, slightly more dramatic depressurization event, is rapid decompression. When this occurs around flight level 350, or in that range, a pilot will have roughly 30-60 seconds to put on a mask before they use consciousness. The concept “Time of Useful Consciousness” outlines different altitude levels and the amount of time needed to put a mask on before losing consciousness. Pilots are trained to put the masks on quickly in emergent situations, especially pilots with the type rating and background this pilot had.