(Retired 757/767 captain)
Can anyone tell me (Rainboe maybe?) how the pilots would know that they were not breathing O2 if their cylinder had disappeared? My memory of performing and watching countless Rapid Depressurisation scenarios in the simulator is that the initial actions were;-
Oxygen masks and regulators...... ON, 100% (and the 100% was set pre-flight)
Crew Communications.... Establish.
So long as the flow to the masks was not restricted in some way I'm not sure they would know that there was no oxygen flow. Perhaps the 744 has a crew bottle contents guage or a Status message will appear.
Any ideas folks?