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[Oxygen masks]..Don’t they automatically deploy?

The cockpit controls are omnipotent, i.e., the cockpit members have full access to the switches, fuses and systems that can override or turn off the oxygen mask deployment. At least this has been my limited observation of the cockpit environment.

In my flight training, we had to do the hyperbaric chamber. Sea level to 26,000 feet in about 30 seconds. The chamber had oxygen masks. You had to grab it fast, strap it on and start sucking oxygen. Time of useful consciousness is about 20 seconds. Act or die. We were training with the expectation that it would happen. The average airline passenger is oblivious. They also have significantly poorer masks. An easy wipe-out of the entire complement of passengers and unprepared crew. Descent to a safe altitude where masks aren't required takes time. Under about 18,000 feet you can do OK without a mask. If cruising at 40,000 feet, the descent time in a decompressed aircraft would likely ensure dead passengers absent a steep dive.

We had training staff that had their oxygen masks on BEFORE the chamber did the rapid ascent. They were present for safety as this was a training exercise. We had backup to ensure nobody died. We were all warned ahead of the training that is was designated "High Risk Training". One project member died on a treadmill doing a supplemental evaluation when an earlier test flagged a concern. It's always hard to have to call an employee's family to inform them of a death on the job.