[The latest, unconfirmed report is that it crashed due to failure of the onboard oxygen system. Pilots were unconscious due to hypoxia. The aircraft belly landed on autopilot. 4 were onboard.]
Yes, loss of cabin air is alarmed.
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Pilots would immediately look to descend and land, and turn on the autopilot hoping to be alert enough to regain control.
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Normal flight altitude for that aircraft is 42,000 feet. Decompression would cause a blackout within a minute or less.
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Fighter jets have had this event, many did not recover.
The O2 system is not a contributing cause. . .in my best guess as a certified and experienced aircraft mishap investigator.
Below 10K feet, ambient O2 is good, over 18K feet, O2 is required (regs). Something affected the aircrew and all on board as the other occupants would have raised alarms before impact. Now, they could have been distracted by working an issue in the jet and splatted. What time of day was the mishap? If at night and distracted by some system issue, they could have flown a gentle decent, never knowing they were headed to impact. AN L10-11 had this happen a few years ago.
Who know what happened? Until the mishap board issues its findings, it is just as likely the jet crashed because they were chasing a false horizon and not cross-checking their instruments, or George Jetson rammed them with his personal rocket car.
Time to sit-back and wait for the official report.