@PaulHoles Human error! It took 18 minutes before they noticed. By then, then pilot (or someone else, or a fire) had turned off the transponder. They hoped it was a malfunction and the plan would land. When it didn’t, they didn’t admit their mistake for a while, probably ashamed
@PaulHoles The theory I tend to believe is : the pilot asked the copilot to step out of the cockpit.
The door locked. Once alone, he depressurized the cabin, killing everyone, then either flyed it or left it on autopilot until the plane ran out of gas, which would have taken abt 6-7 hours