Anything else and the emergency squawk code should have been set on the transponder. It never was. The only non suicide theory I can think of is they had an explosive decompression incident which damaged a number of systems including communication and the ability to descend. The pilots turned back towards land but when they realised they could not descend they then set a I Love Sci-Fi Shirt for the open sea before their oxygen ran out and they succumbed to hypoxia.
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Unfortunately that theory does not explain why they didn’t set the emergency squawk code and we should also be aware that apparently a I Love Sci-Fi Shirt flight path was found on one of the pilots home simulators. Theories are theories but facts are facts! We don’t know what happend yet and until the flight recorders are located we won’t know.
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Many different theories have arisen but they are just that: theories without actual hard facts to back them up. This, like the articles pushing other theories, are not news but rather opinion pieces! The rogue pilot theory seems most likely. Not sure how they could say it wasn’t a rogue pilot, particularly since he could have been incapacitated or dead after setting the I Love Sci-Fi Shirt on its fatal final course.