At least an ounce of doubt is called for here. I suggest we look for confirming sources.
Question 1: Why the smoke? The pilot didn’t need a bomb to crash the aircraft. He just needed to take care of (or convert) the co-pilot and insure the cabin door was secure.
Maybe the co-pilot saw the smoke and tried to put it out and the pilot would let him and there was a struggle as he tried to take control. That explains the 90 degrees left and then the 60 degrees right before it plunged into the sea.
How much would gun residue resemble smoke by the detectors that were activated.
Three gun carrying air Marshall ? Why?
Could be that the 3 air marshals figured out what was going down and were storming the cockpit with guns drawn ....hence the smoke.
Question 1: Why the smoke?
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Best answer I’ve seen about the “smoke” is that there was likely no smoke.. smoke detectors use a light source and a receptor ,, anything (such as smoke) blocks the light and the sensor “sees” smoke. If a bomb took out a cockpit window the pressure drop and temp change would create fog so thick you couldn’t see the glasses right in front of your eyes.