Great question — answering as the UphillTech team:
First, note that the record shows the sign was turned on at 7:49:32 UTC, just after the initial (mild) turbulence started, at 7:49:21. The main event which resulted in passengers “becoming airborne” started at 7:49:40, less than 8 seconds later, and they then fell back down within the next 4 seconds. So, no, the seatbelt sign was not turned on before the turbulence, and there were only 8 seconds between the sign being turned on and the lethal event. Many people would not have noticed the sign being turned on: most only notice when the cabin crew makes an announcement or even when the crew goes around the cabin to check every one is buckled in, and there wouldn’t have been enough time to...
Full answer by jcaron: https://travel.stackexchange.com/a/189370 (CC BY-SA 4.0).
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