Except as provided in this paragraph, each person on board an airplane operated under this part shall occupy an approved seat or berth with a separate safety belt properly secured about him or her during movement on the surface, takeoff, and landing.
Absolutely at takeoff, but the emphasis is often missing for the remainder of your time onboard. It's noticeable when crew add extra reminders.
I've never looked for stats on these incidents involving clear air turbulence. The impression I got as a European was that this was a transatlantic and transpacific problem. It's obviously not, but those were the incidents reported on.
There are occasionally "air holes" happening near my local airport. A friend onboard a particularly severe one told me that the captain said that they'd "fallen" some 160 meters.
I also have another friend who, a long time ago, didn't use the seatbelt on a domestic flight - sudden turbulence made him hit his head and injure his neck badly. That's very dangerous. Personally the "worst" I've seen was just enough to break wine bottles in the overhead in front of me. Flight attendants gave out blankets for all of us nearby, as protection for dripping wine..
Its expected that crew will light seat belt signs when they asses its required. Its literal on/off or yes/no, lets not add some 3rd state to it that will confuse people.
This of course doesn't mean one shouldn't wear it as often as possible, but ie with small kids thats nigh impossible, or for some folks on 10 hour flights (I still used to put it on me when I slept, but mainly to not be woken up by crew when they switch it on and check everybody).
Very similar topic is bus seat belts. All intercity buses (at least in Europe) have them, nobody clips in (apart from driver). I think at this point everybody knows about some horrible bus tragedy which would be a set of minor scratches if all folks had seat belts on. Yet our subconsciousness keeps changing perceived risks and consequences to keep us happy/content instead of worried about everything all the time (makes some sense, cluttered mind ain't best performing in life & death scenarios of bygone era).
Except as provided in this paragraph, each person on board an airplane operated under this part shall occupy an approved seat or berth with a separate safety belt properly secured about him or her during movement on the surface, takeoff, and landing.
https://www.ecfr.gov/current/title-14/part-121/section-121.3...