How tight should passengers wear the seat belt?

Question

How tight should a passenger wear the seat belt?

During takeoff and landing and other moments the seat belt sign is on, I usually pull the belt as tight as possible. This is little uncomfortable, so I loosen it a bit when the seat belt sign is off. I think that one or maybe two fingers might fit between the seat belt and my body. Is this a good idea? Would wearing the belt as tight as possible give a greater risk of crushing my pelvis bone or something?

Please specify whether there is a difference between a situation where the seat belt sign is on and a situation where the sign is off.

Answer 1

According to this document published by the FAA (about smaller GA planes but the topic is applicable)

Tests have shown that slack in the restraint system should be minimal. In an impact, your body keeps moving until the slack is taken out of the restraint, but then must be abruptly stopped to “catch up” with the airplane. The restraint should be adjusted as tightly as your comfort will permit to minimize potential injuries.

Here is a video of them testing the seats. You can see the waist lines of the dummies does not move very much.

Here is an interesting research paper on the topic, may be worth checking out.

A study was conducted by the DOT "Human Factors Associated With the Certification of Airplane Passenger Seats: Seat Belt Adjustment and Release" on the difference between belt tension (as applied by a passenger) in both a cruise and emergency setting. This looks like the most conclusive numbers I can find on measured tension.

An analysis of results indicate that most passengers (90%) tighten the lap belt to a tension less than 7 lb. during normal flight conditions and less than 10 lb. for an anticipated emergency.