I was a highway cop for 30+ years, and the only times I ever had to cut anyone out of a seatbelt was when the vehicle was overturned, and it was very difficult to try to hang on to them while reaching under them to find the buckle. I never saw a jammed buckle, but did see one from time to time that failed to stay buckled after impact...

Last private aircraft crash I was at, all four occupants were ejected, still belted to their seats. The seats themselves were ripped from the craft...

Geezes, what kind of aircraft was it?

I've never tried it, but if I needed to turn a plain edge knife into a seatbelt slicer I'd take a sharp knife and give it a toothy edge on coarse stone. From what I've seen, a polished edge slides on nylon webbing rather than biting in. A coarse toothy edge should allow it to bite in. Never tried it, that's why I bought the Spyderco Rescue. An Endura with a 50/50 blade should do a fine job and would be a good general purpose blade. OTOH, I EDC only plain edge knives. The serrated knife I have is in the truck console.

Piper Cherokee if I recall correctly. It took off at night from a private airport, began a left turn, lost altitude, dug the wingtip in, and cartwheeled for a while. One passenger was still alive when I got there minutes later. She had more compound fractures than I could count, in places you are not supposed to see compound fractured, was thankfully unconscious, and died before I could even begin treatment...

Interesting.........
I was dragooned into a trip on the river last year. Being paranoid about these things -according to the PHRASECENSOREDPOSTERSHOULDKNOWBETTER.- I put my foot down about a couple of things:
1) We ain't sitting down stairs.
2) I'm taking my Ritter Mk1 with me.

If it went pearshaped I am going to:
a) Cut a hole in the sun canopy real quick (if Im have to).
b) Not fart about trying work out how the "security" tiedowns on the carley floats/liferafts work. The straps are going to meet their maker real quick.

Of course, this being the UK, I would have been nicked for possesion of an offencive weapon. Reasonable excuse or not.