SPOILER
it is first mentioned when he jumps from the wreckage, but it says in the narrative he wasn't sure when it happened, which I'm not sure makes it any clearer. Even in a crash, I would think the snapping of one's wrist would register. I've been in car crashes but obviously not a plane crash. At any rate, my believability factor was triggered by the narrative of Jack moving around gathering up things and rolling over the dead man etc, with no mention of his injuries which, if they are snapped bones in his wrist and snapped ribs, he would hardly be able to drag anything, with his good hand, and certainly not be able to heft anything of any weight with broken ribs, one-handed.