Continuity mistake: The outline of the heart the little girl drew on the window change from when she drew it to when Jodi Foster breathes on it. This has nothing to do with the inside/outside shot of the heart. (00:14:55 - 00:50:20)
Continuity mistake: When Stephanie is about to check avionics, Fiona holds the flashlight by the lamp end (bulb) as Stephanie takes the flashlight. However, in the next shot, Fiona still holds the flashlight, but she now holds the other end as Stephanie takes the flashlight from her.
Continuity mistake: In the shot when Carson put the bomb in Avionics to activate it, he puts down two parts. The part to the right (from Julia's perspective) is a little inclined. But when Kyle is in Avionics looking for Julia, that part of the bomb is not inclined.
Continuity mistake: Near the end when the Air Marshall is getting off the plane to "get the two agents" there is a shot of him from behind walking off the plane with snow covering most of the ground, but when the shot cuts to a wide shot from the side of him all the snow is gone.
Answer: The hatch they climb into is the hold of the plane, i.e., the section with the coffin, the car, and all the other luggage in. It only appears small because of the way it is filmed. As Kyle would have known, it would have been extra strong and reinforced, as it was a break in two sections of the plane.