He saw the pilot's body and the fish eating it. He was eating the fish and it grossed him out.
Explanation:
In Gary Paulsen's novel "Hatchet," Brian is "stricken with a white-flash of horror" when the pilot has a heart attack because he feels that he can do nothing. However, the pilot had taught him a little about flying and he had read books, so he predicts he can land safely by letting the plane run out of gas and finding a lake to land by.
