How does Brian react to the scene he encounters in the plane in Hatchet by Gary Paulsen?

Brian is horrified and frightened by the condition of the pilot's body, but he continues salvaging the plane because he must. 

Eventually, Brian goes back to the plane to see what he can salvage.  He hopes to be rescued immediately, but is not.  The plane is half-submerged in the lake, and it is not as easy to get to as he first thought.  Brian struggles to get inside the plane.  He is eventually able to hack into it with his hatchet. 

Brian drops the hatchet and has to go underwater to get it.  The hatchet is crucial to his survival, and Brian berates himself for being stupid enough to drop it.  He eventually manages to retrieve the hatchet and is able to get inside the plane, which is half full of water.  There, Brian is shocked to see fish have been eating away at the pilot, who no longer has flesh on his head.

Too much. Too much. His mind screamed in horror and he slammed back and was sick in the water, sick so that he choked on it and tried to breathe water and could have ended there, ended with the pilot where it almost ended when they first arrived except that his legs jerked. (Ch. 18) 

Brian is aware he is reacting instinctively. He knows he doesn't actually need to fear the fish.  Nonetheless, seeing the pilot's body in this decomposed and eaten-away state is too much for Brian.  He has been through a lot, and this is just another blow.  After all, he is just a boy. 

Brian eventually recovers and is able to do what he came to the plane to do.  He retrieves the survival pack and is able to return to shore with it.  The pack has a sleeping bag, matches, cooking pots, and a knife with a compass.  It even has a hat, which Brian finds funny but wears anyway.  The survival pack also contains a fishing kit and rifle. This pack and all its supplies elicit a lot of optimism from Brian:

He sat in the sun by die doorway where he had dropped the night before and pulled the presents—as he thought of them—out one at a time to examine them, turn them in the light, touch them and feel them with his hands and eyes. (Ch. 19) 

Brian has successfully survived on his own for all of this time without any of these objects, but feels these supplies are treasures of “incredible wealth.”  The pack should make his life easier, and it was worth the horror of facing the pilot’s body. 

Why didn't Brian react to the plane or pilot in Hatchet?

I believe that this question is asking about the sequence of events that occur in chapter one when the pilot of the small plane is having a heart attack. Brian doesn't react much to the pilot for two reasons.

1. Brian's mind is elsewhere. He is constantly thinking about the divorce that his parents just got. Furthermore, he knows the reason, and it is devastating to him. Brian is essentially still in shock at the entire situation.
2. Brian is young and not likely to recognize the signs of a heart attack. The pilot is showing classic symptoms of a heart attack. He's got bad gas. Surprisingly, indigestion is a possible sign of a heart attack. If that were the pilot's only symptom, then Brian would be right in thinking that the pilot probably ate something that doesn't agree with him; however, the pilot is showing other signs as well. He keeps rubbing his shoulder, and he's wincing in pain frequently.

If the question is asking about why Brian doesn't react once the pilot becomes unconscious, then we have to look at the start of chapter two. Brian doesn't react because he simply can't believe the situation that he is in. Additionally, Brian tries to convince himself that his situation isn't actually happening:

Then he looked for ways for it not to have happened. Be asleep, his mind screamed at the pilot. Just be asleep and your eyes will open now and your hands will take the controls and your feet will move to the pedals—but it did not happen.