Chapter 2 of Hatchet ends with the plane's engine cutting out, Brian pushing the nose forward, and him throwing up. Brian is scared, and that is right where chapter 3 begins. Brian is terrified and fully believes that he is going to die. This is one of the internal conflicts that Brian struggles with throughout the entire story. He is constantly scared. He is in unfamiliar territory. He's alone, and he has no idea how to solve any of the problems that he is encountering. Chapter 3 is no different. He has no idea how to fly a plane. He has no idea how to land a plane, and he most certainly doesn't know how to crash land a plane into a lake. Add to that the fact that he can't spot a lake at first. He's scared and has to wrestle those fears down in order to think clearly.
The external conflict in chapter 3 is a man versus nature type of conflict. Brian isn't fighting against any other external force other than the situation he is in. He fights with the plane the entire way to the lake, and he learns the hard way what it means to trade altitude for air speed. Brian is desperate to find a lake, and the lake he spots is just barely within reach of the gliding aircraft. The lake is actually just a bit too far for him to reach because the plane is torn apart as it hits some of the trees. This forces the lake "landing" to be quite violent. The plane plummets into the water, and the plane quickly fills with water. Brian struggles to free himself from the plane, and he just barely makes it back to the surface and the nearby shore before collapsing from complete exhaustion.