Other mistake: When Quin is cutting the plane that's stuck in a tree, he uses a saw, that looks brand new. He's just been chased by pirates and jumped off a cliff - where did he get the saw from? Anything in the plane would be rusty by this point.
Other mistake: Watch Harrison Ford's hair throughout the film. When they are making the rafts out of the plane, you can see that he has had his hair cut. I never knew that they had a barber's on that island.
Answer: Once they were in the air, Quinn would be able to navigate by sight. He was an experienced pilot and and was familiar with the territory and the different islands' configurations. He would also be looking for ship traffic, and he had a general idea which of several islands they were on. Once in the air, he would easily know which direction he was heading based on the position of the sun.
raywest ★
But he passed out. And the ocean is a big expanse, not a lot of landmarks to navigate by.