From the book The Cay by Theodore Taylor how does Timothy attempt signal an airplane?

He decides to find some sticks and start a fire for the airplane to see the smoke signal and also he wrote on the sand "HELP" with sticks. But the problem in the book later on is that their smoke was white and the airplanes thought that it was low cloud and kept flying, because they needed black smoke. I really hope I helped :)

he actually wrote help with stones, but either way it didnt matter

:D