This guide will let you know how to fly an aeroplane in GTA 5.
The first thing you will need to do is kill the pilot and steal his aeroplane. Then make your way inside the aeroplane.
You can use W,S,A and D keys to move the plane forward, backward, left and right respectively.
Press W to take the plane forward and then press 5 on numpad to take off the aeroplane.
And then press 8 to straighten your aeroplane in the air.