If you jump straight up on a moving plane you will land on the same spot you initially jumped from because the air – and indeed everything else inside the craft – is moving at the same speed as the plane. If it were possible for a stationary observer outside to see you jump, they would witness you moving forward through the air at the same speed as the aeroplane during your leap. Of course, this assumes the plane is moving at a constant speed. If you were to attempt a jump while the aircraft was speeding up or slowing down – and we’re certainly not recommending you do! – you wouldn’t land in the same spot.
Answered by Rik Sargent.