What is the true mass of a man in a plane that is accelerating towards the ground with an acceleration of 0.8.He is standing on a set of scales that read 51kg?

If he is standing on balance scales, then his mass is 51 kg. If he is standing on a spring scale, his actual mass would be closer to 63.75 kg. This also assumes the plane is not accelerating faster than free fall, but slower than it.