Question

What is the frame of reference for a plane moving at 500 km/h?

Answer 1

the frame of reference is the rotating earth underneath the flight path of the plane, the rate of rotation of the earth is 1036 miles per hour, meaning that the pilot has to compensate this fact when landing the aircraft.

Answer 2

You can define a frame of reference depending on your point of view, but it would normally be the Earth.

so your answer is THE GROUND