According to special relativity, being inside a plane going at a constant speed is like being stationary because time and space change to keep it so. General relativity extends special relativity to explain gravity by changes in space and time as well.
QR5.4.1. Free-fall is Acceleration
QR5.4.2. The Gravity Gradient
QR5.4.3. Unstoppable
QR5.4.4. Matter Changes Space
QR5.4.5. Matter Changes Time
QR5.4.6. Black Holes