RAMP Button

The RAMP Button appears when you point the crosshair at any aircraft. It is below the RIDE button. Pressing this button opens the ramp of the aircraft you are looking at. Pressing it again closes the ramp, even when there is an object blocking it. The ramp touches the ground so you can enter the aircraft.

Trivia

 * There is a bug with the jumbo aircraft's ramp. Even after you crash, if you are looking at the ramp, the ramp button still appears.