You will have to do some kind of second-based (or tick-based) comparison scheme, comparing the previous plane position to the current plane position.
Like:
Every 1.0 seconds:
Set previousPlaneX = plane.X
Every tick:
if abs(plane.X - previousPlaneX) > 10
Set plane.Y to plane.Y + 10
That way the faster your plane moves across the screen (analogous to airspeed) the more lift it produces. Each second it will update the "previous" position and every tick it will compare it to the current position.
Chances are this exact scheme will be jerky at the exact instances in time when previousPlaneX = plane.X, so you will probably have to massage this scheme some to smooth it out.