You have to climb at a light angle, not too steep.
Viewing post in Biplane Champion jam comments
Can you flash some text like "Stall" when the engine kicks out, just to provide an extra clue for dense players like me? I figured that was it, so I tried nosing the plane down and accelerating with the expected, suicidal, results. In retrospect, I think I was trying to give the plane a longer time on the ground to build up speed, forcing me into a steep ascent at the end and trying to immediately recover.