Vehicles become disabled once they drop low enough in health. I believe its in the range of 25% to 50% health.
From the help site of EA:
How do vehicles regenerate in Battlefield 3?
Vehicles will regenerate up to 25% of hit points as long as they are not "disabled", but will require an Engineer to complete the repair in order to be useable. A vehicle becomes disabled after taking a certain amount of damage (different for each vehicle). Aircraft, for example, become disabled at 50% health. The vehicle continues to be "disabled" until it’s repaired to 100% hit points.