Stalling

A stall is caused when the airflow over the wing is interrupted to the point where sufficient lift is no longer being generated. This means the aircraft is no longer flying so much as falling. Control surfaces tend to become very unresponsive, air intake to the engine can be reduced to the point where the engine cuts out, and eventually death.

The below images depict it rather well:

The point of  "uh-oh".

Geronimo! 

In order to get out of a stall you need to tip the nose of your aircraft down a little, increase thrust, and pull up to level flight again.

As for implementing the physics of a stall in Air: Force, I have two options:

1: I check the aircraft's CFrame.lookVector constantly to compare it with the aircraft's speed and turn the lift on/off (simplified physics).

2: I edit the aircraft's lift dependant on the speed of the aircraft (faster whilst headed downward, slower whilst headed upward).

Both are not exactly accurate representations of real-life physics - thanks to ROBLOX's physics engine - but they should look and feel like pretty much the same thing in-game.