Cherroy is right. The belt is distraction.

Imagine this:
A person running on a threadmill, then he gets a push from the back (which is the forward momentum from propeller/jet engine).
The guy either falls down face first, or he moves forward and run in front of the threadmill.

Same thing to the plane:
The conveyor belt will not be effective in keeping the plane stationary.
The plane will move forward DESPITE the conveyer belt.

OR,
there might be a possibility that the plane will just nose dive into the ground before it even moves anywhere (if the force applied is off centre)

The only way to get a plane to fly stationary is in the wind tunnel.