Last night there was an MD90 in ATL and we were looking at the engine mount and noticed what looks like a aileron or flap or trim tab on the very end. So I decided I would ask the expert what that is and what it is for
That's a... shoot, dangit, what's that thing called.
Oh! Pylon flaps!
If the airplane is in a deep stall and you push the control column full forward to reduce the angle attack, those pylon flaps will deflect downwards to give you a bit more pitch authority to bring the nose below the critical angle of attack.
Swept-wing jets act crazy when you stall them and sometimes at high angle of attacks, the airflow from the wings can 'blank out' the stabilizer and prevent you from getting enough energy over the stabilizer surface to reduce the pitch of the aircraft.