A stall occurs when the smooth airflow over the unmanned airplane's wing is disrupted and the lift degenerates rapidly. This is caused when the wing

Explanation

A wing stalls when it exceeds its critical angle of attack: the airflow can no longer follow the upper surface, separates, and lift drops sharply (PHAK Ch. 5). This can happen at any airspeed, attitude or weight; exceeding maximum speed or maximum weight does not by itself define a stall, although weight raises the airspeed at which the critical angle is reached.