Which of the following is NOT a sign of a stall in an airplane?

A decrease in control pressure is indeed not a sign of a stall in an airplane. Typically, when an aircraft approaches a stall, pilots will experience an increase in control pressure, particularly when trying to maintain altitude or control the aircraft’s attitude. This pressure increase occurs because the control surfaces become less effective as the angle of attack exceeds the critical point.

In contrast, the other options are recognized signs of an impending or actual stall. A loss of altitude can indicate that the aircraft is no longer maintaining lift, while the activation of the stall warning light serves as a visual alert that the aircraft is approaching a stall condition. Additionally, pitching or rolling uncontrollably suggests that the aircraft has lost aerodynamic stability, which is directly associated with stalling behavior. Each of these elements serves as a clear indicator of the stall condition in flight.