Harmon Technologies, Inc.

Stall Warning Light

This red light is connected to a small metal sensor on the leading edge of the left wing.  This sensor moves when the angle of the air hitting the wing exceeds a pre-set amount.  If the airplane is slowed too much, the air strikes the wing at an agle from below and may not be able to bend enough to flow smoothly over the wing.  If this happens, an aerodynamic "stall" occurs and the wing produces much less lift.  (This is similar in some ways to prop cavatation on power boats.)  The light serves as a warning to the pilot that the airplane is close to a stall condition and that the angle of attack should be reduced by some combination of lowering the nose or adding power.  In normal operation this light is never on although it may flash slightly during the final moments before landing (a deliberate stall occurs as the aircraft touches down.

Stalls are usually recognized by feel before the light goes on.  Normally, airplanes are not operated close to their stall speed so it would be unusual to see this light. 

While the term stall sounds scary, it is very easy to avoid stalls and to correct for them.  many hours are spent in flight training deliberately causing the aircraft to stall and then correcting smoothly.  While some airplanes have dramatic stall characteristics, well-designed airplanes like N4907J stall, their design handles the condition very well and nothing very dramatic happens.  The airplane simply mushes through the air while pointing up but not gaining altitude.  Some airplanes have horns rather than the light used on N4907J.  Most airliners have a vibrator device that shakes the control yoke slightly in the pilot's hand to indicate a near-stall condition.  All airplanes are required to have some sort of stall warning indicator.

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