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Stall Alarm

April 6, 2014

I get the impression that most people don’t understand what happens when an aircraft stalls.  They think of an automobile when the engine fails.  All that happens then is that the car comes to a stop.  I’ve even seen an article in the press that said the engine stalled, in reference to an air crash.

An aircraft stall is a much more serious event.  It means that the wing loses its lift, as a result of the loss of smooth air flow over the wing.  It’s caused by air speed that’s too low along with a climb that’s too steep.  Once the wing loses its lift, there’s nothing left to keep the plane in the air.  The aircraft falls straight down.  This Wikipedia article describes the stall in great detail.

Commercial aircraft have a stall alarm that sounds when the plane is in danger of stalling.  The pilot is supposed to take corrective action whenever they hear the stall alarm.  Increasing throttle to increase air speed and bringing the nose of the plane down to reduce the steepness of the climb should be all that’s required to silence the alarm and prevent the stall.  If the aircraft does stall, it’s still possible to recover.  All pilots practice stall recovery in a simulator and often in a real aircraft.  Bringing the nose down so that the plane goes into a dive is the first step in stall recovery.  As long as the aircraft is far enough from the ground, this technique will bring the plane back under control.  If it’s too close to the ground, however, it is going to crash.  That’s why the stall alarm sounds well before the plane actually stalls.

The Air France plane that crashed in the middle of the Atlantic in 2009 is an example of what can happen in a stall.  This aircraft was flying at crusing altitude when the pilot attempted to climb to avoid high thunderstorms in the area.  This would not normally be a problem, but the air speed sensors were giving incorrect readings because of icing.  The aircraft was actually going too slow.  The stall alarm sounded repeatedly.  Unfortunately the pilot disregarded the alarm.  The plane stalled and fell downward for four minutes before crashing into the sea.  Everyone on board was killed.

The British Airways flight that crashed just short of the runway at Heathrow airport in 2008 is an interesting example.  It flew from Bejing to London with no apparent problems.  It was only on its final approach to Heathrow that the pilots noticed that there was no power from the engines regardless of the throttle setting.  They remained at idle all during the approach.  Taking the plane up again was impossible.  It was going to crash on a busy highway just before the fence at the edge of the airfield.  The pilot reduced the flaps just enough so that the plane made it over the fence and crash-landed on the grass in front of the runway.  This maneuver brought the plane close to stalling but by then it was already sliding along the ground.  Everybody on the plane survived that crash.  Only a few were injured.

A stall is a complete and sudden loss of control, one of the worst things that can happen to an aircraft in flight.  It has nothing in common with the stall of an automobile except for the name.

 

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