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When Does The Airspeed Indicator Lie?

The airspeed indicator has colorful arcs that advise us about our speed -- and on every flight those markings lie.The airspeed indicator has colorful arcs that advise us about our speed -- and on every flight those markings lie. If you’re not aware of this, you can get into real trouble.

The Problem The airspeed indicator displays stall speeds. The slow end of the Green Arc indicates the stall speed with the flaps up. The slow end of the White Arc indicates the stall speed with the flaps down. As long as your airspeed remains faster than these stall speeds, you won’t stall... right? Wrong.

Why: The stall speeds shown on the airspeed indicator are only correct when the airplane is experiencing 1G. When the airplane experiences greater than 1G, the stall speed goes up. However, the Green and White Arcs of the airspeed indicator do not change. So, anytime the airplane experiences more than 1G, the airspeed indicator displays erronious information -- and the airplane will experience greater than frequently 1G during normal flight.

Physics 101 The most common occurance takes place in a level turn: The centrifugal force of the turn teams up with gravity to impose a greater than 1G load on the airplane. Translation: When an airplane turns, the wings support more effective weight than the actual weight of the airplane. When the wings are asked to carry more effective weight, they must have more airspeed to do the job.

Aerodynamics 101 An airplane can stall even when the airspeed is indicating well within the Green Arc (the normal operating range). This is an Accelerated Stall. All level turns increase stall speeds. A bank of just 45 degrees will increase stall speed by 20 percent. A bank angle of 60 degrees increases stall speed by 40 percent. At 75 degrees of bank you have increased the stall speed by 100 percent. Danger: A stall in a turn will most likely lead to a Spin. This is why you should not make overly steep turns while in the pattern if you are not also prepared to let the nose drop.

BOTTOM LINE: Calculate and know the accelerated stall speed at various bank angles for the aircraft you fly. This will help you develop better safety margins -- so will spin training. Learn both. In the meantime, don’t completely trust the airspeed indicator.

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About This Author:
Paul A. Craig is a Gold Seal Multiengine and Instrument Flight Instructor. He currently holds a total of 11 Flight Certificates including his ATP. Craig is a previous winner of the North Carolina and Tennessee Flight Instructor of the Year award, the NCVT Outstanding Teacher award and has served as the regional representative of the National Air and Space Museum. Craig is an FAA Aviation Safety Counselor and the author of eight books, including Pilot In Command, The Killing Zone: How and Why Pilots Die, and Controlling Pilot Error: Situational Awareness (all from McGraw Hill).
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