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4 Answers

Indicated airspeed vs. Ground speed

Asked by: 1710 views Aerodynamics

Excerpt from book: "crew said they slowed airplane to gust penetration speed 150 KIAS. However ATC radar plotted ground speed of 200 knots." (some airframe damage resulted). Question: Thinking about maneuvering speed or gust penetration speed,  I always thought that you fly based upon KIAS not ground speed. So in this case since gust penetration speed is 150KIAS should the pilot have flown at 100 KIAS to achieve gust penetration speed of 150 knots to account for the 50 knot tailwind? (200 knot ground speed puts airplane at 50 knots above 150 KIAS or stated another way: 150KIAS + 50 knot tailwind = 200 knot GS)   Thanks for the feedback.    

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4 Answers



  1. John D Collins on May 25, 2015

    No, GS is not relevant, just the IAS.

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  2. flyboy737 on May 25, 2015

    Correct only IAS is relevant, Va – gust penetration speed mentioned on the flight manual is for aircraft all up weight. And the number reduces with reduction in weight. I assume the aircraft was flying for a while burning fuel and maybe less pilot+pax weight. So in actual penetration speed for the aircraft with that particular weight was less than 150 KIAS. It should explain the rest of the story

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  3. Dan Chitty on May 26, 2015

    Much appreciated for the feedback.

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  4. Kris Kortokrax on May 26, 2015

    Va is not gust penetration speed.

    Va is design maneuvering speed. It is referenced in 23.335 and 25.335.
    Vo is operating maneuvering speed. It is referenced in 23.1507.
    Maneuvering speed is referenced in 25.1507.

    Vb is speed for maximum gust intensity (gust penetration speed, turbulence penetration speed). It is referenced in 23.225 and 25.335.

    They are not the same. AC 23-19A states:

    “Va should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits, nor should it be interpreted as a gust penetration speed.”

    Looking at a G-IV manual, it lists 170 KIAS as the maneuvering speed. It shows 270 KIAS or .75 Mach as the maximum turbulence penetration speed.

    Simplifying a bit, maneuvering speed is a limit speed for the load a pilot might impose on the structure. Gust intensity (or penetration) speed is a limit speed for the load that gusts might impose upon the structure.

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