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7 Answers

Pitot tube question

Asked by: 5138 views Aircraft Systems

Let's say an airplane is tied down to the runway, engine not running, with 100 kts of headwind straight down the runway. The IAS would be 100 kts, right? Okay what would happen if we untied the airplane? For arguments sake lets say the plane lifts off the ground (due to air going over wings) As the plane becomes part of the airmass in which it is "flying" the groundspeed would be negative (or positive in the opposite direction). What would happen to the IAS? Considering the following, admittedly rudimentary formula: IAS-headwind=ground speed If we have a negative ground speed and a constant headwind of 100 knots wouldn't the IAS have to go down? Or, would IAS remain constant in the face of the wind despite the fact that the airplane's only movement is because of the headwind? I got this wrong on a checkride.

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7 Answers



  1. Stephen Bellacera on Apr 17, 2014

    I would say that the IAS would remain constant at 100kts. I hope you didn’t get failed for that ?!

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  2. Steve Butler on Apr 17, 2014

    IMHO, as soon as the aircraft leaves the ground, the parasite drag is going to deteriorate the IAS at a rate in direct correlation to the increase in negative ground speed mentioned in the original question.

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  3. Kris Kortokrax on Apr 17, 2014

    What bit of aeronautical knowledge was the DPE trying to extract from you?

    If you left an untied airplane out in 100 MPH winds there would likely be no IAS because the pitot tube would be bent and broken just like the rest of the airplane.

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  4. Jason on Apr 17, 2014

    Stephen- Yes, I did. CFI checkride. We spent three hours talking aerodynamics. The answer to the question was the plane would lift off the ground then move backwards until it stalled and fell back to the earth. I just can’t remember why- and he is certain to revisit the question on my retest.

    So that’s the real question- why would the plane stall?

    I thought by making it a pitot question I would get my answer- i.e. is there less air going over the wing (as well as into the pitot tube) as the plane drifts with the wind?

    Steve- I’m interested in your idea there. Could you expand that thought?

    Thanks all!

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  5. Kris Kortokrax on Apr 17, 2014

    If the DPE failed you for this, I would report him to the FSDO.

    There are some assumptions that would have to be made to even begin this discussion, the first being that with the engine not running there would be no thrust.

    The airplane would need to be trimmed for a certain speed. If it were trimmed for cruise speed and rose vertically to maybe 20 feet, the nose would probably pitch down shortly after leaving the ground in an attempt to regain its trimmed speed after it began to slow. This would result in the pile of wreckage I alluded to.

    If the airplane were trimmed for climb speed, the nose would pitch up in an attempt to maintain that speed. It would probably over pitch, which would result in a stall, and the pile of wreckage I alluded to.

    If the airplane were trimmed for a speed faster than normal cruise (unlikely), the nose would again pitch down after leaving the ground, resulting in the pile of wreckage I alluded to.

    If the airplane were somehow able to maintain a level attitude and remain pointed exactly into the wind as it levitated, it would soon begin to move with the airmass and its speed relative to the airmass would decrease, at some point resulting in a stall, and the pile of wreckage I alluded to.

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  6. Steve Butler on Apr 17, 2014

    Kris is right. In an unoccupied aircraft, the way it is trimmed is going to determine how long it is before the aircraft assumes the mangled wreckage state. If it is occupied and the occupant attempts to maintain straight and level flight, it will leave the ground at an IAS of 100 but the drag will immediately cause that IAS to decrease as the speed in reverse (negative ground speed increases). As the airspeed decreases, the hapless pilot would have to increase the angle of attack to remain airborne. When that angle of attack reaches the critical angle of attack, the aircraft stalls while flying at 50 knots in reverse thus giving Kris his pile of wreckage. Do you think that your CFI will buy that explanation?

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  7. Jason on Apr 18, 2014

    Yes, I think that is the answer he is looking for. Thanks Kris and Steve!!

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