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If there is a leak in the pitot tube in the a/c, what would be the readings for a pressurized and unpressurised aircraft? I've read that if there is leakage in pitot tube within the structure of a/c, so for non-pressurised a/c it will under read because air will escape from the tube to the cabin, as cabin pressure would be less than pitot pressure? Is it correct?

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  • $\begingroup$ @mins, it seems quite clear to me. In a non-electric airspeed indicator there is a pipe from the pitot probe propagating the total pressure all the way to the instrument, and similarly from the static port propagating the static pressure, so the instrument can purely mechanically show the difference. So these pipes run inside the cabin (behind the instrument panel) and can be cracked there, affecting the readings. $\endgroup$
    – Jan Hudec
    Nov 24 at 18:35
  • $\begingroup$ @JanHudec: Ok, pitot tube vs. pitot tube pipe, it's the pipe which is leaking. $\endgroup$
    – mins
    Nov 25 at 14:52
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The underlying principle of how an airspeed indicator works is to display the difference between pitot (ram air) pressure and static (outside air) pressure. The larger the difference, the larger the airspeed needle swings.

The instrument will convert that pressure difference into knots indicated and display an indicated airspeed.

For a 1 psid difference between pitot and static pressure, the airspeed will read approximately 145 KIAS. At 2 PSI difference, approximately 205 KIAS.

This background is important to analyze your question.

  1. If the pitot tube cracks in an unpressurized aircraft, the airspeed will show a small difference between RAM AIR and static air. The difference will be too small to show anything on the airspeed and it will read 0 KIAS.
  2. If the pitot tube cracks inside a pressurized aircraft, the airspeed will have a large difference between RAM air (cabin pressure) and static air. The airspeed will be pegged at max airspeed. As you descend, the airspeed will move to zero when you are unpressurized before landing.

In the pressurized airplanes I fly, a 9.2 PSI difference is standard. That is quite a bit higher than what is listed above for 205 KIAS.

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  • $\begingroup$ Even if there is a leak in the pipe, the ambient air is still flowing inside it and it still senses the pressure, however it is different. The ASI does NOT read zero. Incorrect. $\endgroup$ Nov 25 at 11:47
  • $\begingroup$ @SachinChaudhary, it depends on how big the crack is. If it's large enough, it will read 0 (it can't display less than around 30 knots). But you are right that it may not read zero, it may read any value less then actual depending on how big the leak is (or any value more if the airplane is pressurized). $\endgroup$
    – Jan Hudec
    Nov 25 at 15:05
  • $\begingroup$ I would agree with these comments. $\endgroup$
    – wbeard52
    Nov 25 at 15:20

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