What kind of failure would cause an indicated airspeed of say 200 knots on the ground (stationary)?
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2$\begingroup$ Have you ruled out a hurricane or tornado? $\endgroup$– StephenSCommented Oct 22, 2020 at 17:58
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$\begingroup$ @StephenS Can definitely confirm the airspeed was actual airspeed was fine haha. $\endgroup$– synchhCommented Oct 22, 2020 at 18:30
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$\begingroup$ I was about to question your intelligence thank goodness you ruled out heavy winds. $\endgroup$– Air Canada 001Commented Oct 22, 2020 at 21:13
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4 Answers
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A mechanical blockage inside the airspeed gauge could prevent the needle from dropping all the way back to zero -- assuming the aircraft had exceeded 200 KIAS in the first place.
Whether this or the pitot-related failures in the @CamilleGoudeseune answer, the pitot-static unit, airspeed indicator, and altimeter (also connected to the static port) should be checked and repaired as needed by a qualified person.
answered Oct 21, 2020 at 17:37
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This would happen if you were flying at 200 knots, encountered enough freezing moisture to completely ice over the pitot tube and the static ports, didn't have pitot heat for some reason, and then (somehow safely) landed and parked, while still iced over.
Once iced over, IAS won't change.
answered Oct 21, 2020 at 17:00
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$\begingroup$ Any other source of clogging the pitot tube with pressure inside would do the same (give or take the IAS). Even putting a protective cover on it, if that produced an airtight seal, could cause the airspeed to read while the cover was in place. $\endgroup$ Commented Oct 21, 2020 at 17:02
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$\begingroup$ Yes, although any cover for sale that even mentions airtightness say that it's not airtight! Homemade, on the other hand... $\endgroup$ Commented Oct 21, 2020 at 17:07
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$\begingroup$ @CamilleGoudeseune If the static port were completely iced over, I'd expect to see the altimeter not functioning properly, but it is. So at that point, and I guess this is what the other commenter was referring to, just the obstructed pitot tube could still result in this error? $\endgroup$– synchhCommented Oct 21, 2020 at 17:10
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$\begingroup$ IAS comes from the difference between ram pressure and static pressure. A blocked pitot tube freezes the former, a blocked static port the latter. So with only the pitot blocked, a change in elevation also changes IAS. Maybe it might still read 200 kts when parked, but that's trickier. $\endgroup$ Commented Oct 21, 2020 at 17:17
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2$\begingroup$ Descending with a blocked pitot would decrease the difference, reducing displayed IAS. If the IAS had been reading falsely high at altitude, it might still read 200 kt parked. Bottom line here is that a qualified person should be brought in to inspect and repair the pitot/static unit and attached instruments. $\endgroup$ Commented Oct 21, 2020 at 17:34
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Blocked pitot tube left for a few days and a different weather system coming to cause a drop in ambient pressure could cause a differential to register in the ASI and display a speed when stationary.
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$\begingroup$ How much would ambient pressure have to change to result in 200 KIAS to be shown? $\endgroup$ Commented Oct 22, 2020 at 13:29
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$\begingroup$ @Bianfable I think that the idea here is that it was first blocked at some airspeed (say, 100 knots) and then the difference was exacerbated by a change in ambient pressure? So it would depend on at what point the pitot tube became blocked. $\endgroup$– synchhCommented Oct 22, 2020 at 15:06
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If it was something a little more reasonable like 20 or 50 knots, it could be from a headwind.
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2$\begingroup$ "If it was something [else]..." does not answer the question. $\endgroup$ Commented Oct 22, 2020 at 13:11