Help with overreading ASI

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Timothy
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Help with overreading ASI

Post by Timothy » Fri Oct 14, 2022 6:36 pm

The ASI in my aircraft clearly overreads, by between 12 and 15 kts (12 at the slower end, 15 at the higher.)

I know this for a number of reasons:

1. I have compared ASI and GPS groundspeed, on a calm day and going in different directions, and ASI beats GPS by 15kts.

2. Stall speed is 12kts above book.

3. Landing speed is 12kts higher than it should be.

Everything points at the ASI overreading.

So I put a calibrator on the pitot. It is a water tube manometer, so there is little opportunity for it, itself, to be out of calibration.

The remarkable thing is that it shows the ASI to be perfect.

I can't really see that this is a static source issue, as the ambient pressure on touchdown will be the same as when I use the manometer, so even if the static to the ASI is blocked, the result should be the same. Also the altimeter and VSI are working perfectly.

I also cannot see that this is a bent pitot tube, because if it's not pointing straight into wind, the ASI would underread, not overread.

I have ordered a replacement ASI, but I strongly suspect that that's not going to help, because this one seems to be perfect "on the bench".

Can anyone see where I am being stupid?
Timothy Nathan
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Ian Melville
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Re: Help with overreading ASI

Post by Ian Melville » Fri Oct 14, 2022 11:08 pm

What happens if you open a DV panel?
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Timothy
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Re: Help with overreading ASI

Post by Timothy » Sat Oct 15, 2022 9:55 am

I can try sliding back the canopy a bit.
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tnowak
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Re: Help with overreading ASI

Post by tnowak » Sat Oct 15, 2022 2:41 pm

In my Piper Vagabond I show an IAS that if 5 MPH faster when all windows are closed. Open the pilot's side sliding window one inch and the the IAS reduces...
Almost certain that you have a static issue.
I dont have a static system, so just live with it...
Tony
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Timothy
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Re: Help with overreading ASI

Post by Timothy » Mon Oct 17, 2022 3:20 pm

Static pipe broken through. Fixed. Now perfect. Thank you.
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Re: Help with overreading ASI

Post by Ian Melville » Mon Oct 17, 2022 7:00 pm

Good stuff.
Is there a static port on the fuslage, or combined with Pitot head?

Cheers
Ian
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Timothy
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Re: Help with overreading ASI

Post by Timothy » Mon Oct 17, 2022 8:05 pm

On Pitot. But Altimeter and VSI worked fine throughout, only the ASI was affected.
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Re: Help with overreading ASI

Post by Ian Melville » Mon Oct 17, 2022 9:10 pm

Thanks, little bit of info to store should I see that in my travels.

Cheers
Ian
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