Airpeed appears to be way off

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Airpeed appears to be way off

Post by 8466 » Thu Mar 26, 2020 11:14 am

I've been watching several videos and asking several 140 pilots and they say approach speed for is 60-70 mph. I have tried that but it appears to be too fast so I have tried 50-50 mph and it seems to land (settle) better. Also the airspeed and altimeter are both bouncy.The airspeed indicator shows about 90 MPH at cruise but at calm wind the GPS shows consistently 90 knots. Has these aircraft had a history of pitot static problems or airpeed indicator/altimeter failures?

Thayne Inman

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Re: Airpeed appears to be way off

Post by 2356 » Thu Mar 26, 2020 3:36 pm

Read Neal Wright’s article pertaining to this issue.

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Re: Airpeed appears to be way off

Post by 1125 » Thu Mar 26, 2020 9:16 pm

Where do I find Neal's article? I'm curious about my 120's airspeed indicator also. TO :?:

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Re: Airpeed appears to be way off

Post by 2356 » Thu Mar 26, 2020 10:22 pm

On the 1st page of the website at the bottom. Forum archives.

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Re: Airpeed appears to be way off

Post by 8187 » Thu Mar 26, 2020 11:50 pm

I trim for 70 full flaps 1500rpm either 3pt or wheel. I would guess it sets below 45 on 3pt. That being said my ASI is errratic and it’s be rebuilt. I figure it’s a ballpark deal.

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Re: Airpeed appears to be way off

Post by 6863 » Fri Mar 27, 2020 7:15 am

Thane, approach speed and "over the fence speed' are two different things, "over the fence speed" being slower than speeds throughout the previous approach segments. So yes, slowing down to flare and touch down is required and will be slower than 60-70.
John Kliewer

"Make things as simple as possible but no simpler." Albert Einstiein

"Wheels move the body. Wings move the soul."

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Re: Airpeed appears to be way off

Post by 6643 » Fri Mar 27, 2020 8:04 am

1125 wrote:
Thu Mar 26, 2020 9:16 pm
Where do I find Neal's article? I'm curious about my 120's airspeed indicator also. TO :?:
2356 wrote:
Thu Mar 26, 2020 10:22 pm
On the 1st page of the website at the bottom. Forum archives.
It's the one called "Pitot static problems and the cure Cessna never told owners".

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Re: Airpeed appears to be way off

Post by 8466 » Fri Mar 27, 2020 12:26 pm

Thank you for the information

Thayne inman

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Re: Airpeed appears to be way off

Post by 8233 » Mon Mar 30, 2020 7:20 am

6643 wrote:
Fri Mar 27, 2020 8:04 am
1125 wrote:
Thu Mar 26, 2020 9:16 pm
Where do I find Neal's article? I'm curious about my 120's airspeed indicator also. TO :?:
2356 wrote:
Thu Mar 26, 2020 10:22 pm
On the 1st page of the website at the bottom. Forum archives.
It's the one called "Pitot static problems and the cure Cessna never told owners".
I ran into this on my 120. Went through the static lines and made sure there were no leaks and everything was clear but my IAS was always too low. Followed the guidance of the letter from Neal's article and my IAS started reading correctly.
David Freeland - CFII
1972 Bellanca Super Viking and 1946 Cessna 120

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Re: Airpeed appears to be way off

Post by 4004 » Mon Mar 30, 2020 4:17 pm

Thayne, if the Neal Wright article doesn't solve your problem, then you may consider checking/calibrating your whole pitot system using a "water manometer". I won't go into the details/science here but refer you to an article in the EAA Sport Aviation magazine years ago . Search on "I FLY EZ" - should come up with a picture of an experimental Long EZ at the top - move down the left side of the page to "Articles" and click on "Manifold". The article is self-evident. The manifold can be constructed for "few" dollars. I suggest that rather than pressuring the system by " blowing" into the tubing that one purchase a Walmart inexpensive manual blood pressure cuff and use the rubber bulb with the thumb control and use it to control the applied air pressure to the system. One can in situ test the whole system for leaks and check your air speed calibration by attaching it to the pitot tube at the wing leading edge. If one is careful, the gasoline filter is not required due to the length of the overall system tubing run and not attached directly to an instrument per the article. I've had one for years - first time I used it, I found a leak in a system causing an error in the air speed indicator..

Your A&P may be interested in having one. Obviously it's not "official/certified" but a great tool for troubleshooting, etc. and experimental aircraft use. Readily adapted to any aircraft.



PS Thanks John C for the appropriate editing - "that's what 'moderators' are for"! :D

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