ASI Questions...

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4390
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ASI Questions...

Post by 4390 »

I have a few questions about 120 ASI's. 1. Did the 120 have the white flap arc in it (shared with the 140)? Does the pitot tube need to be aligned in a specific manner to work it's best? My ASI appears to be way off, but sometimes more then other times. It could be original and no doubt could use an overhaul. I used to fly a 140 that had to have the static line loose for the ASI to work it's best.
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Re: ASI Questions...

Post by 6277 »

You might try disconnecting the tube from the rear of the ASI and then blow some air through the pitot tube and see if there is any blockage such as a bug, etc.
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Re: ASI Questions...

Post by 1567 »

A different issue but Airspeed indicator topic is something I've been dealing with lately. I used climb out and land indicating about 70 MPH. The problem I had was the needle would stick at about 80MPH when decelerating for landing. I would almost have to stall the airplane to get it to go below 80. I had the airspeed indicator overhauled by Aircraft Quality Instruments in Wichita KS. Now it seems the indicated airspeed is about 20 MPH lower across the board than it was before. I now lift off and the needle barely comes off the peg. I climb out and land indicating about 50 MPH. I used to cruse indicating about 105 MPH now it's between 90 and 95 MPH. I haven't done a thing to the pito static system except blow the line out to be sure their is no blockage. I sent the indicator back to Aircraft Quality Instruments explaining this issue. They said they tested the instrument and it tested good. I've been flying with it like this now for about 12 hours. It's taking some getting used to especially on landing. Have been flying the airplane by feel and when it feels about right the airspeed is indicating about 50 MPH.
Would be interested to know if anyone else has ever had a similar issue.

Matt
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Re: ASI Questions...

Post by 6643 »

First thing I'd look at is the pitot and static lines. Where does the static line terminate? Any leaks in either line? Water in either line can cause unusual readings, too.

It is easy to ground check The ASI using a column of water in a plastic tube. (Manometer) There's direct correlation to the height of the column of water and the IAS. Also, if there is a leak, the column will not be constant. PM me your email address and I'll send you instructions on how to do it.
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Re: ASI Questions...

Post by 4390 »

1567 wrote: Tue Jun 04, 2019 7:47 am A different issue but Airspeed indicator topic is something I've been dealing with lately. I used climb out and land indicating about 70 MPH. The problem I had was the needle would stick at about 80MPH when decelerating for landing. I would almost have to stall the airplane to get it to go below 80. I had the airspeed indicator overhauled by Aircraft Quality Instruments in Wichita KS. Now it seems the indicated airspeed is about 20 MPH lower across the board than it was before. I now lift off and the needle barely comes off the peg. I climb out and land indicating about 50 MPH. I used to cruse indicating about 105 MPH now it's between 90 and 95 MPH. I haven't done a thing to the pito static system except blow the line out to be sure their is no blockage. I sent the indicator back to Aircraft Quality Instruments explaining this issue. They said they tested the instrument and it tested good. I've been flying with it like this now for about 12 hours. It's taking some getting used to especially on landing. Have been flying the airplane by feel and when it feels about right the airspeed is indicating about 50 MPH.
Would be interested to know if anyone else has ever had a similar issue.

Matt
This is about the same thing I was talking about with that 140 I was flying that belonged to a friend. It is a totally restored plane. I had flown it for a few months and the ASI seemed to work right. One day I was working under the panel and noticed the static line wasn't connected to the port on the left side. Thinking I must have knocked it loose, I pushed it back on the port. Next flight the ASI was way off, read low as I recall. I flew by feel and didn't have a problem. I asked the owner about it and he said "Oh yeah, don't connect the static line".. I pulled it back off and the ASI appeared right again. This 140 had all new lines so I know the lines weren't the problem. Try pulling the static line off the port and see what happens. If it works leave it off. Test your ASI with a GPS.
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Re: ASI Questions...

Post by 8233 »

4390 wrote: Tue Jun 04, 2019 9:47 am
This is about the same thing I was talking about with that 140 I was flying that belonged to a friend. It is a totally restored plane. I had flown it for a few months and the ASI seemed to work right. One day I was working under the panel and noticed the static line wasn't connected to the port on the left side. Thinking I must have knocked it loose, I pushed it back on the port. Next flight the ASI was way off, read low as I recall. I flew by feel and didn't have a problem. I asked the owner about it and he said "Oh yeah, don't connect the static line".. I pulled it back off and the ASI appeared right again. This 140 had all new lines so I know the lines weren't the problem. Try pulling the static line off the port and see what happens. If it works leave it off. Test your ASI with a GPS.
I had the same on my '46 120. Had a new transponder installed along with a new encoder and the static line got connected up to the ASI. I noticed something off in cruise since it was reading about 10-15 low. I found one of the old Neal Wright articles that discusses it:

viewtopic.php?f=10&t=133

The article basically says even Cessna told dealers it was a problem for serials before 12202 and the fix was to disconnect all but the ASI but even then, sometimes the ASI was better off using pressure from inside the cabin. The static connection is only to the ASI for SN 12202 and later and in the 140A IPC I think according to the article. I've discussed the topic with a few different folks. I've been told it just works better disconnected per the original dealer letter and I have had others declare it's a static line issue and I have an issue with my airplane. I may but have been unable to locate it I did blow out my static lines and never noticed a difference so I elected to follow the article's advice I've also heard this can happen on early model year 170s. Sounds likely a poor placement choice for the static port yielding bad results. YMMV
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Re: ASI Questions...

Post by 5115 »

4390 wrote: Tue Jun 04, 2019 9:47 am [Test your ASI with a GPS.
No, you cannot test your ASI with a GPS. GPS gives you ground speed and ASI doesn't consider the speed and wind direction.
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Re: ASI Questions...

Post by 6643 »

Actually, you can, but it's not as simple as comparing the two. You have to fly three different headings and record your airspeed, ground speed and heading for each. Then, there's a calculation that nets your true airspeed and the wind speed and direction. It's covered in AC23-8c, Appendix 9.

There are numerous spreadsheets and on-line calculators available.

"Cheater's method" - find a heading where your ground track is exactly the same as your heading (directly into or away from the wind.) record your ground speed. Reverse course 180 degrees. Record your ground speed again. Your TAS is the average of the two.
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Re: ASI Questions...

Post by 4390 »

6643 wrote: Wed Jun 05, 2019 8:23 am Actually, you can, but it's not as simple as comparing the two. You have to fly three different headings and record your airspeed, ground speed and heading for each. Then, there's a calculation that nets your true airspeed and the wind speed and direction. It's covered in AC23-8c, Appendix 9.

There are numerous spreadsheets and on-line calculators available.

"Cheater's method" - find a heading where your ground track is exactly the same as your heading (directly into or away from the wind.) record your ground speed. Reverse course 180 degrees. Record your ground speed again. Your TAS is the average of the two.
Exactly.... It really is pretty EZ to use a GPS to check your ASI if you either fly on a calm day or know the wind speed/direction. I have a magnetic mount on my panel that I attach my cell phone to while running Avare. When I'm landing I know the headwind and direction, which is usually down the runway. If it's a 10 kt headwind down the runway I just subtract the 10 off the GPS and compare it to my ASI. Not perfect but pretty close.
Glass panels do just that in reverse and display "Wind direction / Wind speed".
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Re: ASI Questions...

Post by 5115 »

Yes I agree there is methods to do it, flying cardinal points and doing calculations. But you have to be very very tedious doing it and select a good day, not bumpy, not too windy, etc, etc. this is why I prefer a mechanical solution like the water column proposed by John.
Martin Tanguay
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