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Airspeed Indicator Disassembly

KayS

Well Known Member
Hi All, i have a 3 1/8 indicator that needs the coloured markings on the clock face. The shop in germany says it would be around 300 USD... ouch!

Is there an easy way to disassemble the ASI or is the risk to high to damage something?

Cheers
Kay
 
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I guess it depends on the brand of ASI.

On the first RV my standard ASI from Van?s read ~5kts slow at the low end - so landings were faster than I would want. Taking the can off to adjust the calibration set screws was straight forward. This also provided assess to the ASI face.

I used a simple manometer to calibrate the indication over the normal range. The photo is final check after reassembly. I have a photo somewhere of what the ASI looks like on the inside - but not with me today.

Carl
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It has always been my position that "what man hath putith together, man can taketh apart".

Since a brand new properly marked AS from Van's is $280, what have you got to lose? I guarantee it will at least be educational. I certainly found that when taking apart an old vacuum AI. (just out of curiosity).
I would make sure you understand how to check it and calibrate it before returning to service though.
 
I have done that as well

Done that recently and happy with the end result. It is rather sensitive but quite doable.
 
Carl, if you find your photos of the dis-assembled airspeed indicator, I would appreciate it. I'm interested in seeing how feasible it is to remove the face plate for re-marking the color bands.
 
Here you go.

Notes:
- This is a standard ASI from Van's so no guarantee yours will be the same.
- This is how it slides out the back of the can. No disassembly other than removing the screws in the base that holds it in the can.
- Notice the base O ring. The can shell is connected to your static line. The DP is measured between the pitot input to the bellow and the static pressure inside the can. If the can leaks, you will have a static leak. I added some Teflon pipe sealant to the can base on reassembly to make sure it sealed well. It did.
- One fairly common static leak source is the ASI glass face. There are tricks to fix that as well.
- The reason I took the ASI apart was that in the low speed range it read ~5kts low. Note the setscrews on the second photo. This is how you calibrate the ASI - each setscrew for a range of speeds. Look at the photo in my first post on the simple manometer to do this calibration. It is amazingly accurate.

I have a PDF file for the template that translates manometer height to airspeed if you can't find it. PM me your email address and I'll send.

Carl
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