Airspeed indicator inaccurate

MOCKK2000

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Jun 22, 2019
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#1
I have a 1972 8KCAB with 150 hp c.s prop. I have owned the plane about two months and have always had a problem with the airspeed indicator showing approximately 10 mph faster than actual ground speed in no wind conditions. GPS confirms this and stall horn confirms this. I blew the pitot line out to check for water(none) and the airspeed indicator(none). The pitot line looks good with no signs of holes. Has anyone experienced this? I have come to the conclusion it must be a bad airspeed indicator, but I cant find a replacement. Any advice would be appreciated.
 

Bob Turner

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Apr 4, 2018
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#3
In 1962 I changed my cream colored indicator for a black one. On takeoff (I was a 20 hour solo pilot) the airspeed indicator looked a little off - like 10 mph!

So, I took it home, removed the glass, pulled the needle and rotated it ten mph. It has been accurate ever since - 57 years and 5000 hours later.

By the way, it is illegal to mess with instruments. I only admit to this because the statue of limitations has expired.
 

MOCKK2000

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Jun 22, 2019
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#4
Are you interested in a new instrument or a good used one? Where have you looked already?
I would be interested in a new one primarily but if I couldn't get one I would consider a good used one. I have tried American Champion (they don't have one for the 150HP), Aircraft spruce(not available), Univair (not available).
 

Bartman

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#6
Not PMA/TSO so only good in experimental Decathlons! :cry:

Here's an example of an overhaul shop (not an endorsement, just an example)
http://rudyaircraftinstruments.com/Overhaul_Prices.html

Your local A&P will very likely have a good referral for an instrument overhaul/repair shop.

You could probably have it overhauled or just calibrated back to whatever the original OEM specs were. If a shop says it's already accurate then you've got to find the source of the problem wherever else it may be. If you just buy new you may or may not fix the problem.
 
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Bob Turner

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#7
Go in there and check on exactly how many of your instruments are TSO. You will be shocked - possibly only the altimeter and compass will say TSO. I think ACA is still using auto oil and fuel pressure and temp gauges - not sure on that, but my 1977 Dec had them. I have not replaced them

So the question is, once you spot an instrument without a TSO tag or proper logbook entry, what will you do?
 

MOCKK2000

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Jun 22, 2019
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#10
I believe I have a leak in the pitot line. Doing a syringe test to pressurize the pitot line reveals that it wont hold pressure at the airspeed indicator needle, if this is a correct method of testing.
 

Bob Turner

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#11
Very common, but usually results in a decreased indication. Those rubber hoses go bad - we use fish tank plastic hose. Not PMA, so don't tell any body.

By the way, I have used Rudy for instruments I was not capable of fixing, and was pleased.

And consider - airspeed indicators in light aircraft are sort of reference only. Avoid steep dives, and let the aircraft tell you what is going on. Practice slow flight and stalls, and note the power off attitude on approach. Pretty soon - like us - you will find that the airspeed indicator is quite secondary. Handy, but not terribly illuminating.
 
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aftCG

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Apr 3, 2018
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Tacoma, WA
#12
I would be interested in a new one primarily but if I couldn't get one I would consider a good used one. I have tried American Champion (they don't have one for the 150HP), Aircraft spruce(not available), Univair (not available).
How does ACA sell their 150hp planes without indicators?
 

Bob Turner

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Apr 4, 2018
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#14
Nope. ACA does not sell 150 hp airplanes. That's why! Ask for a 160 hp indicator. And be prepared to pay over a grand.
 

Bartman

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#16
You can really use any of the legal a/s indicators but would have to add your own green, yellow, and red arcs. But if what you really need is new tubing then that's another story.