Airspeed indicator suggestions

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moparrob66

Pre-takeoff checklist
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Moparrob
My son took our 172B up to practice slow flight before his check ride and on climbout, he noticed the airspeed bouncing between 0 and 40 knots. He turned around and landed without incident and the A&P on the field ruled out a blockage or leak in the pitot/static system, recommending we replace the indicator. This is proving to be neither easy nor economical as certified new is around $2k and used but untested is about $200. I got one on the way from ebay and I may send the old one in for overhaul, but hopefully this one lasts long enough for him to get his check ride passed.

Any better suggestions, and can we do the r&r or does an A&P have to? Thanks guys!
 
You can do the R, but the A&P needs to do the &R. I would think you could order a refurb for quick delivery and send your core in.
 
I called several shops including AQI but no rebuilds in stock. Any shops youd recommend?
 
Many years ago I had an ASI doing that. Turned out to be water in the pitot line ...
 
How did the AP on your field determine there was not a leak? I would take the plane to a certified repair station that does pitot static testing. They have the equipment to determine the problem.
 
Definitely worth purging the lines. Disconnect instrument first.

The AMM for my old Grumman has a pitot leak procedure. From memory, slip a piece of large ID surgical tube over the pitot and start rolling at the other end until airspeed registers. Put a clamp on the roll and see how fast it bleeds down. It should be tight.
 
You can always send the indicator out to be repaired/rebuilt. I did mine years ago and they even cleaned up the paint on the dial arcs in the process.
Alas, I can't recommend the shop I used because they got bought out subsequently and went into the toilet.
 
kontiki said:
Definitely worth purging the lines. Disconnect instrument first.

The AMM for my old Grumman has a pitot leak procedure. From memory, slip a piece of large ID surgical tube over the pitot and start rolling at the other end until airspeed registers. Put a clamp on the roll and see how fast it bleeds down. It should be tight.
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My local repair station recommended this exact DIY procedure to me, as well as using silly putty to block the static ports and a hand brake vacuum tool from Harbor Freight to check for static leaks. Slowly applying pressure / vacuum is essential.

moparrob66 said:
I called several shops including AQI but no rebuilds in stock. Any shops youd recommend?
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I have had my tach and MP gauge rebuilt by Century Instruments this year and was very pleased with their service and turnaround time.
 
moparrob66 said:
My son took our 172B up to practice slow flight before his check ride and on climbout, he noticed the airspeed bouncing between 0 and 40 knots. He turned around and landed without incident
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What would the “incident” have been?

Don’t instructors cover the ASI and have the student fly the pattern anymore?
 
I wouldn't confuse maintenance requirements for the static system with those of the pitot system.
 
kontiki said:
I wouldn't confuse maintenance requirements for the static system with those of the pitot system.
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If either of them leak or have water in them the indications will be faulty.

This is in a 172B. A very old airplane. IIRC all the pitot and static plumbing was aluminum tubing that is prone to corrosion and loose or leaky tube fittings. An airspeed that fluctuates between 40 and zero screams LEAKAGE. Those airplanes also had a length of 1/4" aluminum tube as a pitot tube, easily bent and straightened and misaligned and cracked. Some airplanes have that little aftermarket aluminum cover that flips up when the airplane is in the takeoff roll so as to expose the end of the tube, and those have been known to act up.

One could spend a bunch on instrument repair and replacement only to find that the ASI still acts up.
 
Wow good info thanks! I'll do more digging when i get out to the plane and try the surgical tubing on the pitot.
 
Apparently the mechanic disconnected the pitot and blew it out but didnt find an obstruction. My kid was watching and said the asi pegged as soon as it was disconnected. Unfortunately I was at work.

I was glad to hear that although he was concerned about the faulty gage, he could judge an appropriate approach speed and land safely. It seems possible that a new aviator could get fixated on a bad asi or have become too reliant on the it to land properly, coming in too fast, floating, bouncing etc.
 
Dan Thomas said:
Some airplanes have that little aftermarket aluminum cover that flips up when the airplane is in the takeoff roll so as to expose the end of the tube, and those have been known to act up.
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Mine had one of those. My ASI was acting wonky as above. Disconnected pitot from ASI, mx hit it with shop air, and blew out a ****clot of mud.

If you have a 1/4" pitot tube, get a scrap of 1/4" ID fuel hose and put a blob of epoxy in one end. Attach a "remove before flight" streamer if you're feeling fancy.
 
moparrob66 said:
My kid was watching and said the asi pegged as soon as it was disconnected.
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Pegged? At zero or max? Something's not right here at all. Disconnecting it should not move anything unless there was a bit of pressure trapped in the pitot system.
 
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