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.
 
Doc Holliday said:
What would the “incident” have been?

Don’t instructors cover the ASI and have the student fly the pattern anymore?
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I recall an aircraft with a flight crew of three that flew into the everglades because of a burned out landing gear indicator light bulb.

So with what sounds like a student pilot on a pre check ride flight with a airspeed indicator bouncing around, including the phrase "without incident" doesn't sound particularly egregious. At least for most people.
 
donjohnston said:
I recall an aircraft with a flight crew of three that flew into the everglades because of a burned out landing gear indicator light bulb.
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You forgot to throw in the Air France Airbus over the Atlantic. :rolleyes:

donjohnston said:
So with what sounds like a student pilot on a pre check ride flight with a airspeed indicator bouncing around, including the phrase "without incident" doesn't sound particularly egregious. At least for most people.
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Somewhere in the training before a check ride, a student should have been taught how to fly without the ASI.
 
Doc Holliday said:
You forgot to throw in the Air France Airbus over the Atlantic. :rolleyes:
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I didn't forget.

Doc Holliday said:
Somewhere in the training before a check ride, a student should have been taught how to fly without the ASI.
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Pretty sure that "landed without incident" would make it apparent to most that the pilot indeed had been taught how to fly without the ASI.
 
donjohnston said:
Pretty sure that "landed without incident" would make it apparent to most that the pilot indeed had been taught how to fly without the ASI.
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Driving home my low tire pressure warning came on. I managed to make it home without incident. ;)
 
Doc Holliday said:
What would the “incident” have been?

Don’t instructors cover the ASI and have the student fly the pattern anymore?
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Some people have incidents with a working ASI.
 
kontiki said:
I wouldn't confuse maintenance requirements for the static system with those of the pitot system.
Click to expand...
Dan Thomas said:
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.
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Maybe I should have said "wouldn't confuse the return to service requirements for static and pitot systems?"
 
Lindberg said:
What do you even use the ASI for on a normal VFR flight other than occasionally cross-checking your pitch attitude?
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Well, with a constant-speed prop on a carbureted engine, a falling airspeed indicates carb ice. The RPM won't change for some time. The governor will keep reducing the pitch to keep the RPM the same, but that lower pitch reduces airspeed. If the pilot doesn't check the MP, he'll just add more power. And the icing continues.
 
Dan Thomas said:
Well, with a constant-speed prop on a carbureted engine, a falling airspeed indicates carb ice. The RPM won't change for some time. The governor will keep reducing the pitch to keep the RPM the same, but that lower pitch reduces airspeed. If the pilot doesn't check the MP, he'll just add more power. And the icing continues.
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That's certainly good to know and something I don't have experience with, but wouldn't there be other indications, like sound? And we're taking about a 172 in this case.
 
Lindberg said:
What do you even use the ASI for on a normal VFR flight other than occasionally cross-checking your pitch attitude?
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Off the top of my head, pilotage, Vx climb, Vy climb, approach speed, NOTAM compliance, complying with ATC instructions,.....
 
moparrob66 said:
Well, the pilot is my 17 year old son. Student pilot. Every flight "without incident" is one I'm thankful for, especially with a potentially distracting mechanical failure. Its a good experience for him and it wont be the last with our "antique" airplane.
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Agreed.. you are correct sir
 
Half Fast said:
Off the top of my head, pilotage, Vx climb, Vy climb, approach speed, NOTAM compliance, complying with ATC instructions,.....
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I'd hope you can get darn close to Vx, Vy, and approach speed without looking at the ASI.

What kind of NOTAMs and ATC instructions relate to speed of a VFR flight?
 
Lindberg said:
I'd hope you can get darn close to Vx, Vy, and approach speed without looking at the ASI.

What kind of NOTAMs and ATC instructions relate to speed of a VFR flight?
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I can get close, but I still use the ASI, especially on approach as the Musketeer is a bit intolerant of being slightly too slow or too fast. And if you attend a large fly-in (Osh, SNF, some others) you will have assigned pattern speeds.

And I use the ASI in cruise for pilotage and ded reckoning.

But all this is moot as it’s a required instrument anyway.
 
Lindberg said:
That's certainly good to know and something I don't have experience with, but wouldn't there be other indications, like sound? And we're taking about a 172 in this case.
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The change is gradual, and the only sound change is the engine working less. Most pilots won't spot that.

A 172 has a fixed-pitch prop and the RPM will fall as the carb ices up, unless an oblivious pilot just keeps adding more power.

The 172R and S don't have carbs to ice up.
 
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