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Airspeed Indicator problems

Utah-Jay

MEMBER
Heber City, UT
A couple of weeks ago the ASI was showing some abnormal discrepancy to the normal GS reading.

The problem has gotten worse to the point it was reading 72-73mph while my GS was 100mph. I am well aware that the gauge at 8000’ will be different than the GPS ground speed, but it should be much closer and was until recently.

The pitot connection has been checked, as has the connection at the back of the gauge.

Plane (Rans S-20) has less than 200 hours on it and has only been flying for 2 years. This is a problem that has increased in the past 3 weeks to the point it is not reliable for nailing approach speeds. I don’t really want to buy a $300 gauge if this is not the problem, and at this point the actual problem has not been diagnosed. Mark Pringle from Rocky Mountain Kitplanes has looked at it and was not able to nail it down either.

Ideas?
 
You'll need to fly in one direction and note the indicated airspeed and the ground speed as indicated by the GPS. Turn 180º and do the same thing. Unless it is a perfectly calm day, you'll see a difference in noted air/ground speeds in each direction. The increase in ground speed in one direction should approximate the decrease in ground speed in the other. Compare to the airspeed readings to see if there is constant over or under speed showing on the ASI.

As for checking connections, did they get checked with pressure? That's the only accurate way to check for leaks. A rubber hose, over the pitot tube, squeezed with a clamp, will be enough to show an airspeed on the indicator. Waite at least 30 or 40 seconds and see if the airspeed reading decreases. If there is a decrease you have a leak. A blood pressure cuff squeeze bulb works well also. Of course an actual pump made for testing flight instruments is the best, if you can get ahold of one.

Web
 
You'll need to fly in one direction and note the indicated airspeed and the ground speed as indicated by the GPS. Turn 180º and do the same thing. Unless it is a perfectly calm day, you'll see a difference in noted air/ground speeds in each direction. The increase in ground speed in one direction should approximate the decrease in ground speed in the other. Compare to the airspeed readings to see if there is constant over or under speed showing on the ASI.

As for checking connections, did they get checked with pressure? That's the only accurate way to check for leaks. A rubber hose, over the pitot tube, squeezed with a clamp, will be enough to show an airspeed on the indicator. Waite at least 30 or 40 seconds and see if the airspeed reading decreases. If there is a decrease you have a leak. A blood pressure cuff squeeze bulb works well also. Of course an actual pump made for testing flight instruments is the best, if you can get ahold of one.

Web
We have done the 180* testing, and it reads slow in both directions.

Might try the testing method you describe
 
If it reads low in both directions, you most likely have a leak.

LOW PRESSURE AIR ONLY! If you blow into the pitot or use any kind of shop air, you be looking for a new ASI, lol. And even with a controlled unit like a instrument pump, you increase and decrease pressure gradually.

Web
 
I am thinking leak and new gauge.

I will do the water tube test to see if it will hold pressure. Seems all I can do at this point prior to taking a leap of faith and buying a new gauge
 
Troubleshoot first. If you want to check the ASI, have a shop ops check it.

Throwing parts at a problem will do nothing but produce a leak in your wallet.

Web
 
Total cost: $0.17

pharmacist gave me the syringe

$0.17 for the tube at ACE Hardware
 

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Use something like surgical tubing to connect the syringe to the pitot tube. Soft tube like that makes a good seal. Old school soapy water will show very small leaks and is easy to clean up.

Web
 
You can also have water collected in a low spot of tube. Disconnect gauge then blow out line to pitot. Not the gauge.


Sent from my iPhone using SuperCub.Org
 
I hafta laugh. One of my students called in a panic - her engine had lost a lot of power. Test flight showed the symptoms above. Most Cubs have three rubber tubes - one of them had split. I think we fixed it with model airplane fuel line - pma, of course.
 
Bugs like pitots especially small spiders. They build web blocks that collect and store insects....nothing new there.....but they also grow as the eat and can gradually plug things. If suspected try running some soft line like grass trimmer cord through the system to push them out. Leaks are common for the planes I've had so soapy water and light air pressure. As noted water can collect in line dips. If no luck bypass the pitot system and make up something from small copper tubing and hose stuck outboard of the prop wash to test the gauge.

Gary
 
Utah-Jay
Is your pitot system plumbed with plastic tubing or metal? If you are using the white plastic (nylon) stuff like Poly-flo or Nyla-flo, be aware that it WILL leak at various places and at various times. My observation is that over time, or maybe heating/cooling cycles, the B-nut connections will start to leak. You will be able to tighten the connection with some good results but it's usually not a permanent fix. If the connection was over tightened at installation, it probably will never seal correctly. The best way to avoid this is to use good installation practices such as cutting the tubing as cleanly and squarely as possible, and tightening connections as per manufacturer's instructions.

Cub instruments were plumbed from the factory with 3/16" aluminum tubing. This is still the most secure and permanent way of plumbing the pitot/static system. Route the tubing from the pitot mast, to the wing root. Then from the wing root, down the left windshield post, to the back of the instrument panel. Join the sections of tube with low pressure rubber instrument hose. Push it on a goo two inches to avoid leaks as the hose ages. I've seen the tubing bulged and the hose pieces clamped with lace cord or worm clamps in order to prevent leaks, but not sure how much it adds to the security of the system.

When I build instrument panels, for any type of aircraft, I use aluminum tube and AN fittings for all instrument inter connections with the pitot/static system. On a Cub, this means that the only 'soft' connections are the small hose pieces at the instrument panel, wing root, and pitot mast. As AN fittings have a low rate of leaks, this narrows the search for leaks down to the hose pieces.

Web
 
I've had sunlight passing through the skylight or windshield weather that opaque poly instrument tubing. It can then crack and leak. Had it happen in just a few months in my Taylorcraft and earlier PA-11. Trial fix was first to cover it with dark plastic spiral wire wrap but that didn't work - gaps in the wrap allowed light to penetrate. Finally where exposed I covered it with heat shrink tubing - fixed. For the synthetic nuts that leak I put a dab of RTV as a thread and tubing sealer. Real cold weather followed by periods of cockpit heat can get that threaded fitting to eventually leak. Web's aluminum would be better especially if a slip-on spring tubing bender is available to make the curves.

Gary
 
Last edited:
Plane is down for maintenance

I did not feel comfortable flying as it was. The entire pitot “plumbing” will be redone. IF possible I will use the aluminum tubing, and yes I will have access to the tube bending machine, but I am not going to go beyond the inspection holes for the repair. IF taking the fabric off the wing is required for the aluminum tubing that is not going to happen this time around.

Now I need to decide on a gauge for the replacement.
 
Plane is down for maintenance

I did not feel comfortable flying as it was. The entire pitot “plumbing” will be redone. IF possible I will use the aluminum tubing, and yes I will have access to the tube bending machine, but I am not going to go beyond the inspection holes for the repair. IF taking the fabric off the wing is required for the aluminum tubing that is not going to happen this time around.

Now I need to decide on a gauge for the replacement.

Slow down.

Trouble shoot the problem. Don’t just throw parts at it.

It a simple system.

Shouldn’t take a mechanic 5 minutes to locate the problem. Fixing might take longer.


Sent from my iPhone using SuperCub.Org mobile app
 
Slow down.

Trouble shoot the problem. Don’t just throw parts at it.

It a simple system.

Shouldn’t take a mechanic 5 minutes to locate the problem. Fixing might take longer.


Sent from my iPhone using

I get what you are saying, but the IAS gauge would not hold a “speed” with the test I did today, there is a leak. Maybe I will just put in new tubing and see if that will solve the problem.

Plane is down anyway as there was an incident this morning prior to my flight, gonna be a few weeks I suspect for the repairs.
 
Maybe I will just put in new tubing and see if that will solve the problem.

This is the line that bothers maintenance types. Find and verify the problem BEFORE you put parts on the aircraft. I'd bet my own money that there is a leak in your pitot tubing. But if you can find out where the leak is and what component is leaking, you can fix the problem in a logical way. Maybe it will be best to replace all the tubing, but if it's simply a loose B-nut, then tighten it and make a mental note to see about putting in new tubing during the annual (hopefully during the winter months).

And keep in mind that the ASI could also have an internal leak. So do your due diligence in troubleshooting. Take your time doing it if it's down for maintenance already.

Web
 
Pictures of the air speed probe? Did it get bent out of line with where it should be pointing?


Sent from my iPhone using SuperCub.Org
 
Pictures of the air speed probe? Did it get bent out of line with where it should be pointing?


Sent from my iPhone using SuperCub.Org

Looks straight to me. I am gonna check the lines as you suggest first being I have time with the plane doe for a bit anyway
 
Utah-Jay, did you try performing the pressure leak-down check (the one you described over on another forum) on just the Airspeed Indicator, as well? That certainly would tell you if the instrument itself is bad.

If the ASI is good, but the pressure is still leaking, it'd definitely somewhere in the line... But just as a point of reference, it is just as bad to "over-tighten" those compression fittings as it is to have them loose. It doesn't take much torque for them to seal properly, IF the ends are cut square and the tubing isn't crushed out-of-round during the cut.
 
Utah-Jay, did you try performing the pressure leak-down check (the one you described over on another forum) on just the Airspeed Indicator, as well? That certainly would tell you if the instrument itself is bad.

If the ASI is good, but the pressure is still leaking, it'd definitely somewhere in the line... But just as a point of reference, it is just as bad to "over-tighten" those compression fittings as it is to have them loose. It doesn't take much torque for them to seal properly, IF the ends are cut square and the tubing isn't crushed out-of-round during the cut.
Jim

I did the test as per the video, but did not try to isolate things as that was the morning of an incident and things were a bit tense. What I did see was that the syringe did not move the ASI at all, so current thoughts are it is a leak in the line and not the ASI gauge. Mark is gonna confirm and I will fix it once the diagnosis is determined
 
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