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Pitot/Static leak

WingedFrog

WingedFrog

Well Known Member
Working on the PAP to check the Pitot and Static lines for leaks, I find a rate of decrease of both speed and altitude about 10 times the minima given in the PAP. I did not put any sealant on the connections of the plastic tubes given the force needed to slide the tubes on the fixtures. I will start sealing all these connections and any recommendation for the choice of sealant is appreciated. Also, is it possible that the procedure described in the PAP is not correct for the Skyview. I read in a posting that per Dynon, one should turn the EFIS off while changing pressure which is not what the PAP is directing us to do. I also noticed that while increasing altitude during the static port test the speed is shooting up. As this is not mentioned in the PAP I am a little concerned but there may be a logical explanation based on the design of the ADAHR.
 
All pipe threads need to be sealed (I use teflon tape) and all quick disconnect type fittings need to be cut square and smooth, repeated removal will scar the outer surface of the line causing leaks. I use a small amount of DC4 to help seal the fitting and lubricate the 'O' ring.

I follow the Dynon white paper when doing P-S checks on Dynon, I've talked to them about this procedure because it certainly makes the testing a bit more of a pain with all the on/off and waiting for things to stabilize. But not doing it their way may lead to some real problems so it's not worth it.

Airspeed will rise to around 150K when going 1000 AGL for the static system leak check. When doing the full blown test to altitiude (IFR test) the pitot and static systems get tied together during testing to keep the pressures equalized, otherwise you can damage the pressure sensors. You will be ok as long as you don't exceed the maximum airspeed of the Skyview.
 
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I didn't use sealant, but my pitot system was tight. The static system wasn't as tight. I never worried about the static portion reasoning the error due to airflow is probably insignificant to a VFR only plane.
 
RFSchaller said:
I didn't use sealant, but my pitot system was tight. The static system wasn't as tight. I never worried about the static portion reasoning the error due to airflow is probably insignificant to a VFR only plane.
Click to expand...

There is actually no error due to air flow, because there is no flow in the pitot or static systems, only an induced pressure.
There can actually be quite a bit of error depending on where the leak is and how big.
They may be VFR airplanes, but not correcting issues such as this will effect the use of controlled airspace, reported altitude to other aircraft equipped with TIS or ASDB, etc..
You wont be actually flying at the altitude that ATC or other traffic is expecting you to be at. Potentially by many hundreds of feet, depending on the leak.
 
Scott I was referring to the error caused by the vents and the Venturi effect over the canopy. Together they result in a slight overpressure in the cockpit, and I have a small constant outflow around the canopy rail. In my Cherokee the alternate static source is via a valve under the instrument panel and introduces an error of 200 feet when the valve is full open. A small leak should give less error. I built one home built that used a static source in the cockpit.
 
False alarm, no leak!

After reviewing the installation, I found the the nylon screws on the ADAHR unit were not screwed tight. When I did the installation of the ADAHR I followed the instruction to screw them "finger tight". Now what does that mean? What fingers are we talking about and how tight? I was able to do one full turn before I could not tighten more with my fingers but given the awkward position to do that I probably could have tightened more in a better setting. When I redid the test on the pitot line, to my big disappointment the leak was still there... Then I noticed that when I stopped pushing the syringe plunger it was retracting. I needed a helper to keep the plunger in position while watching the Skyview display. That did the trick, no more leak, just a slow bleeding in line with the figures of the PAP.
I hope this may help some... unless I'm the only one that stupid :eek:
 
WingedFrog said:
After reviewing the installation, I found the the nylon screws on the ADAHR unit were not screwed tight. When I did the installation of the ADAHR I followed the instruction to screw them "finger tight". Now what does that mean? What fingers are we talking about and how tight? I was able to do one full turn before I could not tighten more with my fingers but given the awkward position to do that I probably could have tightened more in a better setting. When I redid the test on the pitot line, to my big disappointment the leak was still there... Then I noticed that when I stopped pushing the syringe plunger it was retracting. I needed a helper to keep the plunger in position while watching the Skyview display. That did the trick, no more leak, just a slow bleeding in line with the figures of the PAP.
I hope this may help some... unless I'm the only one that stupid :eek:
Click to expand...

The intent of finger tight is as tight as you can get it with your fingers (but without using tools because you would likely over tighten). If you were able to tighten them more with your fingers, then they weren't yet finger tight.
 
Scott,
Just to clarify. Are we supposed to use thread sealant on those nylon fittings as per Walt's post, or not? The plans only call for thread sealant on metal threads.
 
rgmwa said:
Scott,
Just to clarify. Are we supposed to use thread sealant on those nylon fittings as per Walt's post, or not? The plans only call for thread sealant on metal threads.
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Our experience with these fittings is that because they are a rather soft plastic, they conform to the female threads when installed and develop a good seal.
A small amount of paste sealant could be used for extra insurance, but after installing a lot of these over the years, I have not had any problems getting a good (low pressure air) seal without it.
 
seal the threads at the D-180

I chased leaks in the pitot-static system for quite a whyile and finally found that they were where the plastic plugs screw into the back of the D-180. Installation directions called for them to be "finger-tight" Obviously my 70-year-old fingers weren't as strong as those of the guy who wrote the directions. I removed the plugs, applied a light coat of a teflon paste sealer (Locktite 507????--72 year old memory now) and my readings were perfect. Later replaced those fittings with Stein snap fittings when I installed backup airspeed and altimeter, use paste again, still perfect.

Wayne 120241/143WM
 
Funny story about "finger tight". In my ill spent youth I was on a nuke submarine. We had strict torque limits for closing valves except for the sampling system that had small T-handles and were subject to "good mechanical judgement" to be closedd by hand. If the seat leaked that meant we usually got the guy nick named "Simba" to close them by hand.
 
Pitot test problems

I'm just doing the pitot test in the PAP and have run out of ideas. The line is good from the pitot tube back to the ADAHRS, where it leaks. I'm able to confirm that because I've got a backup analogue ASI in the line (nylon Tee fluid fitting) and the pressure reading is rock solid when I cap the line with my finger at the ADAHRS end. However, when I screw the nylon nut onto the ADAHRS, I can't get better than about 50-55 seconds for a 10kt drop. I've tried finger tight, a bit more than finger tight, a little bit of O-lube on the fitting and threads (same as I did on the ASI tee, which works fine), a lot of O-lube, Teflon tape on the threads, re-cut the end of the line to make sure it is square, replaced the fitting, put on an extra O-ring, etc, all with the same result. I'm beginning to wonder if the leak isn't in the female fitting on the ADAHRS. Not sure what else I can try at this point. Any ideas anyone?
 
You don't mention an airspeed, 10kts/min @ 150 kts is standard test parameter.
However (at least in the US), there is no requirement to test the pitot system except on pressurized aircraft, a minor leak in the pitot system will generally have little if any effect on airspeed indication.
 
Determine if the leak is in the fittings or internal to the ADAHRS unit. If the ADAHRS has an internal leak, call Dynon to get their advice about living with the leak or sending the ADAHRS back for repair. The problem with lowering standards is, where do you draw the line? And what if the leak gets worse?
Joe Gores
 
Walt,

My experience on an Aventura HP was that a leaking joint resulted in a 12 knot low reading, and it wasn't an obvious leak. Doesn't sound like much until you realize climb speed was 50 KIAS. Seeing 38 in a nose up attitude was a bit disconcerting! I confirmed the leak by installing a second airspeed indicator with it's own pitot. I fixed it by smearing RTV over the joint.

Rich
 
Walt, I was following Van's production acceptance procedure (PAP) instructions which are to pressurise the line until you see 130 kts on the ASI, then time the pressure drop in ten knot increments starting at 120 kts. I'm still about 15% below the specified minimum, and I'm not comfortable knowing that the reading is sub-standard.

Joe, any ideas on how to test for an internal leak in the ADAHRS? I would have thought that each unit would have been thoroughly tested for leaks before shipping, but I'll contact Dynon and see if anyone has had similar problems. I'll have to pull the unit out so I can get a better look at the fitting. It's no fun working back in the tailcone. Must have climbed in and out of there 20 times yesterday.
 
I have never tested an ADAHRS for leaks. But I have used a homemade manometer to test other devices (like a fuel tank) for leaks. An inexpensive manometer can be made using plastic tubing bent into a "U" shape. Search the internet for directions. Fill the tube with water, draw vacuum on one end of the tube. Connect the other end of the tube to the ADAHRS, then release the vacuum and vent to atmosphere. The water column should be higher on one side of the "U" than on the other side. Locate the ADAHRS high enough so that there is no chance of water getting into it. If there are no leaks, the water level should not change over time.
Joe Gores
 
rgmwa said:
Walt, I was following Van's production acceptance procedure (PAP) instructions which are to pressurise the line until you see 130 kts on the ASI, then time the pressure drop in ten knot increments starting at 120 kts. I'm still about 15% below the specified minimum, and I'm not comfortable knowing that the reading is sub-standard.
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I agree with you, it should pass the leak test.

The best solution if practical would be to pull the unit, install a new fitting and have an avionics shop that does P-S testing verify its leaking or not before you send it back to Dynon. If you brought it to me it would take 5 minutes to test it.
 
Walt and Joe. Thanks for the suggestions. I'll pull the unit out and see if I can either find the leak, or have a local shop check it for me. It was very handy having the backup ASI in the line between the pitot and the ADAHRS to help find where the problem apprears to be. It was a frustrating day but not all bad. When I had everything connected up and the EFIS turned on, at least both the analogue ASI and the Dynon were showing the same speed :) and pressure drop. :(
 
Leak fixed ...

Contacted Dynon and they said that an internal pitot leak in the ADAHRS unit was rare but possible. They suggested testing the unit by pressing my thumb against the brass pitot fitting, which would provide enough pressure to register on the ASI. I disconnected the pitot line and thumb tested the nylon male insert which was still screwed into the brass port and got the ASI up to about 100 kts - it takes very little air displacement to get a reading. Still had the leak, so unscrewed the nylon fitting (which was tight) and tested the brass port. Used a small piece of cling wrap to ensure a good seal, and the ASI held steady like it's supposed to. Re-inserted the nylon fitting with some O-Lube on the threads and all is well. Simple solution and easy way to test the ADAHRS, although this test may not work for the static and AOA ports as I think they have an internal moisture vent (Edit: oh yeah, and of course they are tested using suction not pressure, so the thumb test wouldn't work anyway....unless you had an unusual thumb, I guess). :)

Now, can someone explain why the EFIS fuse has blown 3 times so far and why the Dynon occasionally won't come on when the master is turned on. The joys of being a home-builder. :(
 
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rgmwa,
Good job fixing the pitot leak.
Others have had trouble with the EFIS fuse blowing. I suspect that if the Dynon internal battery is run down, then the SkyView could draw more than 5 amps on startup. I am not necessarily recommending that others replace the 5 amp fuse with a 7.5 amp, but that is what I would do if I had nuisance fuse blowing.
Joe Gores
 
rgmwa said:
Re-inserted the nylon fitting with some O-Lube on the threads and all is well. Simple solution and easy way to test the ADAHRS, although this test may not work for the static and AOA ports as I think they have an internal moisture vent (Edit: oh yeah, and of course they are tested using on suction not pressure, so the thumb test wouldn't work anyway....unless you had an unusual thumb, I guess). :)

. :(
Click to expand...

See post #2, pipe threads must be sealed or they will leak! Many folks think the nylon fitting will seal themselves, they won't.
 
Mich48041 said:
rgmwa,
Good job fixing the pitot leak.
Others have had trouble with the EFIS fuse blowing. I suspect that if the Dynon internal battery is run down, then the SkyView could draw more than 5 amps on startup. I am not necessarily recommending that others replace the 5 amp fuse with a 7.5 amp, but that is what I would do if I had nuisance fuse blowing.
Joe Gores
Click to expand...

Thanks Joe. I pulled the ADAHRS out thinking I'd probably have to send it back to Dynon, but luckily it turned out to be a simple problem. Walt is right. Seal those fittings. Of course, then I had to put the #$%^ thing back and re-do all the cable ties. No wonder my back hurts!

I wouldn't be surprised if the internal battery is a bit run down at the moment, so I hope that's all that's causing the fuse to blow. I hate it when something like that happens and you're left staring blankly at all the wiring and avionics because you've got no idea what went wrong. Still, they do say we build these things for their educational value don't they. Thank goodness for this forum. :D
 
EFIS fuse

I had the same situation and found that the internal battery was very low. After getting it recharged and up to speed I have not experienced additional fuse failures.
Slane
120043
 
Static leaks

I remember well when I tested my system and it too failed because of leaks. I had to replace the new airspeed gauge because of an internal leak and the VSI gauge was leaking around the glass, but that was repairable with some clear epoxy.
Also I had to tighten some of the plastic nuts securing the tubing. But now it works good and even passed the first annual inspection a year later.
 
Might not apply to you, but I have the D180/D100 dual EFIS setup. With dual EFIS units the 5 Amp fuse must be swapped out for a 7.5 Amp fuse.
 
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