172 Static leak at port

Nathan Miller

Nathan Miller

Pre-takeoff checklist
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Nathan
So, nearing the end of my avionics upgrades in my 172. (Self install under my IA's supervision. Posting to the group since she charges me $100 an hour :p)

Resized_20200411_190923.jpeg

Still have a few misc things to take care of, as you can see.

But, today's conundrum: I've tracked down a static leak to, of all places, my static port! This took probably a day and a half to go through and eliminate small leaks here and there.

(Some back story- During the course of my G5 HSI install, I installed new 1/4" nylon hoses and fittings. My IA recommended I use a manifold style installation rather than the traditional/cheaper convention of putting a Tee behind each instrument and jumping from instrument to instrument.)

Bottom line, I taped over my static port and connected my hand vacuum pump to the threaded elbow inside the plane, just on the other side of the port, and the thing doesn't even TRY to maintain a vacuum. So, I'm guessing there's a crack in the little static port somewhere I can't see. Anyway, I find this quite surprising. How common is this part to fail? Thoughts and advice is appreciated.
 
Nathan Miller said:
connected my hand vacuum pump to the threaded elbow inside the plane, just on the other side of the port, and the thing doesn't even TRY to maintain a vacuum.
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In the step prior to isolating the static port was the leak rate the same? If not, how are you attaching your test pump direct to the "threaded elbow?" And is the "threaded elbow" screwed directly into the static port?
 
Nathan Miller said:
So, nearing the end of my avionics upgrades in my 172. (Self install under my IA's supervision. Posting to the group since she charges me $100 an hour :p)

View attachment 84641

Still have a few misc things to take care of, as you can see.

But, today's conundrum: I've tracked down a static leak to, of all places, my static port! This took probably a day and a half to go through and eliminate small leaks here and there.

(Some back story- During the course of my G5 HSI install, I installed new 1/4" nylon hoses and fittings. My IA recommended I use a manifold style installation rather than the traditional/cheaper convention of putting a Tee behind each instrument and jumping from instrument to instrument.)

Bottom line, I taped over my static port and connected my hand vacuum pump to the threaded elbow inside the plane, just on the other side of the port, and the thing doesn't even TRY to maintain a vacuum. So, I'm guessing there's a crack in the little static port somewhere I can't see. Anyway, I find this quite surprising. How common is this part to fail? Thoughts and advice is appreciated.
Click to expand...

Is the port itself cracked or is it just crappy install? I ended up drilling mine completely off, and reinstalling it wet with PS890B1/2 with CS machine screws and nuts also wet with PS890B1/2. Also needed a little filler in the screw heads and some paint to fair it all up.
 
I wish I took a picture. My IA works Tuesday through Saturday, so I won't be able to get back to the plane until Tuesday.

She doesn't have a test set and we're only verifying leak rate with a hand operated vacuum pump and checking the G5 airspeed and altimeter. Pretty backwoods engineering if you ask me ;) But it gets the job done.

The leak rates were horrid in the beginning. I ended up going back in and redoing every connection to the instruments. I thought I put at least 3 wraps of teflon tape on the connection prior to attaching the fitting to the instrument. Apparently not. Or, I didn't tighten it up enough. I was told it should be hand tight. While that advice proved true for the T-fittings, I found that the straight fitting on the back of the instrument needs 3 and a half wraps of teflon tape and half a turn with a wrench after the point it's snugged up by hand. After that, the absolute pressures held rock steady.

In the beginning, we used the plunger looking fitting that came with the "handi-vac" on the outside of the static port. We couldn't get a satisfactory leak rate, even after taping the plunger completely on all sides and putting a dab of vasoline to aide in the sealing. So, I disconnected the first "upwind" nylon tube I could find (just downwind of the sump bottle) and connected my hand pump to that. While that gave us better readings, it was still excessive (which is when I started re-doing the straight fittings at the instruments). I believe the leak rate after redoing every connection came in at 3 knots in 60 seconds and also 20' in 60 seconds. I've got it in my notes back at the hangar. (According to AC 43.13-1B, section 12-57, the acceptable leak rate is no more than 10 knots or 100' in 60 seconds.) So, happy dance.

So then I turned my attention to the sump bottle itself. It has threaded connections on both sides - one smaller than the other. I attached my pump to one side and covered the other end with my finger and it held 10inHG for 60 seconds no problem.

All that's left is the blue 90° elbow and the static port. Very short run. Since I don't have a fitting for my pump that attaches to the elbow or the port, and knowing the bottle is good, I attach the sump bottle to the elbow, tape over the static port and start pumping down the short little segment of my static system. Complete failure. I mean, it's not even close to holding. So, all signs point to this needing replacement. I guess I'm posting to the group since I find it hard to believe that such a simple piece of aluminum can develop a crack and cause such a massive leak. But, I guess it happens. At first glance, the port looks good. I'll go back with a mirror and a flash light and really get a good look all around this bugger. I'll keep you posted.
 
Bell206 said:
In the step prior to isolating the static port was the leak rate the same? If not, how are you attaching your test pump direct to the "threaded elbow?" And is the "threaded elbow" screwed directly into the static port?
Click to expand...

Bell, I abandoned the static port early in my leak search, assuming the plunger fitting that I was using with my hand pump wasn't giving me a good seal. Turns out, maybe my fitting was good but the port was bad. Who knew? :confused:

Yes - the threaded elbow is screwed directly to the static port.

I had leftover nylon tubing from replacing my entire run of static line for my G5 install. And I had an extra straight fitting. So, I used that to connect to the sump bottle (the bottle itself was tested and held vacuum), which screws onto the threaded elbow which screws into the static port. I misspoke when I said I connected directly to the threaded elbow (different sized threads from the fittings I have on hand prevent a direct connection, unfortunately.)

There's a lot of moving parts and I appreciate you all sticking with me as I fumble through my descriptions. I'm learning as I go and this has me a bit jumbled.

I should also mention that before I removed the plunger fitting from the outside of the static port (we had it taped up within an inch of it's life in addition to putting a dab of vasoline on it to aide in sealing), I removed the threaded elbow and had enough hose length and arm length to cover the inside part of the port with my finger and operate the pump. It wasn't even trying to hold vacuum.
 
Nathan Miller said:
I misspoke when I said I connected directly to the threaded elbow
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Is it pipe thread from the "threaded fitting" to the static port? If so rig up a pipe thread fitting at the port to your pump. Regardless if the bottle tested good it still is a link in your final T/S chain. Also leak check again from the removed "threaded fitting" upstream to the panel and verify good. Or, rig up a line to the "threaded fitting" at the port and apply low pressure to the fitting and port and using a soap solution look for bubbles.
 
Pressurize it, a few psi, then use some soapy water. (The port and short piece of hose obviously, isolated from everything else).
 
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Good job on checking that by the book. Unfortunately, a lot of unpressurized static systems have been neglected over the years. I remember working in a GA shop as a young man, watched avionics guys sign off static systems by taping off the ports and slamming the doors. No big needle jumps = no big leaks. Horrible, I know!
 
First of all..be very careful applying pressure or vacuum so you do not damage your instruments. You can apply a gob of dumdum, AKA putty, over any fitting you suspect is leaky to help locate the problem. Sometimes the instruments are the problem. There are a couple good videos online from EAA on checking pitot static leaks. I helped a friend with a Stinson a few months ago...the airspeed instrument was defective....leaked like a bucket without a bottom.
 
Sorry for changing topic, but I haven’t thought about a Stinson in years! I owned a 1946 Stinson 108-1. It was my first airplane! Bought it in 1986 for $4,000 with a loan from the bank:)
Here’s a pic of me working on it back then. I was 19 years old.
 

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That static bottle has straight threads on it; tube nut threads. The threaded port in the static fitting in the sidewall has tapered threads (NPT). That's already a bad combination, since the sealing mostly happens at the first thread or two. I have found teflon tape less than ideal for this stuff, too. It often gets scraped off on the way in, and it also seals better when there's lots of internal pressure applied to it, forcing the stuff into the tiny interstices between the threads. A pitot-static system has almost no pressure, so no movement of the teflon into leaks. I found the liquid teflon sealant much better for pitot-static systems; just a tiny bit of it seals things up.

serveimage


Or

serveimage


Those static bottles also crack.
 
I'm sure it will be easy to buck those rivets... All you need are arms that are 12 feet long.
 
Nathan Miller said:
Found the leak!
Back side of the static port. Hard to see unless you're a contortionist...View attachment 84733
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That comes from overtightening tapered pipe fittings. It can happen when the elbow is too loose, so the mechanic turns it one more turn, and now it's too tight and splits the boss. Tapered pipe is a pain and should have been abandoned long ago in most aircraft applications. It relies on wedging action to seal it, or a bunch of various goops or tapes. Good stuff has straight threads with an unthreaded section on which an O-ring sits, and a jam nut locks the fitting once it's located satisfactoriy. The O-ring does all the sealing and seldom presents any trouble. Like shown here. The top left picture is tapered pipe, the rest are how it should be.

serveimage


Cleveland and others have been using this method for many years on their hydraulics, and some fuel systems use it too, but pipe is still employed in too many other places.
 
Dan Thomas said:
That comes from overtightening tapered pipe fittings. It can happen when the elbow is too loose, so the mechanic turns it one more turn, and now it's too tight and splits the boss. Tapered pipe is a pain and should have been abandoned long ago in most aircraft applications. It relies on wedging action to seal it, or a bunch of various goops or tapes. Good stuff has straight threads with an unthreaded section on which an O-ring sits, and a jam nut locks the fitting once it's located satisfactoriy. The O-ring does all the sealing and seldom presents any trouble. Like shown here. The top left picture is tapered pipe, the rest are how it should be.

serveimage


Cleveland and others have been using this method for many years on their hydraulics, and some fuel systems use it too, but pipe is still employed in too many other places.
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Or a crappy casting or corrosion.
 
PaulS said:
Or a crappy casting or corrosion.
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Corrosion is possible, but the fitting is machined from aluminum extrusion, not a casting. It has a thin wall that is easily overstressed by too much torque on a fitting that has small taper. When I was in the air brake remanufacturing business, where pipe threads are almost exclusively used, we had plenty of cores come in with cracked pipe port bosses, and many warranty claims for the same thing. Since we tested every single component after rebuild, we knew it wasn't cracked when it was sold. I finally started putting a card in with each component, specifying the industry-standard torque settings for pipe threads from 1/16" up to 1", and they were a LOT lower than almost any mechanic would be happy with. They were afraid that the air or cooling water was going to simply push that fitting out or something.
 
Dan Thomas said:
Corrosion is possible, but the fitting is machined from aluminum extrusion, not a casting. It has a thin wall that is easily overstressed by too much torque on a fitting that has small taper. When I was in the air brake remanufacturing business, where pipe threads are almost exclusively used, we had plenty of cores come in with cracked pipe port bosses, and many warranty claims for the same thing. Since we tested every single component after rebuild, we knew it wasn't cracked when it was sold. I finally started putting a card in with each component, specifying the industry-standard torque settings for pipe threads from 1/16" up to 1", and they were a LOT lower than almost any mechanic would be happy with. They were afraid that the air or cooling water was going to simply push that fitting out or something.
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Yeah, you can actually see the end of the port looks like it has flared out, it probably cracked when it was tightened. Not sure if I would use an extrusion for that part, I'm thinking if it's not cast, then it's probably just machined from a piece of bar stock. I really don't know that much about I've never seen a riveted port and assumed the rivets were integral to the port.
 
So, I'm curious......how much difference in static reading would this particular leak really show? I suppose, if airflow "inside" the sheet metal was a bit different, it might be similar to using an alternate static port that sample ambient from inside the cockpit, but I'd guess the actual difference would be quite small. (Not implying it shouldn't be fixed, I certainly would go ahead with the repair).
 
My homebuilt ride just uses the interior for the static - opening and closing air scoopish vents changes the altitude about 25 feet or so. But I'm not going as fast as a 172. The error in this particular application would be reduced by the ratio of the static opening to the effective size of the leak.

How much change do you see switching to the alternate static?
 
Jim Carpenter said:
So, I'm curious......how much difference in static reading would this particular leak really show? I suppose, if airflow "inside" the sheet metal was a bit different, it might be similar to using an alternate static port that sample ambient from inside the cockpit, but I'd guess the actual difference would be quite small. (Not implying it shouldn't be fixed, I certainly would go ahead with the repair).
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A big leak like that crack will definitely have some effect on altitude and especially airspeed. But the real problem is certification: the system must comply with the 100' per minute leak rate or the mechanic can't certify it.

Pressurized airplanes have to really watch such leaks. The differential between outside and inside is huge.
 
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