Pitot line leak

Hey guys,
Since I didn't build my airplane, I was just wondering how the easiest way would be to pull a new piece of tubing to the pitot tube. Seems I have a small leak somewhere from the pitot tube to the wing root. I would like to pull new tubing. The existing setup, looking from the inspection panel next to the pitot tube, looks like a piece of 1/4" metal tubing curving into the first wing bay. I can't feel the end of the metal tubing to determine how (or where) it turns into plastic tubing. I just know when it gets into the wing root, it's plastic. Whew......that's a long winded question. Any help appreciated.

Well, good luck on this. If the builder followed the plans, the pitot aluminum tube runs along the front side of the wing spar and is clamped in several places. To gain access to this tube requires removing the fuel tank.

It could be that the builder ran the pitot tubing on the aft side of the spar. You should be able to determin on which side of the spar this line runs by looking in the under wing inspection plate located near the pitot line, or by pulling the aluminum wing gap fairing. This will give you access to the tubing pathway and with an inspection mirror you can see some distance behind the fuel tank.

noelf said:

Well, good luck on this. If the builder followed the plans, the pitot aluminum tube runs along the front side of the wing spar and is clamped in several places. To gain access to this tube requires removing the fuel tank.

It could be that the builder ran the pitot tubing on the aft side of the spar. You should be able to determin on which side of the spar this line runs by looking in the under wing inspection plate located near the pitot line, or by pulling the aluminum wing gap fairing. This will give you access to the tubing pathway and with an inspection mirror you can see some distance behind the fuel tank.

Click to expand...

Many thanks Noel. I will look at it this evening.
JJ

Not sure how you measured the leak, but if it's very small it would be insignificant. You can look up several posts on this or talk with someone who certifies pitot/static systems, so don't take my word for it.

This issue for a leak in the Pitot system is a friction loss problem and with a very small leak/flow, the pressure at the inlet and at the ASI would be negligible. Following a debate on this, I set up a small test, positioning the end of a 20' piece of 1/4" tubing in front of a powerful leaf blower and the other end connected to an old ASI I had lying around. With the leaf blower on I had an indication of about 125 MPH. After I punched about ten pin holes in the tubing near the ASI, the reading was the same. So if you're measuring with a manometer and your seeing a very slow leak down, you're chasing something that isn't going to make a difference.

Now if it's a leak in the static system, well that's a whole other ball game.

Like others have said, this advice is worth what you paid for it though.

One last thought. If the pitot probe is the one Van's plans show, it is a bent aluminum tube attached under the wing with a flair AN fitting. Orndorf used to sell the same thing fabricated out of stainless steel tubing.

If the probe looks like one ( or Dynon) from a certified aircraft, there usually is a small drain hole in the probe body, and allows impact water to seep out of the probe before it and the ram air pressure get to the pitot pressure line. Blocking this drain hole is necessary if you are doing an absolute leak test.

The definitive test is to disconnect the line at the output of the probe and also at the air speed indicator ( or any where else the line goes). Cap off one end of this line and pressure test at the other end.

The ASI could also have an internal leak and is easy to test with a homemade manometer. Plans are available from one of the Horton's on this website.

leak

Noel,

Luckily, (most days) I am an industrial instrument tech. That gives me access to all kinds of test equipment. The equipment I used is what we call a "squeeze bubble". It looks like a doctors' blood pressure cuff, minus the cuff. It is graduated in inches of water. Therefore with a conversion chart from in. H2O, I can determine what that correlates to knots. And when I pump up the squeeze bubble with the end blocked at the wing root, I get a fairly rapid bleed off. Also, I replaced the ASI with a known good one. I did find a nick in the plastic tubing under the panel and that alone made a huge difference. It just doesn't hold pressure for very long. Also, it is the original Van's bent aluminum tubing with the flared fitting. I can't see where it transitions to plastic, but will take the handy dandy work mirror and see what I can determine.

Non pressurized aircraft pitot leak rate (AC 43-13):

c. Apply pressure to cause the airspeed
indicator to indicate 150 knots (differential
pressure 1.1 inches of mercury or 14.9 inches
of water), hold at this point and clamp off the
source of pressure. After 1 minute, the leakage
should not exceed 10 knots (decrease in differential
pressure of approximately 0.15 inches
of mercury or 2.04 inches of water).

Thanks Walt

Thank you Walt..........I'm certain it leaks down faster than that. I'll have to dig deeper to find the leak.

If you need to replace, I would consider just fishing in a new 1/4" nylon tube and leave the old one there.

Larry

Abbygirl1 said:

Noel,

..... I can't see where it transitions to plastic, but will take the handy dandy work mirror and see what I can determine.

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The original -6 plans specified metal tubing from the pitot to at least the wing root, usually with no joints along that length.

Abbygirl1 said:

Noel,

Luckily, (most days) I am an industrial instrument tech. That gives me access to all kinds of test equipment. The equipment I used is what we call a "squeeze bubble". It looks like a doctors' blood pressure cuff, minus the cuff. It is graduated in inches of water. Therefore with a conversion chart from in. H2O, I can determine what that correlates to knots. And when I pump up the squeeze bubble with the end blocked at the wing root, I get a fairly rapid bleed off. Also, I replaced the ASI with a known good one. I did find a nick in the plastic tubing under the panel and that alone made a huge difference. It just doesn't hold pressure for very long. Also, it is the original Van's bent aluminum tubing with the flared fitting. I can't see where it transitions to plastic, but will take the handy dandy work mirror and see what I can determine.

Click to expand...

Jim,
Diane and I are heading over to see the grandkids today (Sat). I have a Harbor Freight borescope you are welcome to borrow. I'll try to remember to leave the door pins pulled so you can slide the door to get in. Scope will be in middle of my parking spot.

Consider fuel tank removal

You may find it necessary to remove the fuel tank in order to gain sufficient access to run a new pitoh line and be able to clamp it appropriately to the front of the spar web. Doing so would enable you to replace the line fully with 1/4" aluminum tubing per plans. Not fun but probably worth the effort...unless of course your aircraft was painted with the fuel tanks on.

Thanks

Thanks Gil. and Bill I will be over there the middle of the week. Ill give you a shout and get the borescope from ya. Where you been hiding lately?

Jimmy

Abbygirl1 said:

Thanks Gil. and Bill I will be over there the middle of the week. Ill give you a shout and get the borescope from ya. Where you been hiding lately?

Jimmy

Click to expand...

Hey Jimmy,
I'm only around on the weekends nowadays because I work out of town. I get the privilege of commuting on Monday and Friday with the RV. It's been a good learning experience in regards to dealing with weather.

I put the borescope back in the bottom of my red toolbox. If you can hook up with Dennis Gilton or Jason McKay, they could let you into my hangar during the week.

I need to do an oil change this weekend so I'll probably be around on Sunday.

Many builders were very creative connecting the alum tubing to the nylon tubing going to the gauges. Check in the wing root for the connection.
You can test the alum line from there to the pitot. I don't expect you to find a leak behind the fuel tank.

Bill & Gasman

Bill and Gasman,

Thanks for the help. I agree, probably no leak behind the tank, just need to see what's there. Bill Ill be over this weekend and Ill get up with you then.

leak found

Well guys, the pitot leak was in the wing root. Removed the root fairing and found a piece of fuel line used as a "coupling" for the transition from aluminum tubing to plastic tubing. Just stuck together, no clamps , nothing. Fixed and all is well. Don't know the logic behind THAT innovation.