Avionics Recertification - Pitot Leak

[David Gates]

Has anybody had experience with a pitot system leak getting in the way of IFR recertification?

A new shop to me is telling me that I have to find and correct a pitot system leak before my S35 can be recertified.

It has been repetitively recertified without problem in the past, and the airspeed indicator seems to works fine.

The shop says the static system meets the requirements of FAR 43 Appendix E which governs static system certification with respect to altitude reporting.

There is no mention of the pitot system in FAR 43 Appendix E, nor Appendix F, which governs transponder recert.

The A&P is telling me tracking down the pitot system leak could be very onerous and consume multiple AMUs.

By my interpretation, the static system/altimeter and transponder need to be up to snuff for IFR recert, but the pitot is not a factor.

Anybody have any experience with this?

[Tracy Ryan]

I know of no requirement within FAR 43 Appendix E as you indicated, but if there is a leak, then there is an error in your airspeed. It may be an insignificant amount, or it could be several knots. But I'm sure the FAA could make a case that the aircraft did not meet airworthiness requirements if they chose to be nit picky about it.

Now, when it comes time for your annual inspection, FAR 43 Appendix D will apply. It essentially states that all systems must be in operating condition with no defects. A leak is definately a defect.

A quick way to check for a leak in the pitot, and to test the entire pitot system, is to get a piece of soft rubber tubing, tygon is best, that you can slide over the pitot tube, leaving at least 6 inches beyond the tip of the pitot tube. It should be snug and put a tie wrap on it to hold it tight. Put a couple of pieces of good duct tape or aluminum tape over the bleeder hole in the rear of the pitot mast. This should be accomplished in a hangar or under an awning so radiant heat from the sun cannot cause an air expansion error. While someone is in the cockpit looking at the airspeed, clamp the open end of the tubing and then slowly start rolling it up toward the mast. When the airspeed gets to 160kts, clamp it off and let it sit. If it loses more than a knot or two over a 15 minute period and over two or three testing cycles, then you definately have a leak. Just make sure when you losen the clamp that you slowly unroll the tube so the airspeed bleeds off slowly and doesn't damage the diaphram inside.

[Don Lawrenz]

What autopilot do you have installed? The Brittan's have a small calibrated leak in them. Many shops have forgot this little feature.

[David Gates]

Kudos Don.

I do have the Brittain B5 which works remarkably well depite its age.

The key to the solution was that the avionics guy said they had found a "tee" connector in the pitot line going to the AI.  They thought the tee went to an old "magic hands" system <it wasn't available until 50 serial numbers after mine>.  That got me thinking - where this line must run is where the pitch box is for the B5 altitude hold system.  So I called Gerry Walters at Brittain and he confirmed the box does connect into the pitot system and has the leak you mentioned.  He sent a copy of the service letter relative to this issue to the avionics guy and the issue seems to be no worries.

Then as I looked through my old documentation materials - there was a copy of the service letter.

Happily, the techs hadn't spent much service time on this "leak" - but they were about to do so.

The question I am left with, however, is how then do I know if my airspeed indications are accurate?

You nailed it, Don.

[Michael Amerlan]

Actually, 43.13 states:

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).

CAUTION: To avoid rupturing the

diaphragm of the airspeed indicator,

apply pressure slowly and do not

build up excessive pressure in the line.

Release pressure slowly to avoid damaging

the airspeed indicator.

[John Johnson]

I had a leak once, found during a pitot static check, also, however, the avionics shop doing the work located and fixed it quickly, for not too much money (perhaps $100.)  It was from water corrosion of the aluminum tubing coming from the left, aft static port, down where it makes a 90-degree turn forward, aft of the baggage compartment so it was easy to access.  Apparently water got in the static port there over the years and eventually corroded the aluminum elbow though.  They just cut the section out and put a rubber section in its place with clamps.

[John Torrance]

David;  This may be too late to be of any help.  I had the same problem on my k model once.  First noticed fluctuating air speed (just 2 or 3 knots) during cruise when flying in rain.  That told me that water in the pitot at a low point somewhere (no matter where) was causing pressure fluctuations as the air passed on down the line to a slight leak.  Turns out it was a leak behind the interior panel beside the pilots left leg, caused by one of those old clamps that just wouldn't tighten down enough on the old age hardened hose.  Might have been some corrosion on the aluminun tube.  I hope this is not too late to help.