Here we discuss aircraft pitot-static system repair. For guidance on how to locate pitot-static leaks, refer to our Leak Detection 101 webpage just next door. This Leak Repair page is divided into three sections: General Considerations, Pitot System Fixes and Static System Repairs.
General Considerations
The hardest part of a pitot-static issue is, by far, identifying where the leak is. Unfortunately, too frequently pitot-static systems can leak at more than one location simultaneously, compounding the leak location and repair job. Just because you find and fix a major leak, don’t think all is good. It’s typically just the beginning of the adventure But lets focus on the repair task.
Unfortunately, there is no industry standard for how pitot-static systems should be constructed which leads to a wide variety of solutions, especially for older aircraft that have been “upgraded”. Pitot-Static systems can consist of hard metal/aluminum tubing flared at the ends with AN fittings of various sizes, metal tubing with overlapping rubber tubing that’s clamped in place, 1/4″ OD tubing with Nylon screw-to-tighten fittings and tubing using push-to-connect fittings. So repairing a pitot-static system most likely will require equivalent marerials or some adapting. And keep in mind anytime you introduce another fitting or compoenent, it’s another leak opoortunity.
Fortunately, you have some flexibility in how you repair a leak. You can replace a fitting with a different style or the tubing with a different material as long is it is aircraft quality.
General advice:
• Don’t buy components to repair a pitot-static system from a hardware store unless you know it is aircraft quality.
• Don’t over tighten screw-to-tighten fittings. Typically 1/2 turn (with a wrench or pliers) beyond maximum finger tightness is sufficient.
• Don’t use teflon tape with nylon fittings.
• Don’t over-tighten AN-NPT type fittings into the back of instruments as instrument cases are cast aluminum and will crack if “Pried” open by the fitting.
• Don’t use poly or vinyl tubing in an aircraft. They are susceptible to deterioration from contact petroleum products and fuel fumes. Use nylon or closed cell silicone tubing. Nylon for the majority of the system. Silicone for the last run to the encoder, if the encoder is ACK fitted with an unusually small nipple fitting. If this is the case, also, install a hose clamp around where the tubing overlaps the pipe nipple, preferably of the ring variety.
• Should you need to replace a pitot or static tube, do so with quality nylon tubing and fittings.
– Poly tubing (available from most hardware stores) deteriorates when exposed to Petroleum products and fuel fumes and should not be used in an aircraft.
• Discolored, kinked, scratched or compressed pitot-static tubing should be replaced. Actually, if the leak was due to a crack or break in the pitot-static tubing, you should replace the tubing in the entire system. Similarly, if you find more than one leak in a system, you should seriously consider replacing the entire system with new tubing and fittings.
• When replacing a system with new tubing and fittings, be consistent in the type of tubing and fittings you use.
• If disassembling a system, it is a good idea to label the remaining tubing and/or fittings either “Static” or “Pitot”, since accidentally flipping them can have devastating consequences.
• When replacing the tubing in a complex system consisting of numerous lengths of tubing, to help discern the pitot and static system tubing, consider using two different colors of tubing. Red is typically reserved for static tubing and blue for pitot tubing. And yes, nylon tubing is readily available in both of these colors. Alternatively, label every section of tubing on both ends using heat-shrink labels stating “Pitot” or “Static” as applicable. Later, you will be very glad you did.
• When reusing a section of tubing, it is a good idea to trim back each end of the old tubing about 3/8″ to 1/2″ before re-inserting it in a fitting to ensure a good seal. Note that Nylo Seal fittings use a insert in the end of the tubing to maintain the tubing diameter when it,s being squeezed by the nut.
• When building the pitot-static system of an aircraft, its a good ideas to increase the length of each section a few inches to facilitate trimming/shortening of the ends and system maintenance.
• There are several styles of nylon fittings. They each incorporate a collet in the nut that squeezes the tubing as you tighten it. Nylo Seal fittings (in addition) include an insert that maintains the tubing OD during nut tightening.
• When cutting nylon tubing, cut the tubing perpendicular to the tubing direction with a sharp knife or tubing cutter, like the one supplied with the Leak Detector kit.
• Regarding push-to-connect (PTC) fittings, there is a wide-range in the cost and quality of these fittings. If not properly assembled (inserted completely), they can leak and the tubing can fall out due to vibration or tension on the tube. The majority of PTC fittings are plastic, and like poly tube, are susceptible to fuel fumes and more rapid aging due to heat – like when an aircraft is at tie-down. In addition, if the plastic tubing inserted in a PTC fitting is of low quality, has a scratch, gouge, burr, or is rough, it can leak. And lastly, unlike a threaded fitting with a collet, a PTC fitting can’t be tightened. Use PTC fittings at your own risk.
• If you are sold on PTC fittings, include a few inches of “slack” or spare tube length on each run to allow for multiple trimming of the tube ends over time. Actually this is good policy when installing any tubing in an aircraft, but anchor it like you do with wiring to eliminate vibration.
•
••••••••••••••••••••••••••
• As a general rule, use threaded fittings for pitot-static connections. They can be brass, aluminum or nylon fittings should be used are ⅛” NPT.
• Most sump bottles (discussed in Leak Detection 101) have an AN4 fitting on one end and 1/8 NPT on the other. Care should be taken when installing them for this reason since they look similar.
• MOVE TO 101 – Should you find that your leak detection leads to an instrument, realize instrument cases, like everything else, are fallible and can leak as they age – the front seals and o-rings around the shafts dry out and deteriorate. Instruments are not user or A&P repairable and require calibration and certification after servicing/overhaul.
– You can test an individual “air” instrument by connecting the AviTool Leak Detector directly to it and performing the appropriate leak test. Realize you will need less air volume (mL) for this test.
• If it appears that the pitot tube is leaking, open the inspection plate under the wing near the pitot-tube and check the connection where the pitot-system tube attaches to the pitot tube itself. If it’s not leaking at the connection point, then the problem is with the pitot tube itself. Before you proceed, though, make sure you have not missed covering a drain hole on the bottom or back of the tube and that your tape job is effective.
Heated pitot tubes are susceptible to cracking internally and thus leaking due to the thermal expansion-contraction. to pitot tube, confirm that it’s leaking by direct connecting to. To test the pitot tube by itself, connect the leak detection tool to either end of the pitot tube and cap the other, remembering to also seal any drain holes, mentioned above. Alternatively, with the pitot tube removed from the aircraft, you can submerge the pitot tube in water using the same configuration above, but use positive pressure and look for air bubbles. A leaky pitot tube is not repairable and must be replaced.
Note, If the aircraft has a combination pitot tube/angle-of-attack sensor in the same casting, it will have two connections in the wing, a line for each function. Before removing the old pitot tube, mark the two lines leading to the two connections, “A” & “B”, as once the pitot tube is removed, you will have a 50% chance reconnecting the lines correctly when installing the new pitot tube. Take care to not confuse the two lines when testing or removing the tube. There should be no leakage between the AOL and pitot “ports”.
Pitot system fixes
• If the aircraft has a traditional cast-aluminum “L” shaped pitot tube and you are using the AviTool pitot tube adapter, confirm that you have securely covered all the small (pin size) drain hole(s) with poly tape on the bottom and rear of the pitot tube, including any hole near the top-rear where it intersects with the mount/shell.
– The design of pitot tubes varies, as do the number and placement of the small drain holes. Carefully inspect the pitot tube to determine and cover them before performing a test.
Static system repair
Move into Leak Detection 101
• When using a latex pitot tube adapter, like the one sold by AviTool, ensure it is snug around the pitot tube and shoved on at least an inch or so past the back-end of the nose of the pitot tube. Its a good idea to wrap a zip tie around the latex tube to ensure a good seal.
• If you are working on a Piper or Diamond aircraft with a combined pitot and static blade assembly, and you are accessing the pitot system via a tee fitting that you added; all the holes of the blade must be taped over, including the pitot orifice.
• Airspeed indicator cases can leak, as well as the diaphragm in the indicator (separating the pitot from the static systems). When either leaks it can affect the accuracy of the airspeed indicated.
• Some leakage in the pitot system is tolerable. The rule of thumb is, in a pressurized pitot system to 60 kts, once “capped off”, should not leak more than 10 kts (drop below 50 kts) during a minute. A leaky airspeed indicator impacts the aircraft static system which can cause the altitude instruments (and transponder) to read lower than actual. That said, a leaky airspeed indicator will also look like a leak in the aircraft static system.
Static System Leaks
• If your aircraft has multiple static ports, confirm that any you are not connecting to (via an adapter) are taped over for the test. Later, remember to remove the tape!
• If you are using a static port adapter and discover a leak in the static system and want to eliminate the possibility that the leak is with the test equipment, move the static port adapter to a flat surface (away from the static port) on the fuselage (thus capping the center cup) then rerun the test. If the leak disappears, then you know the problem is not with the adapter and test gear, but truly with the static system.
• If your aircraft has an alternate static source valve (refer to your POH), confirm that it is in the normally closed position. An open valve will appear as a major leak.
• It is not uncommon for alternate static air valves to fail – the internal O-rings deteriorate over time. These O-rings can be replaced but removing the value from the panel can be a challenge on some aircraft like older Cessnas.
• If the aircraft has a static system drain valve (refer to your POH if you are unsure), it is a good idea to exercise it and confirm it is in the normally closed position. These valves too can fail to seal over time.
• All aircraft that have a transponder will have a digital encoder attached to the static system. Newer transponders have these built in. Some encoders (like ACK) use an unusually small nipple fitting on their “boxes” to connect to small tubing which must be adapted to fit standard ¼” OD tubing. Both are opportunities for leakage. It’s a good idea to check the tube connection at the encoder earlier than later.
• Some aircraft, like Cessnas and Cirrus’ have a small plastic (fist-size) “sump” bottle in line with the pitot and/or static system to eliminate moisture and pulsations from prop-wash from entering the system. These bottles are installed in series with the pitot and/or static line where the line enters the fuselage. A photo of the Cessna version is pictured above on page 6. These small plastic bottles become brittle and crack, and their fittings loosen as the plastic ages, causing leakage. Note, these bottles are “polarized” – have different type fittings on each end.