Pitot-Static Leak Detection 101 - Think AviTool for leak detection

Here we discuss aircraft pitot-static system leak detection – how to find leaks. For guidance on how to repair pitot-static leaks, refer to our Leak Repair webpage just next door. This approach assumes you have some form of leak detector that won’t introduce water to your system or blow up your instruments and that you know how to use it. We use the proven AviTool Leak Detector Kit for locating pitot-static leaks.

Check the test system for leaks

Before concluding that you have a leak in the aircraft, confirm that the leak was not caused by your test detection “system” (test-set).
Confirm that the connections at the ends of the testset tube fittings are tight. You should feel two stops when inserting tubing into a push-to-connect (PTC) fitting – push past the first “stop”, a total of about 5/8 in (1.5 cm).
If a pitot leak, confirm that you have taped over all the small drain holes on the pitot tube. There may be more than one. They are found on the rear and bottom-rear of the pitot tube. One may be at the very top where it mounts and may be difficult to tape over. Also check that your pitot tube adapter is tight over the pitot tube. I frequently will use a zip tie to secure the latex tube around the pitot tube.
If a static leak, confirm that you have taped over any open static ports. It is best to use wide, e.g., 2 inch poly tape and cover more than the static port disk and onto the surface surrounding the port. Lower-end Cessna’s have a single port – usually found in front of the pilot’s door. Most other GA aircraft have two static ports found mid-ship on the sides of the aircraft. If you are using a static port adapter over a static port, confirm that the suction cup is fitting securely over the small hole in the static port.
If you are fortunate enough to have a static port adapter that straddles a static port and you are questioning the effectiveness of the center vacuum cup’s seal, you can reposition the static port adapter elsewhere on the fuselage away from the static port to test the cup’s sealing ability.

Don’t overlook the pitot-static system valve(s)

If a static leak and the aircraft has an alternate air valve, confirm that it is closed. Sometime these valves leak too. I like to exercise the valve a few times to cause a “fresh” seal. Properly installed, the valve shaft should be pushed in against the panel for normally-closed operation. This valve is frequently located on the bottom-center of the instrument panel.
If the aircraft is fitted with a static and/or pitot drain valve, confirm that it is closed. These valves are frequently located closer to the floor near the pilot or under (and accessed from outside) the aircraft, mid-ship.

Tighten the obvious reachable fittings

Next, firmly tighten the obvious and reachable fittings that comprise your pitot and/or static system. Frequently the fittings are nylon and you can firmly finger-tighten the “nuts” where the tubing enters the fittings. When you can reach a fitting with pliers, tighten nuts about a half turn beyond firm finger tightness and little more. If the aircraft uses push-to-connect fittings, again for those fittings you can reach, snug the tubing securely into the fittings. When pushing a tube into a push-to-connect fitting, you will feel two stops. You will need to push harder beyond the first stop in order for the tube to bottom-out and be secured and tight in the fitting.

Draw a schematic of your pitot-static system

If you have every looked behind an instrument panel, you know it is a maze of electrical wires and tubes; and much of it is difficult to understand and follow. I like to draw a schematic of what I see to help determine the scope and condition of my pitot-static system and next moves. This schematic may already exist in a GA aircraft POH and be accurate for newer planes, but frequently these diagrams are incorrect for older aircraft that have been repaired and “modernized” over time. Using a cell phone camera to photograph or take a video of the tubing system behind the panel is helpful in making the schematic. Realize that unless it is a new aircraft, no two aircraft will have the same tubing configuration and fittings. The diagram will help you understand your pitot-static system and develop a strategy to diagnose the leak(s).

As an aid to creating a pitot-static system schematic, we created a worksheet containing the majority of pitot-static components, less the tubing. All you need to do is print out the diagram and connect the components as they appear in the aircraft showing any fittings along the tubing. Cross out components that don’t exist and add any that do, like backup instruments. Download this worksheet here.

Check for a cracked or leaking sump bottle(s)

Check for a cracked or loose sump bottle just inside the aircraft of the static port(s). Sump bottles help prevent moisture from entering the instruments and eliminate the pulsation from prop-wash. Not all aircraft have these bottles. On Cessna aircraft, like the 172, these plastic bottles can be inadvertently banged by the pilots left knee making them susceptible to breakage. Over time, these bottles become brittle and the plastic threads shrink, causing them to leak. You can cap the tubing inside the aircraft at the bottom of the sump bottle (an AN4 flair fitting) and test the system from the outside (pitot or static port) to see if the bottle leaks. The top of these bottles is typically 1/8 NPT. Don’t flip the bottles when installing them, as the fittings look similar.

General pitot-static system considerations

Aircraft “air” instruments are very delicate and quite expensive, and it doesn’t take much pressure or vacuum for them to function or be damaged. Typical leak test pressure or vacuum is less than 1 PSI.
The pitot system works on pressure and static system works on vacuum (unless flying below sea level). When testing them, be careful to apply the correct air polarity.
The static system usually is more complicated and has more fittings and opportunities for a leak.
Some leakage is okay as long as it doesn’t exceed what is permitted by the FAA. Pitot systems shouldn’t lose more than 10 knots over a minute when pressurized with 60-100 knots of air. Static systems may not lose more than 100 feet over a minute when a vacuum of 1000 feet over field elevation is applied (at a baro setting of 29.92).
It is not uncommon for a static system (due to their complexity and tubing length) to have multiple leaks. Don’t assume that after you find and fix a leak that all is good. Retest the system.
If an aircraft has been retrofitted with nylon tubing and fittings that use spreader-inserts at the fittings, it is possible that the installer left them out (its easy to do). Spreader-inserts maintain the OD of the tube when the nut is tightened. Without the spreader-insert is an opportunity for leakage. Not all nylon fittings require spreader-inserts.
Realize that the airspeed indicator is also attached to the static system (as well as the pitot system) and that when you only apply static test pressure, the airspeed indicator will move in the positive direction, accordingly. Static pressure is used to compensate for the decreasing air density with altitude. Limit your vacuum to no more than 1000 feet over field altitude (corrected for barometric pressure) so you don’t extend the airspeed pointer beyond the indicator’s range. As a point of comparison, 1000 feet of static (vacuum) air should cause the airspeed indicator to read about 148 kts or 170 mph.
The vertical speed indicator (VSI) is a valuable instrument when testing a static system. It will indicate the extent of your leak(s). And recall that the maximum leak allowed is 100 feet/minute.

Locating the leak

Assuming that you know which system is leaking (pitot or static) and have tightened the obvious and reachable fittings to no avail, you are now ready to apply logic to the debugging task. Simply stated, the best approach to find leaks is one of “divide and conquer”. Applied, this means breaking or separating your system into smaller systems and testing each one or “side”. I like to start by separating the feed tubing side, starting at the pitot or static port that leads to the instrument panel, from the maze of tubing, fittings and air-instruments in and around the panel. If one side tests okay, then the problem lies in the other half.

For the initial test, break the system in two at the first fitting behind or near the instrument where the pitot or static line comes from their respective port. Then: 1) cap the end of the feed tube originating from the pitot or static port and test it from the port in and/or 2) attach a leak detection device to the now open fitting behind the instrument panel. If you don’t have a static port adapter, shown here, (available from AviTool), tape over the static port and attach the leak detection device to the end of tube where you broke it apart.

If you find that one half of the system leaks, look for the cause in that half and address it, then test this side again. It is a great day when only one leak exists. If a leak still remains, divide the section in half again (to the nearest fitting) and test that half, repeating the above process. If you narrow the leak down to an actual instrument, you can test the instrument for a “case leak” by connecting the leak detector directly to it. No instrument case should leak. Some do over time due to the “O” rings drying out.

You may need a range of different fittings (caps, tees and gender changers) to seal and/or attach your leak testing equipment to the aircraft pitot-static tubing and instrument fittings. The AviTool Leak Detector Kit comes with a variety of fittings for handling different tubing-fitting scenarios.

In the rare instance that you find that the pitot or static system tubing is clogged or plugged (e.g., by a mud dauber, insect, small bird-strike, wax, paint or excessive water), DO NOT use compressed air to blow out the system unless you have completely removed ALL the instruments attached to the system, including the altitude encoder. Then blow the air from inside the aircraft, outward.

The plastic tubing used in many pitot-static systems in GA and experimental aircraft is good for about 10 years or so, after-which it becomes less flexible and can crack. When the tubing cracks, it often does so at a fitting or in line with the direction of the tube. Interestingly, tubing cracks are not always detected during a static or vacuum-related test, as the tubing is pulled tighter together, but cracked tubing leaks like a sieve when tested under pitot or positive pressure. When applying a positive pressure to a static system that is still attached to instruments, limit the pressure to the equivalent of only -200 to -300 feet (on the altimeter) since you will be forcing the airspeed indicator (also part of the static system) pointer backwards against its stop. Any more pressure and the pointer may slip on its shaft, dangerously and falsely decreasing the resulting airspeed indication, thus requiring instrument overhaul or replacement, an expensive endeavour.

Putting the system back together

Use zip ties or adel clamps to anchor lengths of tubing every foot or so to eliminate vibration.
If you divided your system to help locate the leak(s) and you didn’t use a static port adapter when testing your system, you can install a tee fitting to connect the two halves back together, then plug the open end of the tee. The Leak Detector kit came with a few plugs. This will make future leak testing easier. Remember to tighten any “nuts” of this tee fitting.
After repairing and reassembling your system, test it as a whole to FAA requirements.
Remember to remove any tape you may have used to cap off the pitot tube drain holes and/or static ports.