Locating a small leak in the emergency gear extend system can be difficult. My system would leak down from 350 to 275 psi over about four days, and this required topping off before almost every flight. The leak was too small to detect with soapy water and there was always the possibility of leaking across the O-ring in the outboard landing gear actuator(s), which can’t be detected at all visually. Using helium and a leak detector will locate the source of the leak.
A helium tank can be purchased from any welding supply store. Small helium tanks have the same 5/8-inch female fitting that N2 tanks have, so you can use the same regulator and fill-valve assembly.
You can rent a helium leak detector from Ashtead Technology rentals (http://ashtead-technology.com/instruments_us/). They go for about $100/day and they ship worldwide from several locations across the US. The typical instrument is an MGD-2002.
Here is the procedure for finding a leak:
Vent the gear extend system and fill with helium to the redline (350 psi). Repeat five times to remove most of the N2 from the system.
Leave the aircraft sitting for a few hours with helium in the system. This will ensure that helium is present in all parts of the system and that any leaks across the gear actuator O-ring make it up to the gear reservoir.
Plug in the MGD-2002 and charge it for a few hours.
When ready to start checking for leaks, unplug the instrument from the charger and allow it to sit for 15 minutes so it stabilizes with the ambient temperature. There are a few procedures to follow when using this instrument:
a) The instrument detects helium using a temperature-sensitive element. It is best if the instrument and aircraft are at approximately the same temperature. In other words, let the instrument and the aircraft sit together in a hangar before checking; otherwise, you might see false results.
b) Assemble the unit using the small detector probe and the handle assembly. Make sure the quick-disconnect fittings are fully engaged and locked. Verify the instrument is drawing air into the probe by putting your finger on the end of the probe. When the sample airflow is blocked you will hear a change in the sound of the pump.
c) Turn the unit on and allow it to boot up and find its zero point. While this is happening don’t move the probe, and make sure it’s in an area where there can be no helium contamination. Do not adjust the pump speed, and leave the unit in Auto mode.
d) When the unit is ready you’ll see 0 ppm on the display. Verify that the unit holds 0 ppm in uncontaminated air. If the instrument steadily counts up in concentration there is a problem with the unit. (photoC)
e) Test the unit by blowing some helium from the tank into the probe. You should see the value on the display spike up. The unit displays lower concentrations in the ppm range and high concentrations in the % range. The minimum sensitivity is 25ppm and the maximum is 100%. The value then should come back down to 0 ppm within 2 minutes. If this does not happen there is a problem with the instrument.
f) Begin looking for leaks by moving the probe around the top part of the fittings. Note that it takes 16 to 18 seconds for the instrument to detect helium, so you need to keep the probe in one spot for enough time to detect a leak. Go slowly.
An important property of helium is that it easily leaks through and across just about any material. If you have a system that is relatively leak tight with helium, it will be extremely tight with nitrogen. This is why helium is often used to detect leaks, along with the fact that it is not normally present in the atmosphere, which makes it easy to detect. If you detect helium in the 25 – 50 ppm range, it may be a very, very small leak, or it could just be leftover helium in the vicinity of the area you are checking.
But if you see concentrations above 100-200 ppm then you likely have located a leak. Anything in the % range is a substantial leak, although still probably too small to see with soapy water. Lastly, helium is lighter than air, so hold the probe above the fitting you are checking.
The emergency gear extend cylinders are outboard of the MLG. Check all the fittings along with the valve and pressure gauge assembly. Remember to go very slowly as it takes 16-18 seconds for the sample to make it all the way through the probe and into the detector.
Apart from the fittings and valve/pressure gauge assembly, the other path for leakage is across the O-ring in the outboard MLG actuating cylinders. If there is a leak across the O-ring, the gas will be forced into the hydraulic fluid. Eventually the gas will make its way up to the hydraulic fluid reservoir. This is why it is important to let the airplane sit for a few hours.
Place the probe just inside the reservoir and partially cover with the cap. If there is a leak across the O-rings, you will immediately see helium in high concentrations.
Note: Be careful not to dip the probe in the hydraulic fluid! It will suck fluid into the instrument, causing permanent damage.
After completing the leak check, flush all the helium out of the system by charging and venting with N2 five times. Helium is non-flammable and completely inert, so a small amount leftover in the system is no problem.
Scott Dickey says he was fighting this leak in his own Commander for over a year at the same time he had a leak in his water line at home. “A guy came by with this exact setup and found the leak in about 15 minutes. That’s when the bell went off.”
