Air in Hydraulic Fluid

[wslade2]

Over the years I’ve had opportunity to own a number of hydraulic dependent equipment (trucks, construction). Frequently the cause of air in a system is a leak on the suction side of the pump or at the pump itself (the seal). My model uses engine driven pump and is known seals can go out leading to sucking of air. I believe your model uses Electric power pack. You might want to run the problem by one of the power pack overhaul shops (like cox airparts).

Yes, lines don’t last forever, can break down, leak and regarding the “microperfs”, “sweat“ fluid as they leak. However the lines are under pressure and thus unlikely to be a source where air is sucked in.

[mshac]

IIRC = If I recall correctly.

I didn't find the one I mentioned, but I'm still looking.

Here is a Forest Service 337 incident:
August 8, 2018, Lewiston, Idaho
Cessna T337 Turbocharged Skymaster

At about 1828 Pacific time, the airplane sustained substantial damage when it landed with its landing gear retracted. The commercial pilot and his passenger were not injured. The airplane was being operated under contract for the U.S. Forest Service on an “air attack” firefighting support mission. Visual conditions existed for the landing.

When the pilot prepared to land, the landing gear failed to extend. After troubleshooting the problem and cycling the gear, the pilot and passenger determined the system’s hydraulic fluid reservoir, which was accessible from the cabin, was empty. Despite replenishing the reservoir with oil and water, the landing gear could not be successfully extended. Committed to a gear-up landing, the pilot secured the front engine, and “bumped” its starter to position the propeller blades horizontally and prevent damage. The occupants unlatched a cabin door to ensure their egress after landing. After landing gear-up, the airplane slid to a stop within a few feet of the runway centerline. The pilot shut down the aft engine, secured the airplane and both occupants exited. No fire or other problems occurred.

Examination revealed the circlips retaining the actuator rods for both the left and right main landing gear doors had been liberated from their retention grooves. Loss of this circlip allows hyperextension of the actuator rod and permits hydraulic fluid to exit the actuator.

[mshac]

Here it is: https://aviation-safety.net/wikibase/230538

[SkyMac]

Hi Rick

We had some issues when checking my electric power pack, one of the manifolds was not sealing correctly to the pump housing when they initially tested the pump (bench testing) to check functionality, I believe they said air was entering the system.

What they found on disassembly was the O ring seal was being distorted by what looked like a deformation to the manifold housing, this rendered this part unserviceable. The power pack was overhauled a few years before I purchased the aircraft, we believe the damage was done during this time.

Don Neiser was able to supply another manifold, pump was fully overhauled and bench tested in the rig, functioning correctly. Also replaced all flexible lines and rebuilt every actuator and down lock at the same time ensuring the system was in best possible conditions.

Dave

Ps: The aircraft I purchased was not airworthy and had not been flown for three years.

[Rick Erwin]

Originally Posted by SkyMac

Hi Rick

We had some issues when checking my electric power pack, one of the manifolds was not sealing correctly to the pump housing when they initially tested the pump (bench testing) to check functionality, I believe they said air was entering the system.

What they found on disassembly was the O ring seal was being distorted by what looked like a deformation to the manifold housing, this rendered this part unserviceable. The power pack was overhauled a few years before I purchased the aircraft, we believe the damage was done during this time.

Don Neiser was able to supply another manifold, pump was fully overhauled and bench tested in the rig, functioning correctly. Also replaced all pneumatic lines and rebuilt every actuator and down lock at the same time ensuring the system was in best possible conditions.

Dave

Ps: The aircraft I purchased was not airworthy and had not been flown for three years.

Thanks, Dave. I could be headed toward an overhaul myself ...

Rick

[Rick Erwin]

To close out this problem ---

I called Rick Cox, at Cox Airparts in Wichita - 316-945-0737, and described the symptoms to him. He immediately said to check the nitrogen charge in the accumulator. If there was no pre-charge, to pull the Schrader valve and stick the end of a zip-tie in to see if it was wet. It was.

My IA ordered the O-rings to overhaul the accumulator (Rick has parts too), and now all is well.

In retrospect, the TINY bubbles entrained in the hydraulic fluid should have been my clue that nitrogen was in the system (think Guinness, Nitro Coffee, and the bends while diving). There's only one source of nitrogen in the hydraulic system ... and that's the accumulator.

Rick Cox has certainly earned any future hydraulic business I might have!

Rick

[Rick Erwin]

Originally Posted by mshac

Here it is: https://aviation-safety.net/wikibase/230538

Thanks for the accident reports, Mark. They certainly do serve as a reminder just how important it is to get this right.

Rick

[Rick Erwin]

Originally Posted by wslade2

Over the years I’ve had opportunity to own a number of hydraulic dependent equipment (trucks, construction). Frequently the cause of air in a system is a leak on the suction side of the pump or at the pump itself (the seal). My model uses engine driven pump and is known seals can go out leading to sucking of air. I believe your model uses Electric power pack. You might want to run the problem by one of the power pack overhaul shops (like cox airparts).

Yes, lines don’t last forever, can break down, leak and regarding the “microperfs”, “sweat“ fluid as they leak. However the lines are under pressure and thus unlikely to be a source where air is sucked in.

Thank you wslade2. I'll talk with my mech and will probably give Rick Cox a call next week.

Rick