Air in Hydraulic Fluid
 

My mechanic is fighting air in the hydraulic fluid on my pressurized G model.

What's the best way to troubleshoot this issue?

I've thought about trying to isolate each part of the landing gear system and then cycle the gear to see if it's a sucking actuator in that part of the system.

Any experience with that approach?

Any other thoughts?

Thanks,

Rick

Drive you crazy
 

Not a maintainer, a breaker. Experienced this a number of times, mostly in colder climates. High heat not to often. Various Mechs have bleed/drained then did refills with repeated manual crankdown of gear. (Guess whom did the cranking?) Never had the problem on M337s only G&H models.

Thank you, Snake. We’ve done the drain/bleed approach and the air keeps coming back. We’ll keep plugging away trying to find something that works...

Rick

I replaced all the hoses on my P337H and tried to find any reference to bleeding the hydraulic system and found none in the Service manual. I called Cessna and talked with one of their engineers. His only suggestion that was safe was to cycle the gear until all air was out. So that is what I did, I lost track how many times I cycled it, way more than the 7 times I used a stop watch to check the times. Those I wrote down in the service manual. The problem with the system is that all the lines are dead ended in the actuating cylinders. I guess one has to mix the air with the fluid to get it back to the reservoir so that it can get out. Not like bleeding the brake system. If anyone has any other info, I would be glad to here from them on this subject.

One of my better Mechs on the line ...
 

Rick. I am text/email -ing with a Mech and now director of mx that worked for me while flying 337s. He asks is this in the brake or extension/retraction system. I assumed gear Ext-Ret. If you want to email me a full description I can send along to him. gryphonskymaster@gmail.com v/r snake

One of my better Mechs on the line ...
 

Rick. I am text/email -ing with a Mech and now director of mx that worked for me while flying 337s. He asks is this in the brake or extension/retraction system. I assumed gear Ext-Ret. If you want to email me a full description I can send along to him. gryphonskymaster@gmail.com v/r snake

Thanks MD83Driver. Yeah, the air mixes with the fluid so the fluid turns an opaque pink color. The air bubbles are so tiny it takes about 2-3 weeks for the air to come out of solution. We've talked about disconnecting each line and flushing them individually.

Hi Snake, thank you for trying to help with this!

For your Mech / Director of Maintenance:

• This is occurring in the landing gear system on a pressurized G model.
• We've made sure the hydraulic fluid reservoir is full, but not overfilled, when starting the tests.
• There are no visible leaks, that we've been able to find.
• The hand pump works great and does not appear to introduce air into the hydraulic fluid.
• The air is getting "whipped" into the hydraulic fluid so that the fluid turns an opaque pink color.
• The bubbles are so tiny it takes 2-3 weeks for the air to come out of solution.
• With air in the hydraulic fluid the pump makes a loud screeching noise, ...that noise is not present until the fluid is filled with air.
• With air in the hydraulic fluid the landing gear takes MUCH longer to extend/retract.

I'm wondering:
1) Could the pump be introducing the air?
2) Could there be a loose fitting in the system that could be "sucking" in air, in perhaps a return line?

Thank you again!

Rick

Rick - Here is what I have from my best fixer -
Snake
Nice! Sounds like a challenge!!

Without fluid leaking somewhere, this will be difficult to figure out.

Every line, fitting, swivel, actuator and the pump manifold needs to be wiped very well with alcohol and swing the hell out of the gear.

This will most likely kill the pump, so keeping the pump motor cool will help last longer, but still will most likely ruin the longevity of it.

Hopefully a leak will be found after the cleanup. If there's just no leak to be found, I am putting my money on the pump/manifold. If that's not it, the only thing left is the rare case that there is a open in the system that doesn't leak, which will only be found by inspecting each mating surface of every connection.

The hydraulic fluid shouldn't foam up if the pump reservoir cap isn't sealed.

I will keep thinking about it, just in case a few of my brain stones collide and I have another thought. Let me know how it's going

Rick from snake I think he made an error in phrasing re: foaming, I think it will foam and did for me on an improper sealing of cap.

Keep you posted.

v/r snake Happy New Year.

Thank you, Snake. I'll relay this info to my A&P.

I'll let you know what the problem was, ... once I know.

Rick

Just read an NTSB report on a Skymaster that had to belly in. Gear would not come down due to loss of fluid via tiny pinholes in one of the lines, caused by internal corrosion. Its something to consider. May be worth reading. Its from 2019 IIRC.

Thanks Mark, can you include a link to that report? I'm not familiar with IIRC.

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.

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.

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