Air in Hydraulic Fluid

Snake · #1 12-29-20, 10:44 AM

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.

Rick Erwin · #2 12-29-20, 12:29 PM

Quote:

Originally Posted by Snake

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

MD83driver · #3 12-29-20, 02:57 PM

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.

Rick Erwin · #4 12-29-20, 08:42 PM

Quote:

Originally Posted by MD83driver

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.

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.

Snake · #5 12-31-20, 11:41 AM

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.

Rick Erwin · #6 12-31-20, 11:47 AM

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

mshac · #7 01-01-21, 02:21 PM

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.

Rick Erwin · #8 01-01-21, 08:21 PM

Quote:

Originally Posted by mshac

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.

wslade2 · #9 01-01-21, 11:38 PM

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.