Fixing an Hydraulic Leak in the Gear System!

The day after Kathy and I returned from Virginia, I noticed a small pool of hydraulic fluid on the hangar floor under the aircraft.  It was clearly coming from the belly area that housed the gear operating system.  A quick check of the hydraulic fluid reservoir showed the level had dropped substantially since it was checked at the annual inspection a month ago.  Oh oh!

I had two immediate thoughts.  One: crap.  This is the reason why a lot of pilots stick with fixed landing gear.  Retractable gear is complex (especially on Cessnas) and some feel the additional speed gain is not worth the extra cost in maintenance.  Now that I've made my bed (by buying a retractable), I have
to sleep in it!

The second thought was:  this needs to be fixed asap.  One of the big issues with Cessna retractables is the problem with hydraulic leaks.  There are no uplocks on Cessna gear, it's held up with hydraulic pressure.  If a line or fitting should fail in flight, the hydraulic pump would pump all the fluid overboard and there would be no way to get the gear down.  The standby pump needs fluid to work and of course, there wouldn't be any.  Thus, a bent metal landing is guaranteed.

So the first thing to do was try and find out where the leak was coming from.  I removed the back seat and opened the access panel that covered the gear actuators.  There was red fluid on the belly panel under the right gear actuator and the aft end of the actuator was slightly wet with fluid.  Nothing was seeping or dripping even though there is always pressure in the lines (even when the plane is parked in the hangar).

I pulled up the hydraulic schematic in the Service Manual on my phone (doesn't everyone have this handy?) and studied it as I really did not fully understand how the system worked.  It was apparent that the DOWN side of the system is pressured when the gear is down (obviously) and the UP side is pressurized when the gear is up, but of course this can only happen when the plane is flying (or up on jacks).

So I surmised that the leak might be in the UP system and not the down.  How to check that?  Go flying!!  So that's what I did.  I left the access panel off so I could see the actuator when I turned around in my seat.  I took off, raised the gear and climbed to 2500 feet.  Once I had leveled off, I looked back and...ah ha!!!... I saw a small line of red fluid running forward on the floor from the aft end of the actuator!

Back on the ground, I got out my borescope and had a closer look.  There were only two places the leak could be coming from--the threaded cap (or "gland" that the parts diagram calls it) or the AN elbow fitting in the centre of the cap (where the UP pressure line connects).  There was no way to tell which was the guilty party.

I gave my mechanic Chris a call and he agreed that I could bring the plane over to his hangar for him to have a look.  After recounting my story, he agreed with my diagnosis and said the actuator needed to come out so we could have a closer look.

An hour later the actuator was out of the plane and apart on the bench.  It looked to be in good shape.  The 3 O rings and their retaining rings looked fine.  There was no scoring in the cylinder bore and all the parts moved as they should.  Hmmm.  I was hoping to find that the O ring on the gland fitting had been damaged in some way but other than a slight flattening, it looked ok.  Chris said that the fluid might have been getting by it regardless.

I asked about the AN fitting and Chris said it looked fine and that these fittings rarely leaked (famous last words).  So we left it undisturbed.

Actuator on the bench.  The gold "gland" fitting on top
and the blue AN fitting in the centre. 

The actuator was then reassembled (with new O rings) and installed back in the plane.  Again, about a 1 hour job.  The a/c had been hoisted on jacks, so Chris turned on the master switch.  The hydraulic pump ran for about 5 seconds as it filled up the actuator.  He then moved the gear lever to UP and  the wheels swung up into the fuselage.  All good so far.

He then took a mirror to the back of the actuator and oh! oh!, fluid was weeping out of the AN fitting!!  Darn!!  So that was the problem after all!!

To make a long story short, the actuator came back out so we could access the faulty fitting.  A new O ring was fitted to the fitting, everything was reinstalled and the gear swung once more.  And...NO LEAKS!!  Finally!!

So the moral of the story is:  DO THE EASY STUFF FIRST!!  We should have jacked the plane, swung the gear and looked for the the leak in real time.  Unfortunately, we were both convinced that something in the actuator itself had failed and it needed to come out!  The good news is that I now have a fully overhauled right MLG actuator so it shouldn't need any attention for, hopefully, a very long time!!!