Main gear door issue

[hharney]

Bill

The main gear door actuators are held closed with hydraulic pressure. See attached diagram. If only one door coming down it lends that the system is losing pressure. It could be the door actuator on the subject door. There are no uplocks on the doors as I was reading the print incorrectly.

The main gear actuators are tee connected in with the strut door actuators. Of course there is micro switches that have to be satisfied. Sounds like an actuator on the main door is leaking down.

[Skymaster337B]

The way I see the diagram is, the main gear door pressure is trapped between the two actuators. This effectively hydraulic-locks the doors in place. So, my guess is there's an internal leak in one of your gear door actuators. Very easy to overhaul...mostly new o-rings, and of course a proprietary Cessna teflon "back-up seal" that will cost almost as much as socialized health care. The other possibility is the door solenoid in the power pack.

[Skymaster337B]

Does your gear door also creep open while the aircraft is parked and not flown for a few days? If so, there is a thermal relief valve in the hydraulic power pack that helps keep the doors closed during thermal expansion. However, this thermal relief valve was not installed on earlier model 337's, unless the power pack was replaced with a new one. Therefore, during thermal expansion the main gear door will open.

[billsheila]

I have not noticed it after the plane has been sitting, it has shown up (inconsistently) after a landing. I am not clear if the one door is opening in flight on the up cycle...I did not notice an out of trim situation. So maybe it is just on the down cycle?

I searched the forum here where Skymstr02 writes as follows:
The "G"'s (1973 and up) hydraulic systems are different from the earlier 337's and O-2's. The early and military cylinders are internally different in that they have a retracted lock mechanism that locks them in the retracted position without hydraulic pressure, and the later cylinders do not have or need this. This may pose a problem if used on the later model airplanes. The later model airplanes also have a hydraulic accumulator that the early airplanes do not have.

So Herb when you note a main door lock is that what you are referring to (ie some kind of a lock mechanism in the actuator?). I dont see any other kind of lock under there, but I intend to go down to the plane this morning and have a closer look. Re the power pack, with a pile of earlier hassle with gear hydraulics, I did have the power pack replaced with a rebuilt unit a few years ago. It was an exchange situation, so I could check my logs to find out the serial number of the pack that was installed. If I got that number would that tell us whether I have a thermal expansion valve or not?

[billsheila]

A&P did some tests and he says actuators are all fine. He thinks it is the power pack "time delay valve" and related adjustments. One prelim test seems to indicate it may be the internal non-adjustable valve, but he wants to do some added tests and jack it. This is frustrating because this power pack was replaced with a rebuilt unit only 3 years ago. The (original) 40 some odd year pack was working fine, but with a cracked aluminum hydraulic line coming out of it, I decided to replace the pack when the line was being changed. Bill

[Skymaster337B]

Did it turn out to be the Time Delay Valve?

[billsheila]

Still undetermined. The shop shares the proper Cessna mule with another shop and he is waiting to get that before it goes on jacks for more work. He approved it for further flight, suggesting I hold the handle down or up for a further ("5 seconds or so") on each cycle, so this has been the protocol for the time being. Doing this, no doors found open thus far. Timing for the handle to return to neutral on either up or down cycle is "too quick" at about 2 seconds. This is the test he did. First on the ground, then a flight test for the up cycle. Seems like exact same time, both up and down. Do you agree with his diagnosis thus far and his approach?