Main gear door issue
 

On my '65. The issue is every now and again I land and find the pilots side main door wide open like a very effective air brake. If I turn the master back on and hand pump, no problem it closes right up. It doesn't happen every time. Anybody got any ideas?

My understanding of the sequence is that after the light is triggered (gear up or gear down) then the solenoid is powered, moves something on the power pack to actuate the door hydraulics. Doors close, pressure builds until enough is achieved to move handle. My handle is returning and I am getting light...all normal. But then I land to find the one big door open?

Thanks in advance for any ideas.

Bill

Does the door close and then re-open in flight or is it just not closing in flight? I assume it stays closed when the gear is retracted?

Herb I cant tell or rather don't know at this time. I only have a mirror on one side and cannot see the rear big doors with it in any event. With pressure building and the handle returning, my guess would be that it is initially closing. But if the pressure that holds it up is then relaxing somehow, which I guess could be happening, I wonder why the other door(s) would not also fall open? I presume that there is only one main hydraulic line that operates the doors (out of the powerpack) that at some point branches off for the various door actuators.

This airplane is getting frustrating this annual...one issue cropping up after the next! I better get to the bottom of things soon...the fly in will soon be upon us! Thanks. Bill

Bill
You need to see what is happening or swing the gear on jacks where you can determine the scenario. I was thinking that there is a separate actuator for each main door...........I will need to look at the print but that was part of the advantage of removing the main doors to eliminate two actuators.

one door open?
 

The rear doors operate off a tee on the rear bulkhead so there is no reason the pressure would not be the same on both sides, unless there is air in the door close line (top tee) going to the outside of the actuator but normally it should self purge.

I was wondering where the split in the hydaulics was, so thanks for that inofrmation. I suppose the line or actuator or the tee you mention could have a clog or obstruction, so maybe that is worth checking. Related question...what holds the doors closed? Is it just the built up pressure from the handle movement that holds? I ask because I wonder if I am losing pressure somehow, but of course that would not explain why all the doors are not just falling open. Again, any thoughts or theories here are appreciated. Thanks.

Bill

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.

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.

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.

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?

Update
 

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

Did it turn out to be the Time Delay Valve?

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?