As part of my "1 yr annual" we felt it necessary to go over the nose gear system. My concern being a nose gear fold after so many hours invested and that no one had looked at it in who knows how many decades.
So, we removed the nose gear (trunnion, wheel, oleo and rebuilding those) and nose gear hydraulic actuator which has the nose locking mechanism attached to it. I got to examine, play with and send it out for overhaul. Not to claim I'm an expert, just have been over this.
An aside, 337 system has similarities to the 210 system with some shared parts.
Hydraulic pressure pulls the nose gear into position at which point an interesting geometry spring lock mechanism clicks a pin on the nose gear fork/trunnion and locks it down. Hydraulic pressure is not used to keep it there. When you select "gear down" mode it just pulls the system into the lock position. A spring lock engages keeping the gear locked down with no need for hydraulic pressure staying locked down, forever until until the actions to unlock.
As already mentioned, it takes hydraulic pressure from the piston to unlock this mechanism. It will stay locked until pushed from within when you select "gear up". When you select gear up, hydraulic pressure pushes the "jam nut" end to unlock this mechanism. It can't unlock except with positive hydraulic pressure. I know, I tried to do it and could only disassemble the mechanism to get it open when I was without hydraulic pressure. Hydraulic pressure only pulls it into lock or pushes it out of lock (and of course then moves the nose gear up or down). But positive lock is a spring loaded mechanical process.
There is no connection of this nosewheel locking mechanism system to the gear doors. The gear doors are simply "sequenced" to open or close with proper timing. As have been mentioned in other threads, drooping doors can be a sign of weak actuating cylinders as hydraulic pressure does keep the doors closed or open.
A couple things I learned as I sent my nose gear actuating cylinder out for "a just in case" overhaul. First there is a service bulletin regarding the locking pins of this spring lock mechanism. The pins can crack and then fail at which point the nose gear becomes unlocked. As part of my overhaul I was informed of this and reassured (as well as documented) that updated pins were installed when I got my actuator back.
Here's a link to the bulletin:
https://support.cessna.com/custsupt/...df?as_id=22795
The other thing I learned is that the spring part of the locking mechanism is held in place by a "spring guide." I was told apparently at one point this guide was all plastic. It was later updated to have metal reinforcement. I have not found any definitive documentation on this but sure enough when we pulled out my OEM cylinder it appeared to be all plastic. Again new installed. Of course if the plastic becomes old, the spring is no longer securely in position, could "migrate out" and "loose lock".
As I said, there is commonality with the 210 system and I have over time found this paragraph in an AvWeb article for 210 owners:
"IS PLASTIC KEEPING YOUR NOSE UP? Your nose gear has a little “downlock spring guide” to retain a spring that keeps tension on the nose gear downlock hooks. When Cessna originally built your aircraft, they installed a spring guide made entirely of plastic, with two plastic pins that fit into holes in the downlock hooks. These plastic pins have a tendency to break, and this can result in the downlock spring falling out and leaving no tension on the downlock hooks. Taxi over a bump and the nose gear could collapse. Ouch!
Cessna came out with an improved guide, P/N xxxxxxxx, which has steel pins instead of plastic ones. All 210s were manufactured with the all-plastic guide, so unless you have a log book entry that shows installation of the improved part, your aircraft is in jeopardy. The new part costs about $15 and takes about an hour to change. It’s a very small price to pay to avoid a costly nose gear collapse.
John Frank
January 9, 1996
AvWeb"
So, I submit that your mechanic de-energizing the system is not responsible for the nose gear fold. It actually takes pressure from an energized system to unlock it. "Pressure decay" has no effect on lock. Yes, the nose gear locking mechanism needs to be examined or overhauled. (Or you could go through this all again.)
And if your gear doors drifted open, actuating cylinder there needs to be looked at.