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  #1  
Old 07-27-21, 09:03 PM
bjornfb bjornfb is offline
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Question about Skymaster landing gear system

Unfortunately for me, my 1976 P337 suffered a nose gear collapse during taxi tests during its annual :( Fortunately the shop's insurance is paying for all the repairs (new propellor, prop strike engine teardown, sheet metal, etc); the unfortunate aspect is that it's going to be three months (or more) until I get my airplane back...

But I have a question about the incident: the shop wrote up a root cause analysis for their insurance company and I'm curious about it, hence my question for you all: I'm puzzled that the downlocks wouldn't hold even if there is no hydraulic pressure -- I thought the point of downlocks or over-centered construction was to stay down even if there is no hydraulics.

Here's what they said (abbreviated slightly):
Identified circuit breaker ‘LG MTR’ for the ‘Landing Gear Motor’ was ‘tripped’. With the pump motor de-energized, the hydraulic pressure in the landing gear system would not be maintained at operating pressure and would have decayed over time, resulting in the landing gear doors to begin to open and the nose landing gear actuator to unlock during taxi. Once the actuator was unlocked, the movement during taxi caused the nose landing gear to collapse.

... the circuit breaker had been manually pulled to the ‘OFF’ position during maintenance activity four (4) days before the occurrence and not reset. ...
So, in your collective experience and wisdom, is there a problem with the downlocks? I.e. should the downlocks have stayed locked even without hydraulic pressure and thus this collapse should not have happened? I care about the answer because if there's a problem with the downlocks, then the landing gear system has some problem that needs to be fixed (in addition to all the damage).

Thanks for your knowledge and advice,
- Bjorn
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File Type: png collapse.png (413.2 KB, 95 views)
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  #2  
Old 07-27-21, 10:43 PM
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mshac mshac is offline
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Bjorn,

I'm sorry about what happened to your plane. I bet a prop strike on a composite prop is much easier on a motor than a metal prop strike, so I bet your engine is OK.

There's a hydraulic downlock actuator for the main gear, but not the nose.

I've got no idea about the nose gear's mechanical or over-center downlock (if there is one), but I'll be watching this thread for someone much smarter to explain the nose gear system to us!

Last edited by mshac : 07-27-21 at 11:33 PM.
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  #3  
Old 07-28-21, 08:57 PM
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Skymaster337B Skymaster337B is offline
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There is an internal locking mechanism inside the nose gear actuator. It can only be released by hydraulic pressure. That’s assuming it was set correctly. The small jam-nut at the end of the actuator rod is all that keeps the internal rigging set correctly. It’s a good design but also intolerant of faulty settings and lack of understanding of its operating principles.

I always check that internal lock when it’s on jacks. Manually push the nose gear down (into locked position) and listen to “click” of it locking. Then check the external down lock hooks ensuring the pins are fully seated.

Last edited by Skymaster337B : 07-28-21 at 11:20 PM. Reason: Added words to clarify.
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  #4  
Old 07-28-21, 09:19 PM
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Skymaster337B Skymaster337B is offline
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Jam nut pic.
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Old 07-29-21, 12:06 AM
wslade2 wslade2 is offline
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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.

Last edited by wslade2 : 07-29-21 at 01:12 AM.
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  #6  
Old 07-29-21, 12:23 AM
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mshac mshac is offline
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I don't understand how the locking mechanism works. I've attached what I think is the correct diagram. Perhaps you would explain further?

FURTHERMORE drooping gear doors are a symptom of the gear accumulator needing service! It has nothing to do with the actuators (usually).
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File Type: jpg Nose Gear Retraction & Drag Strut Actuaor Assembly.JPG (106.1 KB, 85 views)

Last edited by mshac : 07-29-21 at 12:30 AM.
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  #7  
Old 07-29-21, 12:52 AM
wslade2 wslade2 is offline
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I purposely used the phrase "interesting geometry spring lock mechanism" as to try and manually operate it, it is not completely straightforward. But it is stout. The locking part is easy enough. The cylinder pulls the top of the trunnion forward. Spring loaded hooks on the end of the rod that ride with the trunnion engage horizontal pins on a "fork" which is part of the end of the actuating cylinder. Since the other end of the cylinder is anchored, this rigidly locks the trunnion in place. "Interesting geometry" comes from I could only get it to unlock by either "the jam nut end" pushing on the mechanism from within or by taking the spring lock mechanism apart, unhinging it from the side with bolt removed, spring flying off and everything. Someone knew what they were doing when they designed it.

Forgive me for using doors (pleural). I did have a drooping door (singular) which was weak cylinder. However, there is only one actuating cylinder operating both of the front wheel well doors. So one bad cylinder there could cause both of those doors to droop. As always, investigate thoroughly and properly. These are just personal experiences and not a repair manual. (But I try to be as correct as possible.)

Last edited by wslade2 : 07-29-21 at 01:06 AM.
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  #8  
Old 07-29-21, 06:28 AM
wslade2 wslade2 is offline
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I have tried to upload some pictures. Hopefully it works.
you will see the hook mechanism engaged with the end of actuating cylinder. The bolt hole is for a bolt that goes through the mechanism into and attaching to the trunnion. The rod (with the lock nut) is what moves with the hydraulic pressure. You can see the plastic spring guide.
Even off the airplane with the bolt out it is still locked on. That's what I consider interesting geometry.
Attached Images
File Type: jpg nosegear lock 1.jpg (405.3 KB, 83 views)
File Type: jpg nosegear lock 2.jpg (391.2 KB, 81 views)

Last edited by wslade2 : 07-29-21 at 06:44 AM.
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  #9  
Old 07-29-21, 01:34 PM
bjornfb bjornfb is offline
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Thanks! This is all very helpful. I forwarded the whole thread to the shop and I'll report back on the progress. Thank you.
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  #10  
Old 07-29-21, 09:46 PM
Kim Geyer Kim Geyer is offline
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I’m calling BS on the cause of the collapse. Hydraulic pressure will and does hold the down lock hooks over center but they are also spring loaded and once they snap into place it is almost impossible for it to unlock. 337b is correct about the actuator having a internal lock but later model 337’s didn’t have that actuator. I’ve resealed and rigged both types of actuators this year. I have seen the hooks get dirty and sticky with hyd fluid and not lock well but you will get a warning horn.
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  #11  
Old 07-29-21, 09:47 PM
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Skymaster337B Skymaster337B is offline
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The real magic of the downlock system is on the opposite end of jam-nut rod. It is an internal mechanical lock with checkballs and a spring. Only hydraulic pressure can unlock it. You have to disassemble the entire actuator to see it. If those internal checkballs aren’t locked it is possible that enough force could overcome the hook mechanism.

The nose gear is held down by three separate mechanisms.
1. Internal checkball system
2. Over center hook
3. Hydraulic pressure of the actuator

If any one or two doesn’t do it’s job it’s possible it could collapse.
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