Question about Skymaster landing gear system

[wslade2]

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.)

[wslade2]

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.

[bjornfb]

Thanks! This is all very helpful. I forwarded the whole thread to the shop and I'll report back on the progress. Thank you.

[Kim Geyer]

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.

[Skymaster337B]

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.