Gear "bouncing" at the top of the cycle?

[Quint Van Deman]

Comanche community--

Good old N7406P has developed a new quirk - one that I've not been able to find referenced in any of the tip manuals or other forum posts - so I'm hoping someone here might have some tribal knowledge.

Essentially, when I bring the gear up, everything operates just as expected, bar goes to the floor, and the light comes on. However, right in that exact instant - instead of staying there (so that I can move the gear selector to neutral) - the gear does what I can only describe as "bouncing". It extends just a bit (I'd estimate that the bar raises 1 inch off the floor) and then tries to retract again, the up indicator flashes on just for a split second and then it repeats over and over. Not wanting to make matters worse, I put the selector into neutral to stop the up-down cycle - at which time the gear partially extends back down - in the mirror I see the tops of the tires hanging out and the bar is now about 4 inches off the floor.

Does anyone have any ideas and/or heard of anything like this before?

Much appreciated,

-Quint

[Timothy Poole]

Sounds like your up limit switch might need to be adjusted.

[Quint Van Deman]

The shop I'm working with (owner is a former Comanche owner) is pretty confident that the limit switches are correctly adjusted - but what's the best way to verify their adjustment?

Much appreciated,

-Quint

[Kristin Winter]

It could be a problem with the uplimit switch. From your description, it would probably be internal to the switch. ComancheGear can sell you a replacement.

However, the first thing I would check is the brake. If that is not operating properly, it could give you the symptoms you describe. If it is the old style gate-type, rather than the plunger, you might consider replacing that as well. Either way, call Matt Kurke at ComancheGear. No one knows that system better than Matt.

[William Hughes]

There is also a clutch, or brake, on the gear drive motor. If that is worn out, and it *is* a wear part, then when the motor stops the motor could back slide a bit. That would release the upper limit switch and it would engage the motor to bring it up again, and then when the motor stops it winds back a bit, and the gear drops, and the cycle repeats.

I'd check that brake on the gear drive motor first, myself.

edit -> What Kristin said! I should read to the bottom before posting.

[Zach Grant L1011jock]

If it is jumping down like it is jumping a cog on a gear, you probably have much more serious problem than a maladjusted switch. I would certainly look at the gear transmission, and if it is jumping that much, you probably have a worn ball screw or jack screw, or you could have a blown out transmission housing that is allowing the thrust bearing to move back and forth and when it gets to up it is shifting back under the load. I suggest grounding the aircraft until you can get the airplane on jacks and pull the transmission out for disassembly and inspection.

-Zach

[Quint Van Deman]

Thanks everybody for the tips - the initial word came back from the mechanic today with a diagnosis of a "broken spring on the motor brake assembly". I'm going to remain hopeful for now that it turns out to be that simple.

Zach, these various movements (while unexpected) are always smooth and at the speed of normal gear movement - they are not sudden/violent like my mind envisions gears jumping a cog - does that make sense? Would what you described below occur in that more dramatic fashion?

Regardless, the plane is certainly grounded until all checks out on jacks & in the pattern.

Cheers & thanks again for the tips,

-Quint

[William Hughes]

That sounds like the issue. When the power comes off, unless the brake engages, the motor would work back. Even though there is a worm drive the vibration on a well lubricated gear system will let the gear work down. The limit switch would eventually engage and bring the gear back up. Leading to the pattern of behavior you describe.

[Quint Van Deman]

Thanks to everyone for your replies & suggestions - to close it out (in the hopes that it might help someone in the future) the motor brake spring seems to have been solidly convicted as the culprit. 4 flights in after the repair & nothing but a welcome yellow light at the top.

Thanks again for the tips, it makes owning an old bird so much more doable with the community of ICS.

Cheers,

-Quint