Bent retract rod

[A64Pilot]

I think the NBS works both ways, it seems if it breaks nothing can get the gear down, so it’s used it seems in both directions

[Shadrach]

40 minutes ago, A64Pilot said:

That’s been my understanding too, sort of a mechanical Chinese finger trap, free running in one direction, but maybe that’s incorrect?

But IF the rest of the retract system is very similar and the J bar requires a down lock to keep from collapsing, wouldn’t the electrical system also require one? If not, why not?

I’m not disregarding the overcenter mechanism / springs in this, I think they play a large part as the tubing itself clearly isn’t up to the task in compression all by itself, but it seems that you need both, the overcenter plus something that locks the movement of the tubing. The gear need that spring pressure to keep from collapsing it seems. IF the actuator didn’t have something that locked it in place then that spring pressure could be lost?

Again, I’m not stating this as fact, many times I’ve come up with what seems to be logical theories only to find they were way off base because I didn't have all the data, an assumption got me.

 

Down lock holds gear over center. Actuator does the same.  I have heard that a little slop in the jack screw will sometimes allow just enough play in the the limit switch to activate the actuator a fraction of a turn when the master is turned on.    I do not believe that gear loads transmit to the actuator or J-bar unless it's out of rig or not over center. 

 

[PT20J]

The actuator stops in the down position at a point where the preload is correct. There is a spec on max preload when extended electrically to account for actuator overrun. The over center mechanisms at each of the three gears are the down locks.

The no-back spring prevents the gear from back driving the actuator when the gear is retracted, so it can be thought of as the up lock.

[N201MKTurbo]

One way to bend those rods is during an emergency extension. If you don’t stop when the gear down light comes on or the floor indicator is lined up, you can easily bend the rods.

[PT20J]

1 hour ago, N201MKTurbo said:

One way to bend those rods is during an emergency extension. If you don’t stop when the gear down light comes on or the floor indicator is lined up, you can easily bend the rods.

I believe this is true for the Dukes actuator. The Eaton (and I think the Plessey) have mechanical stops on the actuator so over driving with the emergency extension pull cord will jam the ball screw nut against the stop but not bend a rod. Likewise, if the gear down limit switch fails on a Dukes, I believe the actuator may well bend the rods.

[N201MKTurbo]

5 minutes ago, PT20J said:

I believe this is true for the Dukes actuator. The Eaton (and I think the Plessey) have mechanical stops on the actuator so over driving with the emergency extension pull cord will jam the ball screw nut against the stop but not bend a rod. Likewise, if the gear down limit switch fails on a Dukes, I believe the actuator may well bend the rods.

I have very little experience with the Eaton and Plessey actuators. I would love to take one apart some day.

[A64Pilot]

23 hours ago, Shadrach said:

Down lock holds gear over center. Actuator does the same.  I have heard that a little slop in the jack screw will sometimes allow just enough play in the the limit switch to activate the actuator a fraction of a turn when the master is turned on.    I do not believe that gear loads transmit to the actuator or J-bar unless it's out of rig or not over center. 

 

If what you say is true, then the gear collapse on J bar Mooney’s have nothing to do with the locks and there is no need to replace a worn lock. It’s a rigging, not a worn lock problem.

If an over center mechanism is over centered enough then a force attempting to collapse the system just tries to force the system further over center.

I don’t know if our gear is that far over center or not. I suspect it may not be or there wouldn’t be an issue with J bar down locks, and I believe there is.

I believe your first two sentences in the above quote to be correct.

On edit, if something didn’t prevent the actuator from being back driven, then it could back off the pressure holding the gear over centered, once past center it will of course collapse bending rods to do so.

 

Edited September 13 by A64Pilot

[Shadrach]

46 minutes ago, A64Pilot said:

If what you say is true, then the gear collapse on J bar Mooney’s have nothing to do with the locks and there is no need to replace a worn lock. It’s a rigging, not a worn lock problem.

If an over center mechanism is over centered enough then a force attempting to collapse the system just tries to force the system further over center.

I don’t know if our gear is that far over center or not. I suspect it may not be or there wouldn’t be an issue with J bar down locks, and I believe there is.

I believe your first two sentences in the above quote to be correct.

On edit, if something didn’t prevent the actuator from being back driven, then it could back off the pressure holding the gear over centered, once past center it will of course collapse bending rods to do so.

 

You make a good point. I think it is often a combination of issue when a J-Bar fails. I also think it is exceedingly rare.  Some of these "failures" may be the Electric gear equivalent of the putting the gear switch in the down position after a gear up landing.  There may be some who have released the J-Bar from the up lock block after a gear up, and then claimed the down lock failed. 

I think most actual failures occur when the bar is not fully seated in the lock. Even a severely "egged out" lock will have substantial engagement from the thumb lock (see image). There is some resistance in the system as the J-bar goes over center into the down lock. It's not a lot but on undulating terrain or during a hard landing, it could dislodge if it's not seated.

It only takes one gear leg being out of rig to pull the rest out of over center and cause everything to retract if the down lock is flakey.