172 RG gear problems

172 RG gear problems

A friend of mine owns a 172Rg and the left main gear is not locking in the down position. The retraction system works fine on the ground. The problem is when flying the air pressure working against the left gear prevents the downlock from engaging. You can see this when applying pressure against the gear during a retraction test. After the pump switches off there is no green light and you can pull the gear forward and it will snap into the locked postion with a green light. It appears as if there is not enough hydraulic pressure to push against the wind and go to the full down position. The system works fine when retracting. The pump is supposed to shut off at 1500 PSI, and what i am wondering is that maybe the left gear actuator is binding causing the pressure to build up before reaching full travel. In a system like this shouldn't pressure be equal through the whole system? Could a weak actuator cause this to happen or do you think that it is something else?

Any ideas would help, thanks.

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Re: 172 RG gear problems

There are hydraulic actuated "gear down" locking devices located below the seat deck for each main gear leg.  I would remove the front seats and look under the floor to see what that item is doing. There is pleanty of pressure to move that gear against the airstream. That is where I would start - going from your description of the problem.  (the actuators are located toward the rear of the front cabin - on each side)
jack.

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Re: 172 RG gear problems

Jack, Im not too familiar with this gear system but Allen mentioned that pump switched off even before the green gear down light illuminated.  I would think the motor would continue to operate untill safe gear down indication.  Just my 2 cents...

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Re: 172 RG gear problems

Yea, there is really no way to troubleshoot this without doing some specific checks with a multimeter..  There are some switches in the downlock curcuits that I would want to check, also the pressure switch control on the powerpack. I would, for sure, put the aircraft on jacks and see what the emergency hand pump does with this situation.  It could be, and very unlikely, that the system is low on fluid.  The powerpacks are really well made so unless it has a bunch of cycles, I wouldn't suspect it until everything else was checked out that has an effect on its operation..

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Re: 172 RG gear problems

The downlocks are spring loaded into the locked position. Hydraulic psi unlocks the hooks before routing to the actuators. All the switches are working fine. I can see that the left downlock is not grabbing the strut. On jacks the gear comes all the way down and locks. It does not lock in flight or when you manualy push against the strut while putting them down (similating air pressure). You can see that the downlock hook is at the apex of the lock, and by pulling it forward it snaps into place. The downlock springs have been replaced. Hard to explain. 

Thanks, Allen

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Re: 172 RG gear problems

While on the jacks - can you hand pump the gear down and into the locked position?  This would be another hydraulic pressure source to try. The hand pump emergency extension system it is designed to provide enough pressure to lock the gear down and give you the down and locked gear indication light.  if this turns out to work ok, then you can probably assume that the powerpack is not providing enough pressure - and go from there...good luck and be careful with that airplane up on jacks..

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Re: 172 RG gear problems

Well we found out what was the problem. The left actuator was taken apart a year ago and was put together incorrectly. The end cap was screwed down too far and this caused the piston to contact it sooner than it was supposed to, restricting the travel but still building up the pressure. The cap was supposed to be flush with the cylinder and it was below the edge. Now works like a charm.

Allen

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Re: 172 RG gear problems

That could be expected...I suppose it was removed for that housing (crack) inspection  AD that was later resinded..what a boondoggle that was..

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