The darn GEAR, darn it!

[billsheila]

I have been struggling with a gear problem on my 1965 Skymaster, serial #31

The power pack was changed at annual due to evidence of a hydraulic leak. Turned out the leak was a stress crack in a hydraulic line near the pack (and not the pack itself) but they changed it anyway because it was original equipment.

Question is this for those gear experts out there, as follows:

When the handle is up and gear goes into the locks, is pressure supposed to be kept on the gear even after they go into the up-locks (ie while the doors go into motion to close)? Mechanic has determined (by disconnecting the doors) that after the gear go into the locks it appears that pressure is staying on the gear while the gear doors go into motion...the extra "up" motion is making the switch disconnect, which then re-opens the doors as the switch contact is lost and the door solenoid on the pack drops out. Without the doors closing long enough to work the time-delay valve, the handle never moves. When I manually move the handle to neutral the gear relax into the locks and then make proper contact with the switch, thus the light comes on (but with no hydraulic pressure, the doors remain hanging open).

The answer to the question will help determine whether we have a simple rigging problem with the switches, or whether there is something wrong with the replacement power pack that is not sequencing properly (keeping pressure on the gear while the doors go into motion to close)

Any help would be much appreciated.
Thanks

[Pete Somers]

1. When the gear is up is the orange light ON.
2. When you put the lever in the center position is the orange light ON.
3.Does this happen on gear extension.

The gear handle should return to it's center position upon gear retraction the doors operate as a result of the gear being up and locked. So the gear is resting on the uplocks when the handle is center.
If you push one of the legs up and the doors open you must have a problem with the rigging of that switch, (the orange light will go out).
The door solenoid will not operate until you have either a orange or a green light on as this shows the gear is where it should be.

So you need to check the HYD pressure when the gear is in travel and when it is in the uplocks as you should have a pressure rise to put the gear handle to center. (what is your mechanic using to cycle the gear)

Check the handle micro swiitch and make sure that is ok.
Then check the uplock switches to see if they are ok, they should not have changed as I presume only the power pack was replaced.

Regards
Pete

[billsheila]

But let me try to answer your questions as briefly as I can:
1) the light is intitially out when gear are in transit (of course) it comes on momentarily when the gear go into the home (but then goes out), the gear doors swing shut and then immediately swing back open. Gear stay up and in their locks, door stay open. Handle doesn't return on its own, but if you manually pull it to centre, then the orange lights comes on steady and bright.
2) micro-switch has been carefully rigged and we don't think that is the problem
3) power pack was the only change but the gear were behaving badly before the pack change (intermittent problems with handle not returning) so I thought it was related to the obvious hydraulic leak at the pack that I was having...which as I said actually turned out to be a line rather than the pack
4) mechanic has used both just the hand pump and a separate mule (no pressure gauge on the mule, but I guess he could rig that up if we need it) during this long saga diagnosis.
5) gear and doors work as advertised on down cycle, either on jacks or in the air (thankfully!)

The description in #1 above is what happens in the air on up cycle. On the ground with both the hand pump or mule it has seemingly worked as advertised. Latest diagnostics have involved disconnecting the doors so they can see what is happening. This has led to the question about whether the main gear are supposed to be hydraulically loaded or not while the doors are in motion and before the handle moves. My understanding is that on these early planes the final handle movement arises from pressure build up after the doors have closed and are "forced" against their stops on the airframe...pressure builds in the "time delay valve" until it rises sufficiently to move the handle.

Personally I am convinced we have a rigging problem with the main gear up switches. They seem to work on the ground but not in the air. Perhaps slipstream air is pushing the switch or the gear or both and we are losing connection. My mechanic is focused on whether something is going wrong within the new (rebuilt) power pack that is hydraulically wrong, which is causing the wacky action of the switch.

I guess I wasn't exactly brief!

[skymstr02]

You may want to check the rigging between the gear select rod to the powerpack. A lot of times there is some play in the cross shaft that can be compensated by shortening the length on the clevis fork by a turn.

[Pete Somers]

OK here we go -

1) Suggests the handle microswitch is either faulty or not rigged properly.
But it could also mean that once the gear is unloaded from the the uplock, ie the handle returns and the gear rests on the uplocks that one of the uplock microswitches is intermittant.

2) Doors will not operate until all the micro switches are made, the door solenoid is operated thru the gear handle microswitch if you have the doors open power must have been removed from the door solenoid so the only thing that can cause that is a uplock microswitch or the handle switch.

3) The handle will not return to its center position until the HYD pressure has built up and the doors are closed.

Yep it does look like one of the uplock switches, with the cam shell doors disconnected pull down each main leg and see what happens, if ok go to the nose gear and do the same.
Check the wiring to the nose gear as it get worn due to the movement of the leg.

BTW the clue is that if you put the handle manually to the center position what happens, you unload the HYD and you move the handle switch.

Good luck and let me know.

Regards
Pete

[billsheila]

Well I went up to the shop and me and my mechanic went through a few cycles. Now that I see the motion up close, I can see just how finnicky the switch set up is. When the mains go into the locks and when the system is still pressurized hydraulically it puts quite a bit of extra upward pressure on the saddles and walks the gear leg away from the switch. With the mule it was not quite enough to make it deactivate the switch, but it was pretty close. We wonder if with more pressure from engine driven pump (versus mule) plus some slipstream, if this is de-activating the switch and causing the problem. The rigging of the mains in the saddles was checked and this was within spec. He brought the switch in closer so as to make it less likely to de-activate with upward pressure on the saddles. But the tradeoff is that when the gear unloads hydraulically and the legs settle into the saddles, you don't want too much inward pressure on the switch for fear of crushing the switch. There is a suggested 1/8 inch for rigging this (ie when unloaded). So basically we re-rigged the switches. It works fine with the mule (but it always did) and with the hand pump. Not enough time to get it back together and down from jacks, so that is the next step.

We still want to try to find out if it is correct that mains should remain pressurized while doors go into motion. There is nothing in the manual clearly spelling it out, so perhaps a phone call to the pack rebuilder can confirm.

[Skymaster337B]

The answer to the question: "does hyd pressure remain on the gear after it is up, when the doors start to close," is no! Look at the service manual for 65-73' figure 5-48. It shows "static pressure" in the main gear actuator when the doors are closing under pump pressure...static pressure means whatever was in the line when the priority valve switched over so the doors could close at a lower hyd pressure. So, the engine driven pump isn't holding the gear in the up position after they are locked up. But in theory, trapped hyd fluid (static pressure) does resist the gear from falling down.

By the way, how much did a "new" hyd pack cost?

[Pete Somers]

The main trouble is that these are more than likely original switches and they have springs on them an the springs loose there tension and cause you to have to put the switches a bit nearer to the operating arm so that they can work. Unless they were replaced they are 48 years old and have gone through a lot of abuse in there life with oil, grease, water and ice.
So if you have a problem with these switches it pays to have all the up and downlock switches replaced and rigged.
See what happens when you test fly it.

Regards
Pete

[billsheila]

By the way, how much did a "new" hyd pack cost?
ANSWER: $1500 plus $1000 core

Thanks for the clarification re the pressure, obviously me (and my mechanic!!) are not reading the manual carefully enough. But to save me some reading (manual is up at the shop anyway, so I can't reference it at this moment) and perhaps lots of mis-interpretation which I am prone to..a few more questions if you can be patient with me:

What tells the priority valve to move, and assuming it moves and does something other than load the mains while the handle is still up (ie pump is loaded) where does the pressure go? Does it just come off the mains and reside in the pack waiting until the door solenoid activates the door hydraulics? I believe the only electrical action going on with the pack (on these early models) is the door solenoid.

It seems to me all this stuff happens pretty quickly, so doors open, locks open, gear move up, switches make momentary contact, but then due to the rigging issue already noted move past and re-open (with the light coming on for this split second, this actuates the door solenoid allowing pressure into the door hydraulic circuit), with pressure, the doors close but as soon as they hit the airframe, a bit of pressure builds and slides the door valve to the other side saying "open" because of the solenoid dropped out when the switch was lost, therefore doors immediately reopen. Handle never moves to neutral because this is a function of steady pressure build up with closed doors, and the time delay valve, which we are never getting to in this sequence of events.

If the above description is mostly right, it explains most if not all of what has been observed in flight, with the exception of perhaps a few things or a few remaining questions:

If the main gear are not remaining loaded (ie just residual pressure on the system) why is it when the handle is manually moved to neutral (during an in-flight test) do I get a steady light, but not when the handle is in the up position? NOTE: with the mule or the hand pump on the ground, we do get steady up light (with handle up) once mains go into the locks. So one additionaly theory is that the engine driven pump is putting out more pressure than the mule or the hand pump, forcing mains up a bit further and we thus loose the switch. But this theory only holds if the pressure is somehow remaining on the mains, or perhaps it is just higher residual pressure? But again if it is just higher residual, why in flight do we get a light when the handle is returned to neutral.

The reason for our original confusion and question was to determine if we have some kind of fault in the pack internal hydraulics (which remember is only happening in flight with engine driven pump doing the work) or whether we can isolate the problem to switches and electrical.

Thanks very much for your patience and help with this persistent, frustrating, (and by now quite expensive) problem.

Bill

[Pete Somers]

Bill
The gear up cycle is---
1 Gear handle to the up position which takes the power off the door solenoid and the doors open.
2 HYD pressure is supplied to the downlocks and unlocks the gear and the gear downlock switches open (very important)
3 Gear swings and HYD pressure is supplied to the gear jacks and the gear is lifted into the bays.
4 Gear lifts into the bays and passed the uplocks and pressure takes the gear up to ensure it goes passed the uplocks, the uplock switches should be closed to put on the amber light and pressure should put the handle back to center which takes the HYD pressure off the gear and the it hangs on the uplocks and power off the door soleniod which closes the doors.

Weather you put the gear up or down, power has to be removed from the door solenoid to close the doors, so any open circuit in the uplock circuit will cause the doors to open after retraction, it will also happen with the gear down as any open circuit will cause the doors to open as power is removed from the door solenoid.

As for the handle going to center, if is works in flight on a down cycle but not on the up cycle the time delay valve and priority valve must be ok.

Maybe this power pack being OHC has a better flow through it and the valves operate a little bit better than your old unit, the better flow may mean a higher pressure at the gear actuators which raises the gear a bit faster into the uplocks causing the switch problem.

But I do think you should replace these switches as they will cause you the same problem later on.

Just my views.
Regards
Pete