Gear doors won't close on ground...

atsiii · #1 09-22-08, 10:41 PM

This is a '68 T337C. My a/c just got out of annual, during which the gear swung fine. In prep for the first flight after annual, I opened the gear doors for inspection by using the emergency hand pump. However, when I turn on the master, the doors won't close when pumped. Does anyone have any ideas or things to check? I've cycled the gear handle from down-neutral to down and back several times to no avail.

I'm inclined to fire up the engines and try cycling the gear handle again, but I wonder if that could hurt anything if a solenoid was stuck. On a previous T337 I once had a similar intermittent problem of the doors not closing, which turned out to be the micro-switch at the hydraulic pack. But I've never had a problem with the doors on this a/c, and it seems strange since the a/c has not flown (or even started) since its annual. What thinks ye?

Thanks in advance!

Allen
atsiii@ceoexpress.com

Pete Somers · #2 09-23-08, 03:08 AM

I think the first thing you do is take it back and get the engineering to repair it, after all part of the annual is to cycle the gear.
You have a green down light on?
The gear solenoid in the HYD pump is stuck or the wiring to the door solenoid is bad.
Let me check the diagrams and I will come back to you

Pete

Pete Somers · #3 09-23-08, 09:16 AM

Looking at the diagram this looks like you have a sticking door solenoid valve or a wire broken on the solenoid itself, need need to check if there is 24 volts on the solenoid, if so the problem is the power pack, if not check the wire from the gear handle to the solenoid.

Pete

skymstr02 · #4 09-23-08, 09:49 AM

The same thing happened on my O-2A, and it turned out to be the microswitch on the powerpack that controls the solenoid. The switch mimics the solenoid troubleshooting, and this particular switch is not called out in the airframe wiring manuals, but is in the illustrated parts listing.

atsiii · #5 09-23-08, 09:52 AM

Thanks, Pete! Yes, the gear down light is on. The gear did swing fine during the annual, multiple times. The a/c was then towed back to my hangar, sat for a couple of weeks until I had time to put it back together, and then, after replacing the inspection covers, the carpet and seats and getting ready for the post annual shake down flight, I pumped the doors open for inspection and they would not close.

I'll pass on your suggestions to my IA. Question: Is the door solenoid valve attached to and located on the back of the hydraulic pack above the pilot knees?

If so, I can't see how to get in there and check voltages or to look for a broken wire. If stuck, I read that I should hold a steel ruler next to the solenoid to check for a magnetic field, but I could not see how to get in there and do that, either. There are three wires to a micro-switch on the front of the pack, but I can't find the wiring diagram in my service manual to see what these wires should be, nor track them to the back of the pack.

Since the gear was swung during annual, I thought maybe I had done something while putting the carpet back in. But if the door solenoid is up by the hydraulic pack, I didn't replace anything in that area. FYI: after the a/c sits overnight, the emergency hand pump pressure drains off, but then with the master on or off, and the gear handle down or down-neutral, it only takes a partial pump or two for the handle to be firm with pressure again.

Thanks again! Let me know if you think of anything else to check. Question: do you think if the valve is just stuck, that I should fire up an engine and try cycling the the gear handle between down and down neutral?

Allen

atsiii · #6 09-23-08, 10:04 AM

Thanks Skymstr02! Back in 1999, that same micro-switch had to be replaced on the '69 I was flying then, so I was suspect of it. It's a 3-wire switch but I can't find a diagram to know what each wire should be. I still have the '69 as a parts plane, so maybe I should just change it...

skymstr02 · #7 09-23-08, 12:16 PM

I just bought the switch from Yingling for $30, they had it in stock. There are actually four terminals to this switch, and the actuator button is in the center, which makes this unique. Two of the terminals are wired together so this makes this a single pole, double throw microswitch.

Take the white plastic drain tray off from under the powerpack, and you will see it towards your instrument panel. There are three wires going to a terminal board on the bottom of the powerpack, so you can take the switch and wires off as a unit.

stackj · #8 09-23-08, 04:31 PM

I am attaching a scanned diagram given to me by a friend. This looks like part of a text from some mechanics school who teaches Skymaster systems, but I can't find any reference on this package to either ask permission or credit them. Anyway, I'm chancing making this available because it is a much simplified diagram of the gear door permissive systems... Here it is. When you zoom in on the picture the text is more readable.

The Key to remember. If power is off at the Landing Gear Door Solenoid and Hydraulic pressure is applied, the doors will open.
If power is on at the same solenoid and hydraulic pressure is applied, the doors will close.

atsiii · #9 09-23-08, 05:03 PM

Thank you, Jim & Skymstr02!

Bandit 153 · #10 08-16-10, 10:34 AM

1974 G: Gear raises and doors close on retraction and amber light illuminates; however when gear down is selected gear extends and locks but gear doors do not close. Green light does not illuminate and gear warning horn sounds with throttles retarded. Power pack motor runs for 30 seconds and then circuit breaker pops as it should. Anyone know what is causing this? Micro switches have been checked good. Thanks.

stackj · #11 08-17-10, 09:51 AM

Bandit,

I do not have diagrams for your particular model, however all the diagrams for the models I do have indicate that the gear door circuit works as follows:

28 VDC Power from the circuit breaker goes to the Nose Gear downlock switch, then to the Right Main Gear downlock switch and then to the Left Main Gear downlock switch. The output from the Left Main Gear switch is fed both to the Green Light and the Gear Door Up/Down switch.

Your indication that the green light does not turn on indicates that the circuit has failed somewhere between the circuit breaker and the Gear Down (green) light.

You should be able to trace power by turning on the battery switch while on the ground and testing for 28 volts at each of those switches (both sides of each switch).

The circuit diagrams on later model aircraft appear to have some connectors in the circuits in the vicinity of the main gear. The parts list I have defines them as "housings", but the diagram uses a connector symbol. You may want to look for these. If they are connectors any one of those could cause the problem.

It is also possible that a switch just isn't quite in the right position to close when the gear is in the locked down position even though the switch is electrically good. The voltage checks defined above would also reveal this problem.

Good Luck!

Dale Campbell · #12 08-18-10, 09:50 AM

I just replaced my 2 Main gear down lock switches. They gave me a lot of problems with intermittent gear doors failing to close. I checked the resistance with a meter across both switches and found the resistance to be about 100 ohms when it should be zero. I think the inside of these little mico switches get very dirty and the contact resistance goes up until they fail. After changing these 2 switches all my gear failure problems disapeared. Dale