Gear door malfunction

[Mark Ashiku]

I fly a 1965 Skymaster 337 (normally aspirated, front engine mechanical hydraulic pump) and had a gear door issue that recently occurred.

Briefly, on run-up as per the POH (gear handle in the down position with green light) all systems were normal. Upon retarding the throttle the gear warning horn sounded, the green gear door indicator light was off, and the gear indicator circuit breaker had tripped. Once parked and inspected the following was noted:

• All gear doors were open.
• Landing gear down and locked.
• Full hydraulic fluid reservoir on power pack.
• No hydraulic leaks.
• Tripped gear indicator circuit breaker.
• Gear handle in the down position with green gear down light off.
• No obvious electrical or mechanical issues.

Emergency landing gear hand pump would not pump using the “closed circuit closed doors, open circuit open doors” POH method.

Normal operation of the emergency landing gear hand pump system was reestablished by pushing the gear door indicator breaker in. The green gear down light came on when the breaker was pushed in.

Run-up as per the POH was completed uneventfully and the engines were run for several minutes with no tripping of the circuit breaker or any detectable anomalies in the system.

The Cessna service manual suggests checking for a short circuit in the landing wiring but, has little to offer other than this.

A review of the visible electrical circuit was negative including all gear up and down lock as well as squat switches. A more thorough examination of the electrical system is planned.

We plan to jack the plane and swing the gear, first by hand pump and then cycle it by using a hydraulic slave and observe for anomalies.

About 75 flight hours before this issue the entire landing gear system was inspected, hydraulics overhauled including all hydraulic cylinders, front main and power pack. The power pack was rebuilt by a certified shop with service to current specifications.


Any insight regarding this issue that I can pass on to the mechanic?



Thanks for your help,


Mark

[skymstr02]

Did you try to reset the breaker? Without electrical power, the gear doors will not close. It takes electricity to move the solenoid valves on the power pack, which divert the hydraulic pressure to the door actuator.

Tell your mechanic to check for a short in the door solenoid valves. Other than that, its good old troubleshooting techniques.

Keep us up on what you find, please.

[Ernie Martin]

I am puzzled by your incident. I'm probably not going to be very helpful, but let me share what views I have.

1. The infrequent gear problems I've had have all been on my 1973 due to a weak battery, and doesn't apply to your model*.

2. It seems that your resetting of the breaker resulted in normal operation of the hand-pump process -- as it should -- but your message suggests that after resetting you were not able to repeat the failure (upon retarding the throttle after runup the circuit breaker did not trip). That's a mystery but it may offer a clue. You don't say where you are in California, but could temperature (warmer on the second runup) or higher battery voltage (from the alternators having more fully recharged the battery) been the reason?

3. One intriguing point is that the failure was accompanied by the gear warning horn sounding. That's supposed to happen only when the system senses that the gear is up when you retard the throttle. Again a mystery that might offer a clue.

4. Another clue for your troubleshooting is that the breaker tripped. That only happens when the current is too high (and a short would cause it or when battery voltage is too low, as in the footnote below) or when a breaker is faulty. I say faulty because with age some breakers can deteriorate and trip at a current less than specified; if your runup was soon after starting the engines and/or it was cold and/or the airplane hadn't been used for a while (so voltage was lower, requiring more current), and the breaker has deteriorated, that could explain it. But faulty breakers are infrequent, so I'd put this at the bottom of the list.

5. I'm always suspicious that anomalies happening after service may be the result of a problem caused by the service (e.g., a mistake or damage done to wiring while servicing), but you've flown 75 hours since servicing the gear, so I don't think that's the problem.

6. Therefore, your plans and the suggestions of skymstr02 seem like the right approach.

Sorry, I couldn't be more helpful.

Ernie

______________
* This is for those with newer models. My 73 has an electric powerpack, not an engine driven hydraulic pump like Mark's 65 and my old 69, and draws considerable power. A weak battery, especially if it's hot, can cause the sequence to stop in midstream when the alternators alone can't meet the power demand and the breaker trips. This is especially true upon landing, when your alternators are putting out less juice due to lower engine RPMs and the battery is hot; my solution has been to service the battery every 3 months (refill the cells and trickle charge) and to momentarily increase MP and RPM to 25/25 on the approach during the gear-lowering cycle.

[Pete Somers]

This can be a power pack problem or a micro switch on the gear intermittent allowing the gear solenoid to operate in the power pack.

Check all the micro switches and the wiring for any rubbing and short circuits as the circuit breaker popped for a reason.

Check the rigging of the micro switches as per the service manual.

Pete

[Mark Ashiku]

Thanks for the information. The plane is scheduled for inspection as per my original post tomorrow. I will post the mechanic’s conclusions on this gear door issue.

Thanks again,

Mark

[Skymaster337B]

I have had similar problems with my 67' 337B. The problem is always the downlock switches. Either main or nose gear.
First, how is the aircraft ground handled? Do you use a tug? I have found that certain tugs will raise the nose up, causing an aft CG...which then extend the nose gear strut. The maintenance manual warns against towing with the nose strut near full extension--and in my case it actually broke the nose gear downlock. The nose gear might not collapse either, since there is an internal downlock inside the nose gear actuator (a second downlock). Must people don't know about the second downlock and therefore only adjust/set the primary downlock. But if your primary nose gear downlock is broke the micro switch might not make solid contact with the plate...even though the secondary (or internal) downlock is preventing the nose gear from collapsing. Thus the gear doors will stay open. And once hydraulic pressure is applied everything might look normal again.
So, if you use a ground tug for towing...check the nose gear down lock really well.

The other problems with the main gear downlocks switches are worn "wear pads" that the switches are supposed to contact. This is very noticable while taxing on uneven surfaces.

If you need to talk to me my number is 505-480-7775. I'm in NC and stay up late.

Rob

[Mark Ashiku]

Rob,
I do use a tug but, have never had the nose of the plane rise while towing nor have I had any significant nose strut extension during towing. I have made a very close inspection of the nose gear without any findings. Given your experience I will re-inspect the nose gear including all switches and wires. The mechanic was scheduled for the general inspection of the landing gear system today. Unfortunately this did not occur.....So nothing to report.
Thanks for the additional input and thank you for your phone number.

Mark

[Skymaster337B]

Another note about the primary nose gear down lock. It is hard to see! And it may look like it is locked down, but mirrors and gap gauges must be used to verify.

Another "got ya" about the primary NG down lock. There are two halves, a left hook and a right hook. Connecting the two halves is a piece of plastic on original equipment (replacement parts are metal). If that plastic pin is broken the two halves will not work properly. So, to check, wiggle one side of the down lock (while it is locked down); the other side should move identical to the side you are wiggling. If you wiggle one side, but the other side doesn't move then that connecting pin is broken.

[Mark Ashiku]

Update on the landing Gear issue:

A thorough review of all components of the landing gear system has been completed. This inspection included the following:


• Examination of all landing gear Up and Down indicator switches.
• Examination of all landing gear circuits including power pack and gear/stall warning circuits.
• Thorough inspection of the landing gear mechanical systems.
o Up locks.
o Down locks.
o Push rods.
o Hydraulic cylinders.
o Hydraulic hoses and connections.
o Power Pack.
o Power Pack solenoid.
o Gear control handle and linkage.
o Hydraulic fluid.
• The plane was jacked and the gear was:
o Retracted using the emergency had pump.
o Extended using the emergency had pump.
o Gear doors opened and closed (without cycling the gear) using the hand pump as per the POH.
o Landing gear was cycled ten (10) times using a hydraulic slave pump.
o Landing gear retraction and extension was stopped at various points in the cycle and examination of all of the mechanical and electrical components was performed.

Results:
• Negative. No detectable anomaly of the gear system was found.

Conclusion:
• Cause unknown.

Possible Causes:
• Defective circuit breaker.
• It may be possible that the (green) gear down indicator light/gear horn test switch malfunctioned resulting in excess load on the gear indicator circuit tripping the breaker. Subjectively, the gear/stall warning horn sounds much loader than before.

Action taken:
• Replacement of the gear indicator circuit breaker.
• Lubrication of the landing gear system.
• All electrical connections tested and verified secure.

Any other ideas???

Thanks for your input. It made a big difference in the level of confidence I had approaching this issue.


Best to all,

Mark

[Ernie Martin]

I think you've covered all the bases and the only point I would make is that you shouldn't be overly concerned. Specifically, don't let this mystery keep you from using and enjoying your aircraft. You know that as long as you have fluid in the system, you can always put the gear down with the hand pump. It's a reliable, rugged system. I understand if you're frustrated but don't let it affect your aircraft use.

Ernie

[Skymaster337B]

I agree, go fly and enjoy the airplane...especially if you have insurance, haha.

But if something happens again please let us all know.

[basloane]

Hi Mark

The circumstances and details of your problem sound very, very similar to a problem I had with my landing gear. The only difference is that I was in the air when it happened.

While approaching to land, I selected gear down, the doors opened, the gear came down,…. and then the horn started blaring. I paused for a few seconds, uttered a few expletives, and then selected gear up….the gear remained extended. I performed all the required checks, gear up, gear down, hand pump, and even a few (utterly silly ) checks, but the horn continued with no green light. We looked in both wing mirrors; the nose wheel seemed to be in place, and we could see both mains. I could also see that the gear doors were open. The horn continued with no green light.

As I began to think about my landing circumstances, suddenly the horn stopped and the green light came on. The subsequent landing was uneventfull.

With the assistance of a local mechanic, I inspected the gear system, checked hydraulic levels, etc. and found nothing amiss.

When the problem reoccurred a few flights later, I decided to "take the gloves off" and meet this head on!

We jacked the airplane and performed 10 flawless retractions. We were about to deem it a "non-problem" and declare "Miller Time", but then I decided to do one more test using the hand pump. On this final try we were able to duplicate the problem. After 10 minutes of searching for the problem, we discovered that the Gear Indication Breaker had tripped. We reset the breaker, and everything returned to normal.

I called my ( previous ) mechanic, and he pointed out to me that the reason the doors did not close and that there was no green light, is because the circuit which contains the three down and locked switches was not allowing current to go thru the indicator circuit. The gear probably is down and the switches are closed. He advised that I VERY CLOSELY examine the wiring within 6 inches of each down/lock switch, looking for any evidence of electrical malfunction or shorting. Be sure to look and feel behind the wire, because the damaged part of the wire may be hidden from view. This can be done without jacking the aircraft.

Near the right Main Gear Down/Lock Switch we noticed a very tiny blackened area on one of the wires within about 50 millimetres from the down lock switch. On close examination we could see the physical and thermal damage ( electrical arcing soot ). We performed a ( mickey mouse ) "test" fix, cycled the gear up and then down as we watched the process. During gear extension, the doors completed opening and the “locking dog” retracted to prepare to accept and hold the gear as it deployed into position. The wire became pinched between the dog and the airframe panel. We saw an electrical spark.

We then removed the Mickey Mouse fix and applied some of that "spiral Teflon wrap" to each wire individually, then to the pair together. We repositioned the wire a few millimeters higher so that it would avoid the dog when it retracts, and then did a couple of retractions and extensions. Everything appeared to work perfectly.

Interestingly, we looked at the left side, and it would appear that a repair has also been made to those wires. We are guessing that someone in the past has experienced a similar occurrence problem.

My experience prompted me to think through the manner in which the Engineers designed and arranged these wires. These wires must be located and oriented no lower than, or ( ideally ) EXACTLY as, was intended from the factory. If that last 6 inch portion hangs a few mm too low, it could become vulnerable to pinching between the downlock "dog" ( when it activates to accept and release the gear ) and the adjacent airframe panel.

In my case, the switch appears to have been replaced by a previous owner, and rather than threading the wire up into the local "P-Clamp", it was merely attached to the outside of the clamp with a tie-wrap. This caused the wire to hang a few mm lower and make it vulnerable to pinching.

My only explanation for the mysterious silencing and restarting of the system is that the cloth of my pant leg brushed against the tripped breaker and reset it. This particular breaker is a low current breaker and requires very little pressure to push it back in.

An interesting experience to say the least. I hope this helps you with your problem.

Best Wishes,......Barry Sloane, N187

[Pete Somers]

This is getting a serious problem now on older aircraft, wiring that moves with the landing gear and switches are getting brittle and the outer insulation is cracking with both movement and oil. I know inspecting this wiring is almost impossible and the costs would be extremely high.
But to replace the wiring on the nose gear switches is quite easy as it is all wired to the terminal block in the top right nose gear bay, these are the only wiring that moves with the gear, the main consist of the up/down micro switches which are fixed.
I you do get intermittant problems with the gear indications and suspect the wiring on the nose gear, it would be best to rewire it, then you have it covered.

Regards
Pete

[n9121x@aol.com]

Hello there, I have a 1965 337 and have had gear trouble in the past. the problem was whenever the L/G C/B would get hot, it would open without popping the breaker. This situation would normally happen when a prolonged ground-run. The green gear down light would go out, the doors would go open, and could not silence the horn. Once the temperature went down, the problem went away. I replaced the C/B and the problem went away. I have never had a problem since. I have owned this airplane for 20+ years and really never had any major problems. I hope this helps.

Wes Grady
KCCB Hangar 208

[stackj]

The original CBs in the Skymasters seem to be reaching end of life. I don't know if it is internal corrosion or just what's going on, but I had several become unreliable over our 15 years of Skymaster ownership.

On our last avionics upgrade, I chose to wholesale swap out all of them. I also chose breakers that I could manually turn off. We also manufactured a copper bus to replace the old aluminum one.

This cost about $800, but after having the mains (both of them) pop on a couple of IFR flights, it was well worth the expense. The major problem with the mains is when one popped, the other could not handle the load and it would slowly heat up and it would pop. By the time I realized what was happening, I could only get one to reset - the second one was still to hot to reset. Then I began playing the game Reset 1, 2 minutes - pop, reset 2, 2 minutes - pop, reset 1, 2 minutes - pop...

I LIKE the new breaker panel!