Landing Gear Issue, 1967 T337B

[OhioSkymaster]

When I placed the gear handle in the Retract position the gear retracted as far as I could see, but the amber "gear up" light did not illuminate and the gear handle stayed in the up position. When I manually moved the gear handle to the neutral position the amber light came on. When I extended the gear, the gear handle worked as it should and went back to neutral, and the green "gear down" light illuminated. I tried retracting the gear again and this time the amber light and gear handle worked as it should. Has anyone else experienced this? Thoughts?

[cessnadriver]

Ohio skymaster.
You might check the micro switches. There is one at the bottom of the hydraulic power pack, and the three gear uplock switches. If any are not making good contact this will cause an intermittent lights.
Hope this helps.
Regards, BILLS

[wslade2]

Granted, poorly operating micro switches can cause indicator light issues. You are a B model with engine driven hydraulic pump. you mentioned the gear handle not returning to the neutral position on its own and you manually moved it to center. Worrisome for a low pressure situation. The causes are numerous. So many times , stung by the basics. Check your fluid level first.

[hharney]

All good comments and warranted likely causes. But don't get hung up on the obvious, it can be a microswitch but it can also just be a bad connection to the microswitch. This system is a daisy chain around the airplane that connects from point to point so it could be any connection along that chain. It is sometimes hard to tell if the connector or connection is bad so take some time and trouble shoot each point. It should be on jacks to do this as you will want to swing the gear to detect the fault. Remember a lot of issues only happen in the air because of vortexes from flying so they may not be duplicated on the jacks.

[Jhogan0101]

I have a 337B. During runup i always push the gear handle DOWN. It will usually pop back up to neutral in cold weather about 7 secs, 4-5 secs during warmer weather.

[GAdams]

To state the obvious even more. Check hydraulic fluid level and be sure the system is adequately purged of air. I was surprised how many cycles we had to do on the jacks to get the air out of the system. And how much more fluid it took after each purging attempt.

[Jhogan0101]

Attached is another way from service manual to purge air from system without placing on jacks.

5-128. BLEEDING TIME-DELAY VALVE.
The
Time-delay valve in the Power Pack may be purged of
air by operating the engine-driven pump, or the
Hydro Test may be used.
2.
Make sure reservoir is full.
b
Start engine and let run at 1000 rpm, or connegt
Hydro Test in accordance with paragraph 5-125
Place landing gear handle in the down position
and hold for approximately one minute, while turning
the master switch OFF
until doors open,
then ON un-
til doors close.
d.
Repeat step "c" four times, waiting one minute
between steps.
e.
Check that time-delay operates properly by mov-
ing gear handle sharply to the down position and re-
cording time as handle returns to neutral.
NOTE
The time delay between closing of the landing
gear doors and releasing the landing gear
handle to neutral should be between 3 and 9
seconds at room temperature. Colder temper-
atures will cause a longer delay.
i.
Shut down engine, or disconnect Hydro Test in
accordance with paragraph 5-126.

https://ibb.co/P9P0bQ9

[OhioSkymaster]

Thanks for the suggestions. My main concern is the gear handle issue. If I understand correctly the switches only control operation of the indicator lights and not the handle, so the indicators may actually be correctly showing that the gear is not fully stowed. We did a number of gear swings at annual time just last month and had no problems. We used a homebuilt "mule" built from a pump removed from a 337 and driven by an electric motor. I'm wondering if the pump may be suspect. We did check the fluid but will re-check and check for air. Had the same problem just before annual, so it wasn't introduced by connecting the mule.

[Jhogan0101]

I recently had a problem with air being introduced into the system, fluid was foaming out of the front hydraulic vent. The lock nut on the return side of the pump was loose and sucking air in like a venturi. I doubt this is what youre experiencing but who knows.

[GAdams]

I had some loose nuts in the system and yes it does induce air. The handle is all about pressure. Put a wrench on every nut you can find. Top off frequently. Bleed the system again and again. Put the plane on the ramp and run the front engine. Put the handle down. Wait for it to retract. If it doesn't purge then fill, purge again. Repeat. Once you get the handle to return from the down position. Go fly it. Land. Do it again. Repeat as necessary. Many Many Many times.

[wslade2]

Check the hydraulic screen and relief ball valve for clogging.

If you’re sucking air, it can make the fluid foam. Foamed up fluid is more compressible and won’t actuate the system firmly. ie-may not get firm up operation leading to unexpected lamp indication. Foamy fluid can come out the vent. Look for hydraulic fluid on the ground under the vent or streaking under belly of plane.

I capped off my pump and hooked a hand operated vacuum pump from auto part store to the other port to check for air leak at pump. If it doesn’t hold vacuum, you’re sucking air at a worn pump shaft and bad internal seals.

If you’re on jacks, you can operate your on plane pump with splined socket from harbor freight hooked to a 1/2 inch drive drill and observe for issues.

If you unhook the lines from your pump, you can fill the intake side with hydraulic fluid (I use a hand pump oil can from tool store), then turn the prop by hand. A good functioning pump will move fluid to the output side efficiently and directly. You will see smart fluid movement without a lot of prop movement once primed. Observe for sluggish or no flow.

Look for hydraulic leak anywhere and everywhere. What wasn’t leaking before can be leaking now. I did high speed taxi this weekend to check some brake work and darn if it didn’t make a door actuator start leaking that wasn’t before.

[OhioSkymaster]

Still fighting this problem. Tried the bleeding process mentioned below, no joy. We recently jacked the plane and, after some false starts, found that the right mains up switch was not functioning properly. The plunger of the switch was depressed per the manual, but we found that it was open-circuit when measuring voltage around the daisy chain. I pressed on the switch plunger a bit and, lo and behold, the switch made contact and I heard the door actuator solenoid engage. We replaced the switch and cleaned the terminals. Under test with a mule (electric motor driving the same type of pump as installed on the plane) we cycled the gear 10 times and it worked perfectly. Then I flew and had the same darn problem. The pump was recently rebuilt by Aircraft Accessories of OK, us thinking that it might be the problem but no change. Any ideas? Also, any recommendations for a shop in or near Ohio with expertise on the Skymaster landing gear system?

[hharney]

Check with Griswold Aviation in Marshall Michigan KRMY
(269) 330-2908 Craig the owner knows the system very well

[wslade2]

Thinking about this problem I am going to compartamentalize.

When hooked up to the mule, system works fine. When hooked up to engine driven pump, it doesn't. Sounds like (at least now after your switch repair) everything from the firewall back works ok-because it does on the mule. The pump has been overhauled. Provided the overhaul hasn't failed, or the pump is not being spun at adequate rpm (unlikely), your problem is basically between the pump and firewall. Not much there, except the connections.

In one of my other lives, having dealt with heavy duty diesels, it's amazing what impact a fuel line suction leak can have on operation of the fuel pump and engine. ie-a little suction air leak into the fluid stream can dramatically reduce the fuel pump output.

Maybe check carefully your suction line. Maybe pressure test the suction line for leaks? What I also recall is that the fittings on the pump involve compression nut, oring and a teflon back up ring. Are these all in good shape/new? How many years has that o ring been compressed under that nut loosing some of its sealing capacity?

Also when you switch from mule to engine driven some air can be introduced.

[B2C2]

when I first got my T337D, I had the same issue. Worked fine on the ground, did not work in the air. Finally figured out that the negative air pressure over the back of the plane was causing the gear to sit a little bit further away from the switches. The adjustment procedure in the manual did not get them quite in the right position. My mechanic adjusted the switches to be in slightly further compression on gear up and this solved the problem. Also make sure to top the hydraulic fluid after using the mule as suggested below. Otherwise you may have issues raising the gear after the testing. Had this happen after annual several times. Now I remind my A&P to top it every year.