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Aero Precision provides OEM part support for military aircraft operators across more than 20 aircraft

Elevator Trim Tab Freezing?


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On the road and had a scenario where the elevator trim tab fails to operate at altitudes above 15000 ft for an hour or more on Normal AND emergency. Then 10 Minutes after descending to around 6000ft, it works again. We've replaced the motor driven gear box and the problem still appears. Our guess is something has collected water and is freezing but other than throwing new actuators/jack screws and flex drives at it, what could possibly freeze up? We have verified the physical surface isnt rubbing or jamming up, all rigging has been checked and it works perfectly on the ground.

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t35, Many moons ago had an identical squawk on my A model. Like you we replaced the drive motor, several times. We were deployed, with the acft. flying almost every day. After the first couple of sorties with inop tabs our aircrews begain to get nervous and jerky, and rightfully so. Sooo my assistant crew chief and I decided it was time to take drastic action. We ordered left and right teleflex cables and tab jackscrews. In essance we replaced everything that moved from the actuator cp out. Once we received the parts, on Saturday morning we removed one elevator and replaced drive cables and jackscrews. We only did one side so as to avoid the requirement for FCF. The acft. flew the next day ok. So the following day we did our thing on the other side. From that day on never had another elev. trim tab writeup. Which part was the culprit? Can't say for sure, but my guess was the teleflex outer covering was broken allowing water intrusion. Why did we overkill and change everything? Because only wanted to remove the elevators once.

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Make sure the "Drip Shield" is in place over the elevator trim tab actuator/motor (held on with 6 or 8 nuts and bolts). I have seen more than once over the years where people have failed to re-install this and water dripping down the vertical stab gets on/in the motor and it freezes at altitude (the whole reason for the shield). Seems like a simple thing but it will cause a lot of grief if it's missing...

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After 8 or so #2 apex tips and finally tracking down some good tips, we got all the panels off and found one of the Flexdrive cables ( rh Inboard) with multiple cracks on the rubber outer layer and rust dripping out of several of these cuts. I guess we will find out when supply gets us what we need. I'll keep you posted. Thanks for all the suggestions.

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t35, Many moons ago had an identical squawk on my A model. Like you we replaced the drive motor, several times. We were deployed, with the acft. flying almost every day. After the first couple of sorties with inop tabs our aircrews begain to get nervous and jerky, and rightfully so. Sooo my assistant crew chief and I decided it was time to take drastic action. We ordered left and right teleflex cables and tab jackscrews. In essance we replaced everything that moved from the actuator cp out. Once we received the parts, on Saturday morning we removed one elevator and replaced drive cables and jackscrews. We only did one side so as to avoid the requirement for FCF. The acft. flew the next day ok. So the following day we did our thing on the other side. From that day on never had another elev. trim tab writeup. Which part was the culprit? Can't say for sure, but my guess was the teleflex outer covering was broken allowing water intrusion. Why did we overkill and change everything? Because only wanted to remove the elevators once.

I agree with Larry. Change them all when it's apart. While in Alaska our H1's were having consistent elevator trim problems in the late summer. We found a drop or two of water in the drive cable would not allow them to spin. They were a time change for us then, but we had them added to the PDM work spec for them to be changed then (more frequent) and our problems became nonexistent.

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t35, Many moons ago had an identical squawk on my A model. Like you we replaced the drive motor, several times. We were deployed, with the acft. flying almost every day. After the first couple of sorties with inop tabs our aircrews begain to get nervous and jerky, and rightfully so. Sooo my assistant crew chief and I decided it was time to take drastic action. We ordered left and right teleflex cables and tab jackscrews. In essance we replaced everything that moved from the actuator cp out. Once we received the parts, on Saturday morning we removed one elevator and replaced drive cables and jackscrews. We only did one side so as to avoid the requirement for FCF. The acft. flew the next day ok. So the following day we did our thing on the other side. From that day on never had another elev. trim tab writeup. Which part was the culprit? Can't say for sure, but my guess was the teleflex outer covering was broken allowing water intrusion. Why did we overkill and change everything? Because only wanted to remove the elevators once.

I agree with Larry. Change them all when it's apart. While in Alaska our H1's were having consistent elevator trim problems in the late summer. We found a drop or two of water in the drive cable would not allow them to spin. They were a time change for us then, but we had them added to the PDM work spec for them to be changed then (more frequent) and our problems became nonexistent.

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