Elevator Trim Problem

Pat Schmitz · #1 05-16-04, 12:31 AM

Looking to see if any of you have ever experienced this problem?

Today, while finishing up the last hours (!) of my required 25 hours dual for insurance coverage in my 73' T337G, I experienced a problem w/ my trim.. upon reflection of other events earlier in the day, I realized that this had been 'building' for several hours unnoticed.

On my last takeoff - as usual, I set the trim to the takeoff position, and flaps to 1/3 as recommended by my 337 instructor.. When I reached 85 for rotation, I immediately realized that the plane had a tremendous amount of Nose Down trim...things did not feel right at all, and I had to apply a lot of back pressure to keep the nose pointed in a climb attitude.. In spite of the takeoff trim position indicated on the indicator, I immediately applied nose up trim to attempt to correct the problem. It helped. I carefully made my way around the pattern and landed without further incident. On the ground, we tested the trim, to find that it would go into full nose down trim when the indicator shows 'takeoff'. Eventually, we concluded that the trim would travel to full nose down, and back to Neutral - but had no ability to travel to the nose up position. The trim tab just stops at the neutral position (Even though the indicator on the panel moves the full range.)

We removed the access panels on the right tail boom to inspect the trim motor and cables - all looked fine, tension, etc...

Then, I realized that on two occasions while flying earlier in the day - I had set my autopilot for a 500ft/min climb (which the autopilot normally maintains w/ no effort) only to see it goto a 1000+ ft/min climb attitude.. Since this did not 'feel' right to me, and I wanted to avoid getting too slow, I disconnected the autopilot - reestablished a manual 500ft/min climb attitude and just assumed that the autopilot did not 'catch' the climb rate I had set it at.. I now realize that this was related to the same trim problem, as the AP uses the trim to maintain pitch.

My plane is grounded (by me) at a distant airport, and I want to get a grip on what is 'likely' to be the cause before I let a different mechanic who has no 337 experience touch my plane. I could probably fly it back without using the trim to my home airport, but I don't want to take ANY chances. Who knows what is wrong, and how that might affect elevator and rudder control???... I don't want to take any unneccesary chances..

Is there another bell-crank, or push/pull location up in the Wing, or forward boom?? It seems like it 'pulls' the trim from one direction, and probably 'pulls' it back from the other (rather than push..) Given that, I am assuming that the other end is where the problem is located..

Let me know if any of you have ever experienced this problem, and how you came out..

Thanks!!

skymaster · #2 05-16-04, 07:59 AM

was autopilot on? sometimmes my left hand hit the trim on/off .
flap has interconnect and should go neutral in the up mode. does manual trim work? 73 p with stec65 . auto pilot will flash if its more than limit up. we built a harness for testing the servos.

kevin · #3 05-16-04, 10:44 AM

I am not a mechanic, but one possible cause of this might be the flap interconnect. Perhaps someone with more knowledge will answer your question.

Kevin

Pat Schmitz · #4 05-16-04, 11:23 AM

We did check the autopilot immediately to make sure it was not engaged - so I know that was not the problem.

You both mentioned the flap interconnect... after landing, I retracted the flaps, and taxied in - as far as I know, the flaps were (and are) still retracted... Once we got back to the ramp, we shut down the engines and both electrically/manually actuated the trim from full up, to full down... The tab travels to full nose down, but stops on the way back to nose up at the 'neutral position'... When the trim tab is in the neutral position, the indicator shows full nose up.. (they do not match up)

That is why I am certain that the cable pulling the trim tab into the down position is working, but perhaps the actuator or cable that pulls it back into the 'up' position may be loose, slipped, or worse??? Since those cables run in the same cavity as the rudder and elevator cables, I will not fly it until we identify the 'real' problem... I dont want to take the chance that the trim cable might be broken or loose, and risk jamming something else at a critical time in the flight process..

I had ordered a service manual for the plane, but of course...it has not arrived yet.. This was to keep with the plane so that I could work with the A&P, and be appropriately informed if I had a problem at a remote airport..and to make sure we had the right documentation/partslists at all times. As you all know, lots of $$ can be spent by a mechanic who has no experience w/ 337's.

I am not unrealistic about expecting any of you to solve the problem, we clearly need to carefully inspect and fix the problem here - just curious if anyone else ever had the same experience, and if not - give you all a heads up...

It is through our shared experiences that we can all benefit from a safe and mutual passion for the Skymaster.

I am FRUSTRATED, but still completely in LOVE with this plane...

travis · #5 05-16-04, 11:41 PM

As previously mentioned there is a flap connection to the trim. When the flaps are retracted for the landing position the trim is automatically rolled to somewhere in the t.o. range (as I remember. Just a minute Ill look for it in my service manual!).

I’m no A&P but hear is what I see. There is a low possibility that the cables can are out of calibration because they are anchored to chains on both ends. There are three things that seem like they could be contributing to your dilemma. Starting with the simplest one first:

1 There is a possibility that the indicator is bent out of shape.

2 There could be a broken cable in the works.

3 The trim stops (metal blocks clamped onto the "trim up cable") have come out of position. On A/C SN after 337-0526 there are three of them for use with the electric trim and the flap interconnect.

4 your electric trim has gone to the dark side and is causing chaos and mayhem.

I find that aviation is a realization of Murphy’s Law so it will probably be a combination of all things easy to fix but expensive to obtain and things hard to get to but inexpensive to obtain. That said hear is what I think you should do.

1 Fully extend the flaps and trim to the nose high stop. Can you feel the stop? Visually verify that the tab is where it is suppose to be?

2 Retract the flaps and watch to see if the trim rotates back to T.O. range. Check the indicator and the tab position for proper setting.

3 Run the electric trim from stop to stop and verify indicator and tab movement.

I think that the problem will revile itself through these tests. Please let us know. Also if you would like to chat about it give me a rattle tomorrow evening at 817 595 3411. I can check with some Cessna guys at the local shop if I need to.

_travis

PS perhaps a peek in to the log books will provide a solution!?!

Paul Sharp · #6 05-18-04, 06:05 PM

I don't have electric trim, but not long after I bought my plane, it appeared that somehow the "jackscrew" arrangement of the trim down in the center console area had "jumped" a few notches. It wasn't fully off like yours sounds it is acting, but the APs had to readjust it so that when the adjustment was centered, it really was centered, and that the pointer pointed about the center of the area at the same time.

Something else to check, anyway...

SkyKing · #7 05-18-04, 06:14 PM

BTW, the elevator control cables DON'T run in the same "cavity" as the trim cables... TRIM cables are in the RIGHT Boom and the ELEVATOR cables are in the LEFT Boom.

Per the Cessna POH "Airplane & Systems Descriptions" of the Trim System:

"The elevator trim tab system is also mechanically interconnected with the wing flap system to automatically eliminate excessive nose-up trim while retracting the wing flaps."

Suggest looking at the interconnect...

Also, does your bird have either a Hortson STOL kit of Robertson STOL?

SkyKing

Pat Schmitz · #8 05-19-04, 10:43 AM

We did find the problem - the manual trim wheel in the panel is bad. Apparently it has been wearing against the aluminum bracket for some time, causing an egg-shaped hole in the bracket which slowly grew in size over time from abrasion... When it got large enough - the cable tension caused the trim wheel to 'pull' to one side far enough that the cable winds up on the spool wrong - this caused the cable to stop prematurely, and indicated the position wrong..

We think we have found a used part for about $50 - so I may be back in business soon.

Thanks for everyone's help, and check your trim wheel!!

Pat

Pat Schmitz · #9 05-25-04, 11:58 PM

Correction of my own comments above - the bracket was not aluminum, it was steel. And there is a plastic or nylon bushing that had worn out - causing the metal-to-metal contact..

Richard · #10 05-27-04, 04:11 AM

Dang... I'll have to check that too. I've just started developing the same issue.

Thanks

Pat Schmitz · #11 05-27-04, 10:05 AM

We were able to locate a used trim wheel / bushing. But the metal brackets where nowhere to be found.. We ended up repairing the old bracket by having a weld shop heliarc weld (I believe) the worn metal back, then re-grinding it to the proper size/shape again.

The whole process was only a couple hundred bucks.. flys like a dream again..