M20J Trim Question

[Tim-37419]

The cost of annual maintenance thread triggered a thought for me.

 

During the cold snap, my trim was so stiff that I immediately landed. To adjust the trim I needed to hit the switch and turn the wheel. It seems cold soaked below freezing is when it gets bad. Non freezing cold to include entering below freezing temps at altitude is fine.

I found some old threads that implied it's not uncommon and something to keep in mind of you live up north.

Should I give the squawk to my A&P during annual? The shop is not a MSC and I want to avoid them fixing something that isn't fixable (and then me paying for it)

 

Thx!

[Ragsf15e]

10 minutes ago, Tim-37419 said:

The cost of annual maintenance thread triggered a thought for me.

 

During the cold snap, my trim was so stiff that I immediately landed. To adjust the trim I needed to hit the switch and turn the wheel. It seems cold soaked below freezing is when it gets bad. Non freezing cold to include entering below freezing temps at altitude is fine.

I found some old threads that implied it's not uncommon and something to keep in mind of you live up north.

Should I give the squawk to my A&P during annual? The shop is not a MSC and I want to avoid them fixing something that isn't fixable (and then me paying for it)

 

Thx!

There’s a reasonable chance your grease is getting “firm” in the chain, or trim mechanism.  It should turn freely.  I would want that fixed as it’s not going to fix itself.  There are some great descriptions on how to fix that on this site. Shouldn’t be rocket science.  Maybe @N201MKTurbo remembers where he posted that last?

Edited February 23, 2024 by Ragsf15e

[Fritz1]

yes, sounds like trim jackscrew needs lube and/or cleaning, better done sooner than later

[PT20J]

Have the shop clean and lube the entire trim system. (It’s supposed to be done at annual anyway). There are jackscrews  at each end and u-joints in between. There are chains at the trim wheel and trim servo that need to be adjusted and lubricated, and a bushing at the trim wheel that need oil. If it is a Bendix-King autopilot, there is a carrier bearing at the trim servo that frequently seems to get missed. 

[Tim-37419]

Yep, I do have a BK AP.

 

Thank you all for the tips. I have my marching orders.

[larryb]

For me the source of that problem was the pillow bearing for the trim tube in the rear avionics compartment. Just needed a shot of tri flow.

[Tim-37419]

@larryb - could you help me find that in the IPC? I searched section 27 but came up dry. I'm 180lbs, can I lay in the tailcone? I'm assuming the bearing is way back there somewhere.

[N201MKTurbo]

Just open the battery hatch on the tail. It will be right in front of you.

[Tim-37419]

Not making much progress other than emptying my can of trilube. I have not yet removed the belly plate but everything I’ve inspected has the appearance of being maintained (ie greased). I am not washing away grease with trilube btw.

I did notice the chain on the trim wheel as a bit of slack (which I would expect) but the chain on the electric trim motor in the tailcone is very tight. 

I checked the service manual, I can’t find anything more specific than “re-tension drive chain” Does anyone know if the chain should be tight or have a bit of slack like pretty much any other chain drive I’ve ever seen?

Also, is the aft jackscrew inside a cloth sleeve at a rearward bulkhead behind an access panel? Not sure how to get in the sleeve and I do not want to tear a 30 yr old sleeve that I probably can’t replace.

 

[rturbett]

Tim,

If you have the chance, please post some photos of the work your doing. I will likely be doing the same soon.

Thanks,

Rob

[Tim-37419]

Will do @rturbett

 

I found the answer to my cloth sleeve question in a thread titled “68 M20C trim jack screw cover” In a nutshell, yes it’s removable but no, I should not expect to find an OTS replacement.

[PT20J]

Details of the servo trim chain tension are in the autopilot installation manual. If it is a KAP/KFC 150, it is supposed to have only 1/4” deflection. The trim carrier bearing (again, if you have a B-K autopilot) should be lubricated per the maintenance manual. 

The boot covering the jackscrew is pretty tough. Run the trim all the way nose up and remove the 4 tiny screws at the forward end and slide it back. Clean all the old grease off with solvent and apply new grease. 

 

[EricJ]

1 hour ago, Tim-37419 said:

Not making much progress other than emptying my can of trilube. I have not yet removed the belly plate but everything I’ve inspected has the appearance of being maintained (ie greased). I am not washing away grease with trilube btw.

I did notice the chain on the trim wheel as a bit of slack (which I would expect) but the chain on the electric trim motor in the tailcone is very tight. 

I checked the service manual, I can’t find anything more specific than “re-tension drive chain” Does anyone know if the chain should be tight or have a bit of slack like pretty much any other chain drive I’ve ever seen?

Also, is the aft jackscrew inside a cloth sleeve at a rearward bulkhead behind an access panel? Not sure how to get in the sleeve and I do not want to tear a 30 yr old sleeve that I probably can’t replace.

 

Trim motor chains tend to not have any slack, so that there is no appreciable lash when it runs back and forth (i.e., changes direction).   This isn't a big deal on the hand-operated wheel, but similarly you can remove a dead spot in the wheel by taking out slack.    Mine does not have a dead spot.

As mentioned, the easiest way to expose the jack screw in the tail is to run the trim all the way up.   The accordion cover on mine is only connected on one end, and with the trim all the way up I can get enough compression on the boot to get my fingers in there and spread some lube around.    Ahem.    If yours is connected on both ends just disconnect one end and squeeze it a little.   It doesn't take much to get some lube in there and you'll be able to see whether it has been dry or not.

 

[skykrawler]

Motorcycle experience taught me overtightened chains wear faster.

[Tim-37419]

Success!

 

Removed the rectangular access panel

took out the top two cloth sleeve screws, I couldn’t get to the bottom two.

Jack screw looked dry but not 30 years neglected dry.

greased it up and sprayed the bearing or maybe it was some sort of sleeve that goes through the bulkhead with tri flow.

run the trim full up and full down twice

reapplied grease

ran the trim again

reinstalled cloth sleeve. Definite anxiety over dropping the tiny screws but didn’t happen. Helps to have slightly magnetized screwdriver.

reinstall panel

clean up

treat myself to fast food lunch

 

thats a great day.

 

I took a few photos for Rob that I will post later. Thank you to all who chimed in.

[Tim-37419]

2 hours ago, EricJ said:

Trim motor chains tend to not have any slack, so that there is no appreciable lash when it runs back and forth (i.e., changes direction).   This isn't a big deal on the hand-operated wheel, but similarly you can remove a dead spot in the wheel by taking out slack.    Mine does not have a dead spot.

As mentioned, the easiest way to expose the jack screw in the tail is to run the trim all the way up.   The accordion cover on mine is only connected on one end, and with the trim all the way up I can get enough compression on the boot to get my fingers in there and spread some lube around.    Ahem.    If yours is connected on both ends just disconnect one end and squeeze it a little.   It doesn't take much to get some lube in there and you'll be able to see whether it has been dry or not.

 

I do have a bit of play in the trim wheel now that I know it’s not supposed to be there it will annoy me until I fix it :-). 

[EricJ]

5 minutes ago, Tim-37419 said:

I do have a bit of play in the trim wheel now that I know it’s not supposed to be there it will annoy me until I fix it :-). 

I don't think there's a spec on play for the manual trim wheel, at least not that I've ever been able to find, so it's really personal preference.    If the dead spot doesn't bother you it's probably not hurting anything.

For the electric trim motor there should be maintenance guidance specific to the autopilot that it goes with, but generally it's unusual to see slack there.

[Tim-37419]

for @rturbett

 

The boot

[Tim-37419]

Top boot screw positions

[Tim-37419]

boot screws removed.

[Tim-37419]

exposed jackscrew

[rturbett]

Thanks!