PDA

View Full Version : TruTrak servo failure (again) & repair (and safety issue)


Tim Lewis
08-12-2020, 07:09 AM
I had replaced my RV-10 TruTrak roll servo on 30 Oct 2019 with a refurbished roll servo from TruTrak, after my original roll servo failed.

(https://vansairforce.net/community/showthread.php?t=173654&page=6&highlight=trutrak+roll+servo)

Less than a year later, my roll servo failed on 7 Aug in flight.

Following the TruTrak sale to Bendix King, technical support/repair seems to have been outsourced to Mid Continent Instruments and Avionics (mcico.com). I called Mid Continent at 316-630-0101 (also 800-821-1212), http://mcico.com. I spoke to Bruce, their Trutrak guy.

Bruce told me Mid Continent has the BS-C servo for $500 (not a bad price, IMO). I would need to work with Andrew (former Trutrak owner, still with BK) regarding warranty coverage. But Bruce suggested I probably have a broken roll pin inside the servo (not the shear screw). He suggested I could replace the roll pin, which would get my autopilot back in operation for an upcoming 10 hour IFR flight. I agreed, I paid for rush shipping of the roll pins.

Open up, per Bruce: 4 hex head screws come out to pull the stepper motor off. I looked inside, saw the roll pin was still intact, but had backed out of the gear. Bruce said use new one, don’t re-use, to get better fit (seems wise, since the existing pin backed out in less than a year).

https://vansairforce.net/community/attachment.php?attachmentid=1174&stc=1&d=1597233840

https://vansairforce.net/community/attachment.php?attachmentid=1171&stc=1&d=1597234011


https://vansairforce.net/community/attachment.php?attachmentid=1172&stc=1&d=1597234214

Be aware Torx screwdriver TT8 bit is required to open servo gear area to get to the gear and roll pin. I opened that, removed roll pin (intact). The roll pin I need is 3/32”x5/8”. Bruce says to use some sort of thread locker (not permanent) on torx and hex screws upon reassembly.

Safety note: If the TruTrak BS-C servo internal roll pin shears or backs out (mine has done this twice), the spindle and control arm can fall right out of the servo housing, with the risk of jamming controls.

https://vansairforce.net/community/attachment.php?attachmentid=1173&stc=1&d=1597234011

On my RV-10, the servo link is installed with the shoulder washers TruTrak recommended. Those large shoulder washers apply just enough force to keep the broken spindle and control arm from falling out of the servo. I added a trapping mechanism to capture the pitch servo spindle/torque enhancer if it should happen to similarly fail. In addition, my checklist includes the warning that if any autopilot component malfunctions I should completely turn off the autopilot and investigate before further flight.

https://vansairforce.net/community/attachment.php?attachmentid=1175&stc=1&d=1597234011

Canadian_JOY
08-12-2020, 09:13 AM
Thanks for the "peek inside" these servos, Tim!

Just wondering aloud... Since your roll pin is backing out, is this an instance of perhaps out-of-spec holes for that pin? Or is it a case perhaps of higher-than-desirable control forces causing some "wiggle" between the components held together by the roll pins, thus allowing a tiny bit of relative motion between those components which ultimately results in the roll pin wiggling loose?

Not trying to armchair quarterback but rather to understand what might be the root cause. Yes, I have a TruTrak autopilot in my aircraft so this information is of great interest and your safety warning is much appreciated.

climberrn
08-12-2020, 12:41 PM
There is a green locktite that is made to glue bushings in place. It would “probably” keep the roll pin in place. Anywhere else i would suggest that. In an AP servo where the controls can get locked, would not want to take that chance.

philb
08-12-2020, 01:08 PM
I just had exactly the same thing happen to me couple of months ago. Roll pin just backed out.

When I talked to Bruce, (i hadn't opened up the servo yet) he suggested no repairs were possible, so I bought a new one. It was only when I opened up the old one (after already receiving the replacement), that I found the roll pin. Just have to find a replacement pin and locktite it in place and I'll have a backup...

Phil
RV-10
Bountiful, Utah (KBTF)

Tim Lewis
08-12-2020, 02:45 PM
Just have to find a replacement pin and locktite it in place and I'll have a backup..

For mine (Model BS-C) the roll pin that fit was 3/32x5/8”. I used red (medium) locktite at both ends of the pin. If it ever comes out again I’ll use a 3/32x3/4 (or longer) roll pin and open the ends of the pin with a punch. And cover the whole pin in locktite.

Bevan
08-12-2020, 04:45 PM
It would seem to me there should be a groove cut near the end of the shaft to put a C-clip to prevent the whole thing from coming out of the housing should the roll pin fail. Does the shaft/control arm assembly come apart completely so that the shaft can be put into a lathe and machined to accept a C-clip?

Shouldn’t this be a SB?

climberrn
08-13-2020, 01:08 AM
For mine (Model BS-C) the roll pin that fit was 3/32x5/8”. I used red (medium) locktite at both ends of the pin. If it ever comes out again I’ll use a 3/32x3/4 (or longer) roll pin and open the ends of the pin with a punch. And cover the whole pin in locktite.

If you are going the Locktite route, the Locktite 640 is made for that. Have used it on brass bushings in a 4130 frame. It is made to bond smooth fittings. They are VERY difficult to remove after it sets. May not be the best fix, but worth a try.

cheeselion
10-26-2020, 04:33 PM
Just in case anybody is reading this thread with their brand new yet-to-be-installed TruTrak AP in hand and having second thoughts as I was... I dropped Andrew Barker a line, and he followed up with their engineering department who confirmed that the shaft size and pin size were adjusted to ensure this doesn't happen with future units.

BK reputation aside, was happy to get a dose of the legendary TruTrak customer service there! Thanks Andrew.

plehrke
10-26-2020, 07:20 PM
Just in case anybody is reading this thread with their brand new yet-to-be-installed TruTrak AP in hand and having second thoughts as I was... I dropped Andrew Barker a line, and he followed up with their engineering department who confirmed that the shaft size and pin size were adjusted to ensure this doesn't happen with future units.

BK reputation aside, was happy to get a dose of the legendary TruTrak customer service there! Thanks Andrew.

Yup, same here. I worked with a Jay at BK with several back and forths on my Gemini not showing the correct track in the DG box. No joy in solution and he ended the trouble ticket with I needed to send it it for repair. Luckily I did not and Andrew saw my post here on the forum and one email later he gave me a fix to the setting in my GX65 and it worked.

Tim Lewis
11-13-2020, 10:24 AM
Safety note: If the TruTrak BS-C servo internal roll pin shears or backs out (mine has done this twice), the spindle and control arm can fall right out of the servo housing, with the risk of jamming controls.



Here's my precautionary modification to protect against a jammed elevator control in the event of a pitch servo failure similar to the above roll servo failure. In normal operation, the long bolt I added to the torque enhancer never touches the sides of the slotted retaining plate shown in the picture. If the servo roll pin fails and lets the torque enhancer start to work out of the servo body, the bolt/slot is intended to act to retain the parts until I can land.