Wednesday, June 30, 2021

Completing Safety Directive SD-00001 Replacement Of Trim Servo Motor

A potential safety issue has been identified with RV-12’s that have the Ray Allen ES MSTS-T3-7A-2 trim servo motor installed. The issue involves the threaded portion of the trim servo motor shaft just forward of the AN315-3R lock nut that secures the AN665-21R clevis. The shaft is bending and, in some very rare cases, snapped altogether. The service directive requires mandatory replacement of the ES MSTS-T3-7A-2 trim servo motor with an upgraded Ray Allen ES MSTS-B6-7T-165 trim motor if the shaft is determined to be bent … or after 1000 hours of flight time.

Drawing from Safety Directive SD-00001 showing the location of the fatigue area.

The replacement Ray Allen trim servo motor has a part number of ES MSTS-B6-7T-165 and MUST to be installed with a bushing (Van’s part number BUSH-BS.188X.313X.222) that slips over the short threaded area of the shaft preventing the shaft from bending. Van’s also suggests when replacing the trim servo motor to also replace the F-1287A servo mounting tray with the newer F-1287A-1 mounting tray. The new F-1287A-1 mounting tray supports the DB9 connector Van’s has switched to … replacing the micro-Molex connector (problematic for some builders not having the proper crimping tool for the tiny pins) used on legacy RV-12’s.   When switching to the new F-1287A-1 mounting tray, you will also need to order an additional AN315-3R nut and Bag 2670 from Van’s which contains the male and female electrical pins, bodies for the new DB9 electrical connectors along with the necessary mounting hardware, plastic bushings and some rivets.
Drawing from Safety Directive SD-00001 showing the new ES MSTS-B6-7T-165 trim motor, bushing BUSH-BS.188X.313X.222 and AN665-21R clevis.

The decision was made that it would be better to take the approach of being proactive rather than reactive so decided to change out the trim motor …. Especially because, to Ray Allen’s credit, the company is offering RV-12 owners a substantial rebate if exchanging the old trim motor for a new one.

The DOG Aviation RV-12’s trim servo motor did not have a bent shaft. However, decided to make the exchange anyway and not need worry about the trim servo motor shaft bending issue anymore. Plus, I liked the idea of switching over to the new F-1287A-1 mounting tray which supports a DB9 connector to replace the micro-Molex connector.
New F-1287A-1 mounting tray which needs to have the doublers cut away and riveted onto the servo tray. Also, my finger is pointing to the new mounting flange for the DB9 connector.

After separating and smoothing the edges of the servo tray parts, they were primed with SEM primer and top coated with white paint for extra protection. Van’s calls for LP4-3 rivets to assemble the doublers onto the servo tray, but I used solid AN470AD4 rivets instead for the assembly. Next the plastic bushings are installed and filed down so they just clear the servo tray. (This is the same process as used on the original servo tray … in fact, I probably could have gotten by using the plastic bushings from the old servo tray).

Below is a photo of the new Ray Allen trim servo motor on the right and the old trim servo motor on the left. The three most notable differences on the new motor are … the round brass portion of the actuator that the new brass bushing will seat against, only four mounting holes as opposed to the six on the older unit and if one looks closely, the four corners of the case are scalloped a little.
Original Ray Allen ES MSTS-T3-7A-2 trim servo motor on the left and the replacement Ray Allen ES MSTS-B6-7T-165 trim servo motor on the right.

On the original trim motor instillation, Van’s covers the trim motor’s mounting flanges with doublers … but they are not called for on the new trim motor. I like the idea of the doublers because the trim motor’s mounting flanges appear to be just a tough plastic. Unfortunately, because of the scallops on the case of the new trim motor the old doublers can’t be used. I tried to file the correct profile in one of old doublers and was not happy with the results … because of the center hole in the old doublers, it leaves a very thin doubler in the area of the center hole.  Although not called for, I decided it would not hurt to just make a pair of doublers as can be seen in the photo bellow.
After trimming the old doubler (top of the photo) to fit the new trim servo motor, one can see how thin the metal is around the center hole (which is not on the new Ray Allen trim servo motor). So a new doubler (bottom of photo) was fabricated to mount the new trim servo motor in the F-1287A-1 mounting tray.
The new Ray Allen ES MSTS-B6-7T-165 servo trim motor installed in the F-1287A-1 mounting tray with my handmade doublers added for good measure.

For the final assembly, the forward threaded shaft temporarily receives two AN315-3R nuts which are tightened together or "double nutted", as they say. The nuts are temporarily used so a wrench can be used to hold the shaft from twisting and torqueing the internals of the trim motor when the AN665-21R clevis is tightened against the bushing. Prior to final assembly Van’s wants the clevis to be 15° from vertical when dry fitting the parts together using only fingertip pressure.
Two AN315-3R nuts are used to double nut the servo motor’s shaft so a wrench can keep the servo motor’s shaft from twisting and possibly damaging the servo when the bushing and AN665-21R clevis are tightened together.

The bushing in the above photo is slightly longer than necessary. To insure a proper fit, Van’s recommends using a drill press with some sandpaper to remove a little material at a time from the bushing so the bushing’s edges remain square. This is one place you don’t want to do any hand filing because Van’s wants the bushing to be a tightly mated fit between the trim motor and the clevis. Material is removed from the bushing until the slot in the AN665-21R clevis is approximately 15° degrees BEFORE vertical when the clevis is hand tight to the bushing. Note: Go slow! … only remove a few thousands at a time because it doesn’t take removing much material to make quite a difference in the positioning of the clevis ( I almost over did it on the second cycle to the drill press where I removed quite a bit more material than I did the during the first cycle). When a finger tight dry fit 15° shy of vertical is achieved, the bushing is ready for final assembly. Permanent red Loctite thread locker is applied to the aft threaded portion of the trim servo motor shaft and while holding a wrench on the double nuts to prevent the shaft from twisting, the clevis is threaded on further beyond the 15 degree point where it should become snug as the clevis reaches its proper vertical orientation.
Completed trim servo assembly ready for electrical connections and final instillation. Note, the two AN315-3R nuts are not yet removed from the forward threaded portion of the Ray Allen servo motor. The two nuts need to be removed at this point prior to installing the assembly back on the RV-12.

As previously mentioned, the new F-1287A-1 servo tray is designed for use with a DB9 connector. Female pins are attached to the wires coming from the trim servo motor and male pins are attached to the wires exiting the tail cone. There is a small change in the colors of the two power wires going to the new trim servo motor (the three trim position wires remain the same colors). The old trim servo motor power wires are both white … the new trim servo motor uses a white and a gray wire. Before permanently installing the wires into the DB9 connector bodies, I thought it best to use a 9v battery to make sure the new motor moves in the same direction as the old motor did … doing this insured the two white wires (servo trim motor power) exiting the tail cone will be connected to the new trim servo motor so motor movement is the same as the old trim motor. After the correct motor movement was established, the trim motor power wires exiting the tail cone were marked. Next, the pins were inserted into the DB9 housings. Van’s suggests sealing all the wires with silicone RTV so that was done prior to final assembly.

After the silicone RTV cured, the DB9 connectors were installed onto the F-1287A-1 trim servo tray. While trial fitting the connectors together, I noticed the DB9 connector was not fully seated. Upon a little investigating it was determined that because the male DB9 connector from the tail cone rests on top of the F-1287A-1 servo tray which has approximately a .060" or so of thickness, the standard sized threaded barrels are a tad too long … so I removed .060" from each threaded barrel and now have a fully seated DB9 connectors (If one chooses not to do this, it is not a big deal. However, I wanted my connectors fully seated and it was easy to accomplish just by removing a little material from both threaded barrels).

The DB9 connector #4 mounting hardware is NOT a piece of cake to install. Access is limited and the use of the tiny #4 MS21042 all metal hex stop nuts makes instillation a real hassle. The hex stop nuts are slightly egg shaped so they really grip the pan head screws … the problem is they grip just a little too tight when trying to install in such close quarters using a 5/32" wrench and only being able to tighten one flat at a time. I finally resorted to placing the head of the screw in a vice and running the metal stop nuts on and off a couple of times using CorrosionX as a lubricant to reduce the bite. The other issue I ran into was my long thin Philips screwdriver was 70 miles south at the southern outpost, so I needed to cobble together another way of accessing the #4 Philips screw heads from above the stabilator while using the 5/32" wrench from underneath the stabilator. Standard Philips screwdrivers are too short to accomplish this task when working by yourself. Below is a photo showing a drawing of how the DB9 connector is to be mounted onto the F-1287A-1 servo tray and the Rube Goldberg use of tools to fashion a way to get on the heads of the #4 Philips mounting screws from above.
Lacking immediate access to my very long thin Philips screwdriver, a Rube Goldberg assembly of various tools from the tool box was used to devise a way to hold the #4 Philips screws from above the stabilator. The photo also shows the drawing for mounting the DB9 connector.
Completed instillation of the new Ray Allen ES MSTS-B6-7T-165 trim servo motor, F-1287A-1 servo tray with the DB9 connector in place. Per Van’s instructions, silicone RTV is applied to the wires to seal the connector.

With the exception of dealing with the #4 mounting hardware for the DB9 electrical connectors, swapping out the RV-12’s trim servo motor went smooth and is not a daunting task. However, one does want to be careful when using the drill press to remove material from the bushing. Go slow and only remove a little material at a time to creep up on that 15° sweet spot.