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.
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.
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.
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.
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.
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.
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.
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.
