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Check Your Autopilot Servo Arm Screws!

4kilo

4kilo

Well Known Member
It was a beautiful day here in the DFW area, so I decided to take my 8 up just to bore holes in the sky. After doing a few clearing turns, I started a loop. Up I went into the "delirious burning blue" and over the top just enjoying the wonderful view, then down the backside reducing the power as the speed increased and releasing a little back pressure.

The next feeling I had is really difficult to describe. As I released the back pressure, the G-load just increased. The first thought that went through my head was "Wow, that feels wierd." Then I pushed forward on the stick, and the G-load kept increasing. As the nose came through the horizon, I put both hands on the stick and pushed hard, and by now my thought was, "Oh $^&#!" The nose kept moving up, so I pushed the power back in to maintain airspeed and rolled the lift vector off to the left to keep the nose down and the airspeed to reasonable levels.

For the next few minutes I was able to modulate the airspeed and attitude with the bank angle, while using throttle to maintain altitude. I distinctly remember thinking, "Why is my #%&$ parachute in my apartment and not on my back!" The airplane just had its first condition inspection about 6 flight hours ago last week, and I was trying to figure out what could be causing the elevator jam. I reached behind me to the rear stick to feel if it was fouled on something, but it seemed completely clear. I felt all the linkages on the front stick I could reach, but nothing was out of place. I looked back at the elevators, and both appeared completely normal, it was just that I could not deflect them down even close to neutral (about the width of the elevator horn was showing below the stab on each side). I was able to deflect them more in the up direction, but it felt like they were hitting a stop when the stick was moved forward. I tried messing with the trim, but the tab is not large enough to even partially overpower the elevators.

I next tried leveling out a little to see if it was possible to fly in level flight, but the airspeed kept decaying too far, and I certainly did not want the airplane stalling with no down elevator available. Running out of options, I put both hands on the stick and shoved really hard. Nothing. Alright, it can't get a whole lot worse, both hands and push for all I'm worth. This time the stick popped forward to about -3 G's, and I was able to recover to level flight with normal elevator feel.

To get back on the ground, I just determined to keep the airplane fast enough to keep the elevators nearly neutral, figuring that as long as I didn't get too close to the same position they jammed in, they would continue work normally. I flew a wide, low pattern at 110 knots, and touched down at about 100, which actually worked out just fine. As I slowed to taxi speed, my habit of pulling the stick aft for ground ops jammed the elevators again.

Back in the hangar, I found the problem. The arm of my autopilot elevator servo is held on by a screw, which was missing in action. The arm and attached linkage then dropped between the elevator bellcrank mounting bracket and the body of the servo and jammed there.

The screw which holds the arm on a TruTrak autopilot servo is a #8 panhead stainless 1/4 inch long, with a nylon washer between the screw head and the servo arm. This is the factory set-up, and I have never seen any indication of people using anything different. I am looking for a source for a screw with a drilled head so that I can safety the screw to the arm. I will not fly my airplane again until there is some positive safety lock on these screws (there is another identical servo for my ailerons).

Please note that there was no failure of any other aspect of the autopilot system, and I continue to have full confidence in my TruTrak autopilot. I just will require a positive safety lock on that servo arm attach screw.

Please check those screws!!! I am off to do laundry,

Pat
 
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A/P arm

Thanks for sharing that harrowing experience. I'll check my screws and do as you suggest. So glad you're OK. Bill
 
WOW! Great post and quick thinking. Thank you.

You might want to email TruTrak - this sounds like serious "letter to the owners with advice to safety a screw" time.
 
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Great post! Thanks for the advice. And most importantly, glad you are okay!

Here's my question...and I'm still drilling holes in the fuse so cut me some slack: I thought anything that has a "rotation axis" has to be safetied? Sounds like a pan head is a poor choice in design (?). And it sounds like you have a good/safe solution.

Thanks again for sharing.

Joe
 
Good job Pat, glad you are okay.

In retrospect, should you have declared an emergency? I'm not trying to second guess you, just asking what your thoughts were.

*************************

I've found two aileron arms to servo (two different planes) installed wrong. The little arms should not be installed on the bottom / lowest side of the bell crank (I don't know the PN) it should be installed on top. To make it simple, the connecting rod with bearings must be straight, and not at an angle.

d100ap74sv32tj3.jpg


To see this better I stole the picture of the servo from Bob (below). The connecting rod goes in between the gap created by the arm, not on top of it.


Maybe be a good time for all to check both.
 
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Servo Arm Attachment

Pat,

That is an incredible story, and glad you're OK. Pretty dog-gone great airmanship, that's for sure!

I just installed the Dynon servos this week, and went to look at some pix to make sure I remembered how the servo arms were attached (as this really got my attention!). As shown below, the arm-to-servo attachment is done with a castellated (sp?) nut and cotter pin:

d100ap74sv32tj3.jpg


Not trying to compare the servos at all, but rather thought it might provide an idea for a safety mechanism for your TT servo. Not sure if there would be a way to install a stud with a cotter keyhole in the existing hole, and a #8 hole doesn't give you much to work with in that vein. Guess the stud could back out too. Don't want to be too much of a shade-tree engineer here...it's serious enough to consult the mfgr, for sure!

That safety wire idea might do the trick...though I wonder if it might have an impact on the servo arm torque or any other impact (just thinkin' out loud). As Kevin said, probably worth checking with TT, which you may have already done!...they have a ton of great products out there, and would undoubtably want to hear about this.

Great job bringin' her back safely!!

Cheers,
Bob
 
Thanks for sharing, Pat.

Lack of pitch control is my nightmare scenario. Thanks for sharing this and rest assured several of us are going to check these servos out before our next flight.

Grace under pressure, Pat. Hope I could react as well.

b,
dr
 
Thanks a bunch!!

......since we're right at the TT installation and I just looked at my TT servos....same screw.

BTW, stuff in the rear fuselage has on many occasions caused serious and some fatal accidents, so check very, very carefully for any tools, nuts and bolts, etc, after any annual.

My last boss went out for some acro in their clipped-wing Cub and after a snap roll or two, the elevator would not budge past horizontal but it would go down. He managed to fly it onto the runway at near cruise speed. Later he discovered a stainless steel Zippo cigarette lighter wedged between the rear longerons acting as an elevator stop!! His pilot had "lost" it two weeks earlier while doing aerobatics and how on earth it got where it was is still a mystery.

Regards,
 
Securing

How do you plan to secure? Inquiring minds want to know (I have the TT and my mind is inquiring)
 
****

4kilo said:
It was a beautiful day here in the DFW area, so I decided to take my 8 up just to bore holes in the sky. After doing a few clearing turns, I started a loop. Up I went into the "delirious burning blue" and over the top just enjoying the wonderful view, then down the backside reducing the power as the speed increased and releasing a little back pressure.

The next feeling I had is really difficult to describe. As I released the back pressure, the G-load just increased. The first thought that went through my head was "Wow, that feels wierd." Then I pushed forward on the stick, and the G-load kept increasing. As the nose came through the horizon, I put both hands on the stick and pushed hard, and by now my thought was, "Oh $^&#!" The nose kept moving up, so I pushed the power back in to maintain airspeed and rolled the lift vector off to the left to keep the nose down and the airspeed to reasonable levels.

For the next few minutes I was able to modulate the airspeed and attitude with the bank angle, while using throttle to maintain altitude. I distinctly remember thinking, "Why is my #%&$ parachute in my apartment and not on my back!" The airplane just had its first condition inspection about 6 flight hours ago last week, and I was trying to figure out what could be causing the elevator jam. I reached behind me to the rear stick to feel if it was fouled on something, but it seemed completely clear. I felt all the linkages on the front stick I could reach, but nothing was out of place. I looked back at the elevators, and both appeared completely normal, it was just that I could not deflect them down even close to neutral (about the width of the elevator horn was showing below the stab on each side). I was able to deflect them more in the up direction, but it felt like they were hitting a stop when the stick was moved forward. I tried messing with the trim, but the tab is not large enough to even partially overpower the elevators.

I next tried leveling out a little to see if it was possible to fly in level flight, but the airspeed kept decaying too far, and I certainly did not want the airplane stalling with no down elevator available. Running out of options, I put both hands on the stick and shoved really hard. Nothing. Alright, it can't get a whole lot worse, both hands and push for all I'm worth. This time the stick popped forward to about -3 G's, and I was able to recover to level flight with normal elevator feel.

To get back on the ground, I just determined to keep the airplane fast enough to keep the elevators nearly neutral, figuring that as long as I didn't get too close to the same position they jammed in, they would continue work normally. I flew a wide, low pattern at 110 knots, and touched down at about 100, which actually worked out just fine. As I slowed to taxi speed, my habit of pulling the stick aft for ground ops jammed the elevators again.

Back in the hangar, I found the problem. The arm of my autopilot elevator servo is held on by a screw, which was missing in action. The arm and attached linkage then dropped between the elevator bellcrank mounting bracket and the body of the servo and jammed there.

The screw which holds the arm on a TruTrak autopilot servo is a #8 panhead stainless 1/4 inch long, with a nylon washer between the screw head and the servo arm. This is the factory set-up, and I have never seen any indication of people using anything different. I am looking for a source for a screw with a drilled head so that I can safety the screw to the arm. I will not fly my airplane again until there is some positive safety lock on these screws (there is another identical servo for my ailerons).

Please note that there was no failure of any other aspect of the autopilot system, and I continue to have full confidence in my TruTrak autopilot. I just will require a positive safety lock on that servo arm attach screw.

Please check those screws!!! I am off to do laundry,

Pat
Click to expand...

GOOD TO HEAR YOU'RE BACK, MY MAN. GLAD I WASN'T ON YOUR WING. THE PFANTOM
 
Dec 1 my annual inspection begins so I will be sure to check each of these screws on my servos. I'm sure you already sent all this to TruTrak and hopefully they will investigate and come up with a fix for all of us flying with their servos.

Thanks for the message and heads up to all of us. Most of all, I'm glad you made it back safely.
 
Someone on the BMA list had a similar problem a couple of years ago.
BMA servo arms are held with a set screw with no good way to safety them. Used loctite and installed a small "fence to keep the arm from sliding off the shaft in the event the screw came loose.
100_2140_Small_001.jpg
 
Glad you're OK.

Thanks for the post Pat. We all now have another item to add to our inspection checklist.
 
There's more......

....to the servos. I just got off the phone with my buddy/multi RV builder, A@P/IA. He said to also be sure and buy three or four stainles steel hardware store screws, 1 1/4" long and go all the way through the four corners and add a nut. He's had two Trutrak cover plates come loose because all that holds the cover on are 4 itty-bitty allen screws.

AC Spruce has screws that are taller and cross-drilled that you can use to keep the servo arm on and then safety through the screw AND the roll pin that holds the arm assembly to the shaft.

Regards,
 
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Great Save!

That is a heck of a story Pat - congratulations on a great save! I have experienced a control jam in a Cub before, and it is simply amazing how time slows down while you're trying to come up with a solution. In reading your post, I was wondering if it was going to be an "over-center-lock" problem, but I think the screw coming out is even scarier.

I'm going to check the screws on all four (2x2 airplanes) of our Tru Trak servos ASAP!

Paul
 
Nice Job Pat!!
You know if this kind of thing would have happened at your work, and you made a miraculous "save", a flight manager/chief pilot/standards guy's only response to you would be: "Thanks for bringing the plane back in one piece, but we're going to have to reprimand you for not contacting flight control first before landing, and for not declaring an emergency".:rolleyes:
 
View Port

Great Job Flying Pat!
The servo arm screws, pushrods, bellcrank etc. aft of the baggage compartment are of a concern. My Trio servos use four small screws to hold the arm on. On my RV-7, I've been contemplating placing a small 3"x3" plexi window in the baggage bulkhead for viewing all the linkage there without having to remove any of the bulkhead. I have a 100 hour coming up soon and your harrowing experience confirms that this area needs more frequent inspections. I'd also like to install a small window on my outboard wing access panel to look at the roll servo. Anyone see any problems with this?
 
(thinking out loud) Could you replace the screw with a drilled an bolt and safety wire it to the arm? That would keep it from turning loose.
 
Maaan ohh Maaaan!!!

What an ordeal! Good thing you didn't lose your head. Great job and glad you made it through. It could have easily gotten worse.

I haven't got to any servos yet but I will be thinking about this when I do.

Your experience reminds me of my buddy at Embry Riddle in AZ who was instructing spins to a would-be intructor. They were flying a new 172 with a glass cockpit he had just flown back from Cessna a week earlier. It was brand spankin new. Once they were fully into the spin they couldn't pull out. They were losing altittude fast. He couldn't figure out what was going wrong either. Nothing was responding as it should much like your situation. But, he remained calm (until he got two feet ont he ground) and just kept going throught the standard procedures and then some. It wasn't until he moved his seat back one inch that they came out of the spin about 1200 AGL (started the spin at 6000 AGL). Cessna later determined that the glass had not been considered in the CG. So, by moving his seat back (who would have known) just one inch corrected the CG.

Point being......he didn't lose his cool and just like you continued to trouble shoot. Other wise, I believe I would be visiting a gravesite today. It is so easy to read these stories and wonder in the back of your mind "would I be calm or would I be freading out?"

I hope I posess the same attribute as you if I ever (knock on wood) have something as such happen to me.

Glad you are safe. Beer is on me. :D

Regards



Evans
 
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4 screws loctited into a Trio servo

This is pretty scary stuff - thanks for posting. On my Trio Gold servos, the crank arm is held on with 4 screws. They are countersunk on the arm side and pick up threads in the servo itself. I don't think there is room to add nuts. The manual does require use of loctite on the 4 screws.

This is a great thing to inspect regularly!
 
pierre smith said:
....to the servos. I just got off the phone with my buddy/multi RV builder, A@P/IA. He said to also be sure and buy three or four stainles steel hardware store screws, 1 1/4" long and go all the way through the four corners and add a nut. He's had two Trutrak cover plates come loose because all that holds the cover on are 4 itty-bitty allen screws.

AC Spruce has screws that are taller and cross-drilled that you can use to keep the servo arm on and then safety through the screw AND the roll pin that holds the arm assembly to the shaft.

Regards,
Click to expand...


Funny you should mention the 4 little allen screws. I found 3 of mine loose the other day. I put some loctite on all of them and re-installed.
Thanks for the tip about using 1 1/4" bolt/screws in place of the other ones.
 
Ok so

What does TT have to say on the matter?
Frank
 
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Follow Up

Here is a picture of the jammed elevator linkage:

ap001.jpg


And here is my solution:

ap002.jpg


The screw in the photo is secured with both locktite and safety wire.

I used a MS24677-14 which is a socket head cap screw with a drilled head in place of the original screw. I had to use three washers because of the length of the screw. A MS35265-41 1/4 in filister head screw would be a better choice, but I couldn't find one locally.

I discussed the situation with TruTrak this morning, and Lucas said that they would be investigating the problem.

Pat
 
What a wonderful save

Congratulations. Everybody's worst nightmare...throw a little fire in the cockpit next time and I'll mess my underpants just reading the account.

On another note. I know we are "Experimental" and all, but isn't there a requirement to notify the NTSB in cases of inflight control system malfunctions? This is the kind of thing I'm sure they'd like to study. I am not casting stones if you don't want to report it, and I'm not even sure where I remember seeing the notification requirement.

I admire your skills and calm under fire. One of the lessons we should all try to take from your experience is to keep working the problem. Panic doesn't do any good at all (until you've tried everything else, then it might add a little strength to the push right before the crunching noise) so keep trying solutions until one of them works.

Thank you for sharing. Alright all you guys with autopilot servos...out to the hangar and don't come back until you've checked them.
 
tcone1 said:
On another note. I know we are "Experimental" and all, but isn't there a requirement to notify the NTSB in cases of inflight control system malfunctions?
Click to expand...

Good point... it says so right at the top of the applicable rule:

49 CFR Part 830 said:
Sec. 830.5 Immediate notification.

The operator of any civil aircraft [...] shall immediately, and by the most expeditious means available, notify the nearest National Transportation Safety Board field office when:

(a) An aircraft accident or any of the following listed incidents occur:
(1) Flight control system malfunction or failure;
Click to expand...
 
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1-glad you are OK
2-glad your plane is OK
3-great job 'working it out' in flight
4-great job getting back down
5-good follow up
6-good fix
7-thanks for sharing with the group

Curious....did you change the screw on the roll servo as well :rolleyes:?
 
Screws

Thanks for the photos. Wish I'd have had the pics early this AM before I left for the airport. I mis-read which screw you were talking about, and now I have it to do all over again tomorrow.:(

Wouldn't be so bad since I'm retired now, but along with retirement comes arthritis etc, etc, etc, which makes crawling into the back of an RV-8 a real PITA. (Or shoulders, knees, elbows, and neck!!!!!!!!).:rolleyes:

Like I tell my friends-----At least I'm still vertical!!!!!!!!!!!:D
 
Pat, Great pictures that detail the event. I'll be heading out to the hanger in the AM to fix mine.

Thanks Pat.
 
Geico266 said:
d100ap74sv32tj3.jpg


To see this better I stole the picture of the servo from Bob (below). The connecting rod goes in between the gap created by the arm, not on top of it.


Maybe be a good time for all to check both.
Click to expand...

Larry,

Just requesting a clarification here. In the earlier post, quoted above, are you saying that the connecting rod goes between the servo arm and the servo body? If I misread that, my apologies. But if not, I looked at the Dynon install manual, and it shows that rod attaching to the side of the servo arm away from the servo body. It looks like Pat's install is that way on his TT as well. Not trying to cross swords at all, just want to make sure good gouge is put out, and no one else mis-reads the post like I did.

Pat, the fix sure looks good. Very nice job again...during and after!!

Cheers,
Bob
 
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Checked Mine!

I got home from work today, and went straight out to the hangar to open up the Val and check my servos. the screws were tight in both servos, but I think I'll order those bolts and change out the screws next chance I get.

It is an unfortunate truth that the little lessons in aviation are written in blood - we do things a certain way because someone died when they didn't. it is a real blessing that pat sorted this one out and brought back an answer for all of us!

Paul
 
Bob,
Can I talk you into turning those AN-3's around? Yes, I know there is not much applied force (servo slip clutch limited), thus a low failure probability in this app. It's just one of those "good practice" things. A single-shear rod end installed as shown applies the bolt bending moment to the root thread. Single shear rod ends should have the nut on the free end, shear and bend thus applied to the bolt shank.
 
Pat,

Incredible story. Thanks for passing along that very useful, potentially life-saving information. Just when I thought I would not have to crawl back there yet again, I now anticipate a bit of rework. Still, (on so many levels) that is a very small price given what could possibly happen.
 
DanH said:
Bob,
Can I talk you into turning those AN-3's around? Yes, I know there is not much applied force (servo slip clutch limited), thus a low failure probability in this app. It's just one of those "good practice" things. A single-shear rod end installed as shown applies the bolt bending moment to the root thread. Single shear rod ends should have the nut on the free end, shear and bend thus applied to the bolt shank.
Click to expand...

Dan,

I understand what you're saying (place the bolt so that the pressure from the servo arm and rod are applied to the shank, not the threads, correct?), and I see that Pat has his TT servo installed as you are suggesting.

I do appreciate you watching my six, and after reading this, I went back to the install guides to see if I had followed them correctly. The diagrams actually show it as I have them installed, with the bolt head on the side of the connection rod bearings, so if I may, let me bounce a Q off you and the group, as I do want to do this right (best practices), and really respect your opinion:

The way the diagram has it, the bolt head is on the rod bearing side, which is captured by the large washer. Then there is a small washer on either side of the servo arm, followed by the nylok fastener. In looking at it, that puts the bolt shank under the push rod. It shows the same assembly at the bellcrank.

I guess my question is, where do you think the most pressure is applied to the bolt, at the connecting rod bearing, or at the servo arm and bellcrank? Since the servo arm is the driver, I sure can see your point. What do you think after looking at the diagram, posted below?

pitchservodiagramrt4.jpg


The diagram for the Dynon roll servo shows the same orientation for the servo arm connection (bolt head on the rod bearing side, bolt shank under the rod bearing), but the opposite orientation at the stick attach point (bolt shank under the aileron push tube connection, nylok at the rod bearing side). Here's a pic of that diagram:

rollservodiagramdq7.png


Any thoughts after seeing the diagrams? Like I said, I respect your opinion, and would like to have this put together in the strongest and most reliable way! Thanks again very much!

Cheers,
Bob
 
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Bob, as the Push rod is attached to a single surface, and not a "fork" shape, the liner force from the servo is actually trying to rock the bolt in the bellcrank.

What Dan is saying does make sense, although in practice I am guessing the force here is probably pretty small.

Best setup would be a reamed hole in the arm, snug fit on the un threaded bolt shank, and the nut on the outer side of the ball end.

By the way, the entire assembly should be tight on the bolt, the inner part of the ball joint is held stationary on the bolt, the outer part of the ball end rotates on the inner. The bolt is a fastening device, not an axle.

This is the reason for the large washer. Just in case the ball end comes apart.
 
Very good discussion on which way the bolt head should go! I agree that good practice would have it the way Dan suggest, and I also agree that in this installation, the forces are pretty small. Both of those consideration being well within the margins, there might be another factor in this case - clearance to structure or the bellcrank. The bolt head is pretty flat, and the washer/nut is pretty "tall". When I was checking mine yesterday, I learned again how tight this area is between pushrods, bellcranks, and control arms - it might only fit in one direction, in which case you don't have much choice, and can then rely on the forces being very small.

This certainly will vary with model of RV, as the geometry is just a little different in each (at least I know the -8 and the -6 look the same, but are slightly different).

Paul
 
If the servo fails and the servo shaft is broken would the same situation exist? That is, even safety wired, wouldn't the control arm still be free?
 
Pat, I applaud your grace under pressure!

I've had the screws come loose on the cover plate and also have broken (how I don't know) the shear screw on the bellcrank.

From memory, I recall that the TT bellcrank has three holes on the part that surrounds the shaft from the gearbox/motor. In only one of the holes is a brass shear screw that is designed to shear if certain loads are exceeded. The shear screw keeps the bellcrank from rotating on the gearbox shaft. When it shears, you get another brass screw and stick it in one of the remaining open holes.

So my questions are:
1 - did the shear screw fail also? I would think it must have or the bellcrank would not have slid on the shaft.
2 - in a separate but related scenario, if the shear screw fails and the center screw is safety wired to the bellcrank, could the safety wire hold the center screw in a manner that holds it secure while the gearbox shaft rotates? This could cause the screw to back out while the motor runs continuously (note: I do not know the details of the TT design and this may or may not happen). In this scenario, having the safety wire could be worse than not having it.

So at this point I have more questions than answers.

I'm tempted to call TT, but it sounds like a few people have already and I'm really intereseted to hear their response.
 
Bob,
Take a look at post #16 in this thread:

http://www.vansairforce.com/community/showthread.php?t=26487&highlight=single+shear&page=2

Second everything Mike said.

Dynon diagram #2 (top view) is an interesting illustration. Note the 0.600" spacer, which lengthens the lever (the arm) applying bend to the bolt. On the other end of the pushrod the bolt (and thus the arm) is short.

As a practical matter the short arm is unlikely to break the bolt at the root thread because the loads are so low in this particular application. The longer arm makes things a little more risky.....so note how they installed the bolt with the nut at the free end.

A lot of what we do in aviation is based on "best practice" generalization. We encourage best practice even when the particular application doesn't strictly require it. Your servo install is probably fine forever. However, later you might be building something else, for example one of the single shear aileron bellcranks found on a popular biplane design. When bolt orientation is critical, knowing best practice is a good thing.

BTW, bit of a thread hijack here, sorry. Pat, may I add my compliments on your cool under fire? The right stuff indeed.
 
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Pat - Great job maintaining control of the aircraft, and getting it back on the ground.

And thanks for bringing this control jam to our attention. It has specific interest for other folks with the same servo and installation details. But, the event should also serve as a wake-up call for everyone to review all aspects of the flight and engine control systems. Think about all the things that can come loose or break, consider the potential ways the disconnected linkage could jam etc, and make installation or design changes as appropriate.

The servo of my wing leveler connects to the flight controls just above the front stick pivot. The servo has a slip clutch, but I can also reach down and pull a quick release pin to disconnect the wing leveler push rod at the stick, in case the servo ever jams. This type of design obviously won't work in many cases, but it an example of the types of things we should consider where possible.

I am thinking about putting springs at the throttle and mixture arms on the fuel injection servo, to push them to full throttle and full rich in case a linkage ever disconnects.
 
4kilo said:
And here is my solution:

ap002.jpg
Click to expand...

Pat, so very glad to hear you are OK.

We at Dynon worried about this situation a lot when we designed our servo. It's why we went with the castellated lock nut. We thought about doing exactly what you did with saftey wire, but it has a bad failure mode too:

If the shear screw breaks, the servo will spin the arm off, or it will re-lock the shaft, rendering the shear screw ineffective.

When the shear screw breaks, the purpose is that the shaft and the arm are no longer related to one another. The arm should be able to spin 360 degrees all day around the shaft without putting any torque on it.

If you break the shear screw, the servo has no idea of this. With the TT AP, the servo motor will just run one direction all day, since it's trying to command the aircraft with no effect, and it has no idea of its position. Now the shaft is spinning, the arm is not. No big deal if the screw in the shaft is not hooked to the arm, But in your case, the screw is now hooked to the arm, so as the shaft spins, the screw will tighten or loosen. If it loosens you're in the same situation where the arm falls off, and if it tightens, it will re-engage the servo to the controls. Even without the servo moving, it's possible for bad things to happen because the pilot is moving the controls and thus the arm, and this may re-tighten a screw down onto a jammed servo.

Theoretically, the shear screw should not break until a servo has mechanically failed, but in reality, metal fatigues over time and stress, and it's possible to have a shear screw failure with a servo that is working 100%. It's happened to us in testing, and I have heard of it happening to TT servos in operationas well. In normal situations this is a minor inconvenience, with just a loss of AP control of that axis, but with the screw safety wired to the arm, this becomes very dangerous very quickly.

As I say, we thought about this a lot, and the only solution we could come up with was the castellated nut. I do not believe there is any way to safety wire the screw without causing another failure mode. Locktite and frequent inspection is the only solution. Please consider removing your safety wire, or at least understand the increased risk this solution causes.
 
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Mike S said:
snip...Bob, as the Push rod is attached to a single surface, and not a "fork" shape, the liner force from the servo is actually trying to rock the bolt in the bellcrank...snip
Click to expand...

Ironflight said:
snip...there might be another factor in this case - clearance to structure or the bellcrank. The bolt head is pretty flat, and the washer/nut is pretty "tall". When I was checking mine yesterday, I learned again how tight this area is between pushrods, bellcranks, and control arms - it might only fit in one direction, in which case you don't have much choice, and can then rely on the forces being very small...snip
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DanH said:
Bob,
Take a look at post #16 in this thread:

http://www.vansairforce.com/community/showthread.php?t=26487&highlight=single+shear&page=2

Second everything Mike said.

Dynon diagram #2 (top view) is an interesting illustration. Note the 0.600" spacer, which lengthens the lever (the arm) applying bend to the bolt. On the other end of the pushrod the bolt (and thus the arm) is short.

As a practical matter the short arm is unlikely to break the bolt at the root thread because the loads are so low in this particular application. The longer arm makes things a little more risky.....so note how they installed the bolt with the nut at the free end.

A lot of what we do in aviation is based on "best practice" generalization. We encourage best practice even when the particular application doesn't strictly require it. Your servo install is probably fine forever. However, later you might be building something else, for example one of the single shear aileron bellcranks found on a popular biplane design. When bolt orientation is critical, knowing best practice is a good thing.

BTW, bit of a thread hijack here, sorry. Pat, may I add my compliments on your cool under fire? The right stuff indeed.
Click to expand...

Dan, Mike and Paul:

Light came on with Mike's rocking comment as well as your post Dan in that other thread (illustrates it very well). Thanks much.

Headed out to the airport to turn them around, after checking the clearance that Paul brings up.

Thanks again guys...appreciate it. And my mantra on the drive out will be, "I like taking out bulkhead and floor screws, I like taking out bulkhead and floor screws!" ;) (Worth it, of course!)

Cheers,
Bob
 
To all,

We are aware of this situation. This is most definitely an isolated incident. We have over 10,000 servos in the field at this time, and to our knowledge this is the only account of this ever happening. We are reviewing options for a change in the way the arm is attached. When we reach a decision we feel comfortable with, we will post up with the solution. This solution will be available to all existing customers. Thanks everyone!
 
THANKS!!!!!!!!!!

Lets see, 1 failure in 10,000 units, that is a .001% rate, if my math is correct.

And the company is willing to look into, and correct it:D

Question; How many company will even acknowledge a .001% problem??? Much less spend the time/effort to fix it???

That my friends is what customer service is all about.
 
I must be stupid. Please help.

Some pictures of servos show the bellcrank arm secured to the shaft with a screw threaded directly into the shaft, right down the axis.

Other pictures show the bellcrank arm secured to the shaft with a castellated nut and cotter pin.

If I'm reading this correctly the castellated nut is a design update. If someone has the castellated nut do they have anything to worry about?

Thanks
Doug
RV6
TT owner
 
The black servos with a castle nut are DYNON servos.

The gold servos with no castle nut are TRUTRAK servos.

You cannot get a servo with a castle nut from TruTrak. We would argue that if you have a Dynon servo, this is not a failure you need to worry about.
 
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