View Full Version : Navaid "AD" SAFETY ALERT!

12-19-2005, 08:47 PM
I had a scare last weekend that anyone with a navaid autopilot with a wingtip-mounted servo should be aware of.

During preflight, I lifted the right aileron and upon reaching full displacement, found it locked in place - fully displaced. I removed the right wingtip and found that the drift pin in securing the servo arm to the servo shaft had fallen out. The arm had fallen off the shaft and the pushrod dropped onto the lower edge of a hole in the end rib. The pushrod was segmented with telescoped sections of tubing. When I lifted the aileron, the pushrod pulled back through the opening dropped, over the step of the telescoped pushrod and was effectively locked against return to neutral. I had noticed that the Navaid was nonfunctional for the prior 10 hours or so. Gives me the creeps to think what would have happened if the same would have happened in flight.

It appeared that the pin was a simple friction fit from the factory.

Two fixes were employed. The pin is now a TIGHT friction fit with loctite and the step in the pushrod gone.

If it happened once, it could happen again - to you.

12-20-2005, 10:33 AM

Whoa. May I suggest a book called, Fate Is The Hunter, By Ernest K Gann.

BTW, Trio autopilot uses the SAME SERVO!!!


12-20-2005, 10:49 AM
Should be required reading before taking the PPSEL written exam. I feel the exam should contain questions from the book.

BTW, we never did find out what O'Connor meant with the big X in the snow....


P.S. I should make it a point to fly AM-21 someday....

12-20-2005, 04:34 PM
Great book (Fate...) and good catch on the servo arm! Yet another reason to move the control surface full throw during yor preflight.


12-20-2005, 05:09 PM
Good point. I try to make a Left turn and verify that the left aileron goes all the way up and the right all the way down. Then a right turn. Right all the way up and left down. It takes 1.7 seconds longer than just wiggling the stick :-).

I read an accident report years ago where a C-310 came out of annual with reverse rigged ailerons. The pilot was killed on the takeoff. That stuck with me......

Also, never use experimental spark plugs on three of your engines :-).


12-20-2005, 05:14 PM
I'm having a little trouble visualizing the drift pin. Could you maybe describe where it is located on your servo. I have the EZ pilot servo which is similar, if not the same.

Also, what type of telescoped sections of tubing are you using and how long? I'm sorry to ask for more info, but I want to understand what happened and take preventative measures if necessary.

Thanks for reporting this problem. It very well could save a plane and pilot. Pictures would be great if you have them. Jack

12-20-2005, 06:08 PM
I'm having a little trouble visualizing the drift pin. Could you maybe describe where it is located on your servo. I have the EZ pilot servo which is similar, if not the same. Jack

Jack look at the s2servo picture in above link. Coming out the side of the servo is the shaft, mount ring and the "arm". There is a pin that goes thru the mount ring, which just slips on the shaft and is held in place with a "drift pin". The arm itself is screwed on the mount ring with those 4 screws. If the drift pin comes out the whole arm assembly (mount/arm) and push/pull rod, I suppose, could just slide off the shaft. Correct me if I am wrong anyone.

I would think a secondary pin or hole with safety wire at the end of the shaft could be a mod to consider so the whole assembly can't slip off the shaft. If the pin falls out out shears the arm assembly would just slip on the shaft but no jam.


12-20-2005, 06:12 PM
I'm having a little trouble visualizing the drift pin. Could you maybe describe where it is located on your servo. I have the EZ pilot servo which is similar, if not the same.

The pin is just below the drilled hole in the shaft, just past the servo arm.

Looking at mine, it would be easy to have another metal plate, much like a washer with four holes drilled in it, to retain the pin. Could be a 1/16" thick, or just thin with tabs bent up. The four holes are for the small screws that position the servo arm.


12-20-2005, 06:49 PM
Holding the pin is one goal. That could be as simple as a metal band around the ring (hose clamp). Stopping it from sliding off it shears is more difficult. I'm not convinced necessary either. Eliminating the steps in pushrods, in this and other aplications, is the other lesson.

12-20-2005, 08:37 PM
Sorry about the typos and brevity. Working on a pocket PC. To elaborate on my earlier lack of conviction that it is necessary to protect the ring from sliding off if the pin shears, it would seem to take a lot of torque to make that happen. It is a pretty thin pin though (and soft) and if worn from vibration etc., it could have some significant stress risers to start with.

I'll get a pic the next time the tip is off.

12-21-2005, 09:16 AM
When I was installing my wing tip servo, I noticed this potental and decided to plan for the control rod to come off the servo in flight some day. What I did to plan for this failure, was to make a plexiglass restrictor mounted around the control rod where it passes through the second from the last outboard rib. I did not like the look of the small rod end, the servo arm ect... so with the restrictor, if the rod falls off from any possible failure point around there, and the rod end happened to become free, the restrictor keeps the rod from jambing in to anything and inhibiting the controls.
Look at the installation and say to yourself... "what if..."