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  #1  
Old 11-19-2008, 08:34 PM
4kilo's Avatar
4kilo 4kilo is offline
 
Join Date: Nov 2005
Location: Fort Worth, TX
Posts: 154
Default Check Your Autopilot Servo Arm Screws!

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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Last edited by 4kilo : 11-19-2008 at 09:05 PM. Reason: Spelling
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  #2  
Old 11-19-2008, 08:42 PM
Bill Dicus Bill Dicus is offline
 
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Location: Shorewood, WI (Milwaukee area)
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Default 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
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  #3  
Old 11-19-2008, 08:46 PM
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kevinh kevinh is offline
 
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Default

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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Last edited by kevinh : 11-23-2008 at 11:02 PM.
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  #4  
Old 11-19-2008, 08:50 PM
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jferraro17 jferraro17 is offline
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Location: TX
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Default

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
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  #5  
Old 11-19-2008, 09:13 PM
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Geico266 Geico266 is offline
 
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Location: Huskerland, USA
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Default

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.



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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Last edited by Geico266 : 11-20-2008 at 05:32 AM.
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  #6  
Old 11-19-2008, 09:56 PM
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rvmills rvmills is offline
 
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Location: Georgetown, TX
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Default 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:



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
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  #7  
Old 11-20-2008, 12:23 AM
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az_gila az_gila is offline
 
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Smile Glad you are OK...

Quote:
Originally Posted by 4kilo View Post
....... After doing a few clearing turns, I started a loop.....

..... "Why is my #%&$ parachute in my apartment and not on my back!" ......

Pat
...but if it had got a little worse, perhaps that is why FAR 91.307.c is actually a good idea...

http://www.flightsimaviation.com/dat...rt_91-307.html
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  #8  
Old 11-20-2008, 05:41 AM
DeltaRomeo DeltaRomeo is offline
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Location: Highland Village, TX
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Default 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
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  #9  
Old 11-20-2008, 06:08 AM
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pierre smith pierre smith is offline
 
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Location: Louisville, Ga
Posts: 7,909
Default 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,
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  #10  
Old 11-20-2008, 06:12 AM
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Webb Webb is offline
 
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Default Securing

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