Bellcrank bolt loose for stabilator control rod

[Tom Baker]

I just looked a 180 Cherokee that is in the shop. Where the rod ends attach to the ailerons have lock nuts. Where the stabilator attaches to the fuselage the bolt that goes through the bearing has lock nuts. I don't think when you have a bolt going through a bearing that the bolt is considered to in rotation. A bolt through a bushing is a different story.

[FlyingMonkey]

The A&P and I were working on the annual today, and we checked all the points in question in this thread. We found nothing loose.

[WmInce]

The A&P and I were working on the annual today, and we checked all the points in question in this thread. We found nothing loose.

Andy,

 

Check your PM's, your mailbox may be full.

[FlyingMonkey]

Andy,

 

Check your PM's, your mailbox may be full.

 

Thanks, should be fixed now.

[Doug Hereford]

Anticept,

I looked in ASTM T2245-11. Para. 6.4 requires a friction type locknut to be backed up with a non-friction type in bolts subject to rotation. I don't know if you have seen this or not, but it might be worth mentioning to FD. I suppose that they may not consider this bolt to be subject to rotation. In my opinion it should be treated as it is subject to rotation.

 

I have let my membership to ASTM lapse, so I cannot confirm that I have the latest revision, but that should be irrelevant anyway.

 

 

Doug Hereford

[Tom Baker]

Anticept,I looked in ASTM T2245-11. Para. 6.4 requires a friction type locknut to be backed up with a non-friction type in bolts subject to rotation. I don't know if you have seen this or not, but it might be worth mentioning to FD. I suppose that they may not consider this bolt to be subject to rotation. In my opinion it should be treated as it is subject to rotation.I have let my membership to ASTM lapse, so I cannot confirm that I have the latest revision, but that should be irrelevant anyway.Doug Hereford

Doug, I would not consider this bolt to be in rotation. In the standard category world lock nuts are used in the same manner as those in the CT.

[Anticept]

Wrapping up a 100 hr on this aircraft. The bolt still seems to be holding pretty well. It is a metal bearing in the rod end, and now that I have better light, I have noticed there's a thin layer discoloration on the rod end bearing, as in JUST starting to corrode. I can't even feel it with my finger. It's not binding at all, but I threw some lubricant at it anyways and will continue to monitor. There is also a bushing on the bellcrank (plain bearing).

[Doug Hereford]

 

I like the way Cessna and Lear do it. Corey if I get a chance to take a picture of this on one of of the aircraft I maintain, I will see if I can post it here. Without exception, they employ a double lock feature on flight control rod ends. The nut that Cessna uses is P/N MS17825-*. Lear uses MS14145L*. Both of these I seem to see at every rod end connection point to bell-cranks and control rods. They are also used on pulley bolts. Both of these employ MS24665-* cotter pins.

I realize that these MIL spec parts aren't necessarily applicable in ASTM, but I would think that they meet the standard requirements. Also, they probably do not come in metric variations (-* sizes).

If you are not familiar with these parts, you can Google them and see pictures of each. It may be worth sending to FD as well, as a possible suggestion for a better connection system.

 

Doug Hereford

[Runtoeat]

Corey, I recall seeing a slight gold color but don't recall if this was in the outer bearing race or the inner sleeve which the bolt goes thru.  Perhaps this is the coloration you see?  This coloration was noticed on both old factory original and new rod end bearings that I bought from FD.

[Doug Hereford]

Here are some pics of fastener configurations on a couple of different systems on Citation 560 XLS+ aircraft. As a matter of reference, this is a Standard Airworthiness, Transport category aircraft.

 

I need to correct a previous post where I stated that there have to be double safety features for bolts that are subject to rotation.

The requirement for SLSA (ASTM) and Standard are very similar. If the bolt is subject to rotation, and a self locking (like a fiber lock) nut is used, a mechanical (non-friction) safety must also be employed. This requirement is in ASTM F2245-11 para.6.4 that I mentioned earlier. 14 CFR part 23 and part 25 (small and large aircraft Airworthiness Standards respectively) have an almost identical requirement. From what I have been able to find so far however, ASTM stops short of part 23 or part 25 in that there is an additional requirement in both Title 14 rules for double safety features on critical systems (systems where if the fastener came off, safe flight/landing  could not be achieved).

 

The first and second pictures are of the attachment from the elevator tab actuator to the tab horn. These are AN 320 nuts (free running castellated, with cotter pin). This system only incorporates one safety feature. I would say that this fastener is subject to rotation (non-friction safety), but because it is not considered critical, one safety is good.

 

The third picture is of the elevator cable dampner assy. The pulley bolts are retained with self-locking nuts (MS17825) and cotterpins (MS24665) for the double safety required in critical systems.

 

The forth picture is of a support strut for the ACM. You can hopefully see the standard rod end/bearing, and the bolt and nut. This is a self locking MS21044 nut. This system is not subject to rotation, non critical (one safety, non-friction ok).

 

The last picture is the nose landing retract actuator attachment the nose gear trunion. It is again, an MS17825 lock nut with an MS24665 cotter pin for double safety (critical).

 

In any of the double safety configurations it can be noted that, even if the rod end bearing or pulley bearing were to seize, the nut could still not be backed off due to the cotter pin. These attachments are also very easy to inspect for condition.

 

I would personally consider the elevator control system to be critical on any aircraft. 

 

To me, a loose nut on a critical system needs to be explained if at all possible. If was maintenance induced, that is not good (tighten and fly on), but better than the alternative which would likely point to a design issue.

 

Doug Hereford

[Anticept]

Would a trim system that could get you landed somewhat safely be considered a secondary system?

 

Anyways I haven't heard anything more on the issue from FD. Just completed a 100hr on the bird and there is no change in movement. Added green torque seal (it had red at one point, it looks like) so this will help in the future for monitoring.

[Doug Hereford]

Anticept,

I have always considered trim systems to be secondary flight control systems. Need to do more research for a better answer to that one.

Your actions may be all that is ever needed to prevent a future unsafe condition. I just hope that if there is any possibility that this was caused by a design deficiency, that FD will issue a Safety Directive to force operators to use the same inspection techniques that you are using. Maybe an additional safety feature as well. That is why I included those pictures. When I inspect attachment features like those, I get a warm fuzzy feeling about their future condition.

 

Doug Hereford

[Anticept]

On one hand, I wish they would, but on the other, it's a pretty big pain in the ass place to get at. Only thing that looks more miserable is stuff in the tail, and I really want to know if they use child labor to reach some of that stuff for layups

[Doug Hereford]

Anticept,

I hear what you are saying. I can think of a bigger pain in the ass though.......... Landing in a crosswind at night with a panicking passenger, and a failed elevator control system.

Poor access might make redundant safety features even more reasonable.

 

Doug Hereford

[FastEddieB]

I think I found a single flight control linkage on my Sky Arrow that uses a jam nut - the elevator trim right at the trim tab.

 

Good time to mention that one might call trim "secondary", but a failure in that area can extend well beyond just not having trim.

 

It could lead to destructive flutter. A few years back I believe that's what led to a crash at Reno, though in a WWII plane at high speed, it might have just led to G's that were so excessive they caused the pilot to black out.

 

So, secondary or not, they deserve the same attention on preflight and in maintenance that primary controls do.

[Doug Hereford]

Noted

[Doug Hereford]

Tom,

I have noted the same thing over the years on some older aircraft. I wonder if it is a CAR vs FAR thing? A PA28-180 is a CAR 3 aircraft (so are other chreokees with the same stabilator attachment features). I looked all through that document and couldn't find much of anything on fastener standards.

 

Doug Hereford

[FastEddieB]

So far, this is the single locknut on the flight controls of my Sky Arrow:

 

 

It's looking up at the elevator trim linkage. The elevator trim motor is in the elevator.

[Runtoeat]

Eddie, just curious.  What is the approximate length and width of the trim tab surface?  Does the trim tab have any cantilevered counter weights that would allow for C.G. balance?

[FastEddieB]

Eddie, just curious.  What is the approximate length and width of the trim tab surface?  Does the trim tab have any cantilevered counter weights that would allow for C.G. balance?

 

No photos that show it clearly, but...

 

 

 

It is not cantilevered in any way. Don't see any obvious weights or way to balance it. I was not aware that trim tabs ever required balancing - they usually seem to be pretty simple affairs.

[FlyingMonkey]

 

It is not cantilevered in any way. Don't see any obvious weights or way to balance it. I was not aware that trim tabs ever required balancing - they usually seem to be pretty simple affairs.

 

A trim tab is like any other aerodynamic surface.  There exists some critical speed at which it will flutter.  It's possible that the flutter speed is far above Vne, in which case it's not a problem...

 

Or, as in my case, flutter can be exhibited at some fairly common speed.  My flutter problem is entirely a tab issue, not a stabilator issue.  The CTSW I have has a large, full-span anti-servo tab which I'm sure contributes to the problem.  More area means more force which in turn means more exaggerated behaviors.

 

In my case I had trim tab flutter at 135kt on an airframe with a 145kt Vne.

[Runtoeat]

Eddie, thanks for the diagram.  Just trying to learn more about what Andy's post describes.  As Andy indicates, some CT's have a trim tabs that run the full width of the elevator.  Our trim tab is also called an "anti-servo" tab.  This trim tab has small counter weights but these weights do not provide for total neutral balance.  High speed airflow over the trim tab may excite the natural frequency (or frequencies) with the result sometimes being what Andy's video shows.  The other factors we deal with on our CTSW's are additive looseness from the pitch control wheel, cables, bellcranks, hinges and rods that control this trim tab.  And, probably, structural weakness is involved in those areas where the trim tab hinges at the elevator - thus the SB which called for reinforcement of the hinge attachment area of the elevator on certain CTSW's was issued.  Your Sky Arrow trim tab appears to be relatively small and stiff and has a single link that connects directly to the servo in your elevator.  These factors should constrain the tab and prevent excitation of it when buffeted by airflow.  

[FastEddieB]

My Tiger and Traveler each had an "anti-servo" tab.

 

As you ran the elevator through its range of motion, the trim tab moved in the same direction.

 

As I recall, that has two effects:

 

1) It makes the controls feel "heavier", since you're fighting increasing trim - which may desirable.

 

2) It makes the elevator more effective, since as its deflected the trim tab moving in the same direction can deflect more air.

 

A servo tab moves in the opposite direction, aiding the pilot in the deflection, but moving less air.

 

Each have their place in aircraft design.

[Doug Hereford]

Fast Eddie,

That is an excellent point! Servo and Anti servo features are very aircraft specific, and speak to their critical nature. In the part 23 and 25 world at least, these will have redundant safety features for their control attachments.

For those who are familiar with it, that mysterious King AIr aileron trailing edge bump is another example of how something subtle related to a flight control can make a huge difference in performance and safety.

 

Doug Hereford

[FastEddieB]

Speaking of trim tabs...

 

The trim tab on the elevator of the Skycatcher has right angle extensions on both the top and bottom of the trailing edge.

 

Best photo I can find:

 

 

Seems inefficient, and I can't figure out the purpose.

 

Anyone?