Re: COZY: Major Control System Issues found in canard aircraft examination


cozygirrrl
 

Makes me wonder about the shortened main gear strut if the bottom 3-4" got badly overheated during taxi testing and the "fix" was to just cut the bad part off and shorten the gear?

Chrissi
CG Products      
 


-----Original Message-----
From: Marc J. Zeitlin
To: cozy_builders ; Canard Aviators Mailing List <canard-aviators@...>
Sent: Wed, Aug 17, 2016 10:57 pm
Subject: COZY: Major Control System Issues found in canard aircraft examination

Folks:

I was recently asked to help evaluate an Aerocanard aircraft that had a very hard landing - one which resulted in a broken nose strut, propeller tip damage and damaged brake calipers. While all of these things are semi-interesting, the most interesting thing about this incident (NOT an accident, as there was no injury or "substantial damage") was the root cause.

The owner of this plane had asked a member of the canard community - someone who had assisted with the purchase and helped fly the plane from it's purchase place to its current base - to assist with transition flights. However, in the interim between purchasing the plane and getting these transition flights, the owner had a local A&P shop (not one familiar with canard aircraft) perform what turned into, for no apparent reason, a $3K / 6 month Condition Inspection. During this CI, the owner also asked the shop to install a pulse width modulator for the electric pitch trim system, as the trim system was reported to be overly sensitive at higher speeds during the ferry flight.

During the incident/hard landing flight with two aboard, a number of issues came to light. First, the A&P was not able to successfully install the PWM unit, so that was not in the plane. However with substantial effort he WAS able to successfully reverse the original wiring to the electric trim motor so that when the "Nose UP" trim button was depressed on the stick, the trim created a "Nose DOWN" trim. And vice versa - when the "Nose DOWN" trim button was depressed, the trim created a "Nose UP" trim condition. Now, this would be merely annoying, once analyzed and figured out, as the two buttons still worked - you just would have to choose the right one.

HOWEVER, in concert with this issue, the trim system had a few other quirks - it had relatively poor geometry where it attached to the left elevator pushrod, leaving the effect of trim pretty nonlinear, depending upon where the elevator was positioned, and there was a ball retract pin that held the trim piston rod-end to the elevator pushrod rather than a bolt and limiting large area washers. This setup had a LOT of rotational play and looseness, so there was substantial hysteresis in the system, where a lot of trim motion command would have to occur before any trim change would happen.

Those issues were not the worst part of the trim system, however much we might want them to be - the worst part was that the trim system did NOT allow for full elevator motion when at its extent of trim. When trimmed for full "Nose DOWN" condition, it was impossible to move the elevator to a position where the TE of the elevator would be below the "in trail" position, meaning that there was no way to raise the nose when the trim was commanded or ran away to the "Nose DOWN" position. Before the reversed trim issue, this never caused a problem, as no one ever (in anything but an O-540 powered canard) commands full nose down trim, although it certainly COULD have been an issue if anyone had commanded it, or if the trim ran away.

Maybe you can see where this is going - with the reversed trim commands, the relatively unfamiliar pilot tried to trim the plane, but with the poor trim feedback due to hysteresis and geometry, had a difficult time figuring out that the trim was reversed, and while attempting to slow down and trim for slower speed, was actually commanding "Nose DOWN" trim, which then led to limitations in the ability to override the trim because of mechanical limitations in the elevator motion due to a very poorly designed and implemented trim system. VERY BAD.

Along with this, to add insult to injury, unfamiliarity with the AP system and it's switchology (a Navaid system which had the mode switch left in "Track" mode due to a poor checklist and a power switch on the main stick where it could be easily bumped without knowing it so when the AP turned on it would automatically try to go somewhere OTHER than where the aircraft's pilot was pointing it) caused the pilot to also be wrestling with the plane in roll while simultaneously wrestling with the absurd pitch issues.

They were very lucky to get back on the ground safely (for them, if not totally for the plane).

Now in my view both the owner and the pilot bear a bit of responsibility in not understanding the functionality of all aspects of the control system of this plane prior to taking off in it. But MOST of the responsibility rests with the A&P who did not check his work, understand what he was trying to accomplish or note the functionality before digging into the system so that he could at least put it back the way it was, and even more so with the original builder of the aircraft/trim system who implemented a trim system which, if it ran away to the full "Nose DOWN" trim condition was almost guaranteed to kill the occupants of the aircraft.

This plane will not fly again until the trim system (as well as the damaged parts of the plane) is repaired to correct working condition.

Draw your own conclusions here, but if nothing else, understand the corner cases of the safety critical components of your aircraft.

Oh - by the way - for reasons that cannot be fully understood, the builder of this aircraft also cut the main gear strut down by 3" - 4", so that the aircraft sits far lower than a standard COZY / Aerocanard / Long-EZ. This gives far less ground clearance for the prop and was clearly a contributor to the loss of a $2K prop in what otherwise would have been merely a hard landing - fixing the nose strut is relatively cheap.

Get good Pre-Buy examinations, folks, and check every damn thing after any maintenance, especially that done by someone other than yourself.

--
Marc J. Zeitlin                      marc_zeitlin@...
                                            http://www.cozybuilders.org/
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