Michael wrote:
Exactly – it’s a non-issue.
Maybe, but it shows that no thought has gone into the design – zero. It’s the kind of error that sometimes can be found on a brand new experimental kit of a brand new design, but is fixed rather fast when builders start to rivet the thing together, coming up with better solutions. Besides, unless it is a permanent connection (no, or seldom movement, but mainly used as a method to allow easy dismantling), these hinges are designed to have the piano wire free floating, using a cotter pin on each side to prevent it from falling out. On the other hand, I guess one could argue that a door hinge is used relatively seldom compared with a trim tab, or an aileron for instance.
LeSving wrote:
Maybe, but it shows that no thought has gone into the design – zero
It will have been built to last a x number of hours. Overdesigning an aircraft is terribly uneconomic.
LeSving wrote:
these hinges are designed to have the piano wire free floating, using a cotter pin on each side to prevent it from falling out.
No, it’s a fairly common practice across manufacturers. Cessna 206’s and 208’s have the piano wire fixed as well on the trim hinges.
Just changing the design is not always a good idea. As part of continuation training this safety advisory from the Commanders owners group was discussed as a nice example. While it seems logical to have the nuts visible some rotated a bolt 180 degrees, such that the nut is always visible. This bulletin showed that in this case, that wasn’t a good idea, and it was installed the other way around (which seems less logical) for a good reason.
Excellent example Jesse.
Once again, one shouldn’t be so quick to judge a design feature – there most prob is a very GOOD reson why it was done the way it was done and that may just include the TB20’s tab hinge …
I have no issue with which side the pin is. What I don’t get is using screws for the part which doesn’t wear and using rivets for the part which does wear.
Incidentally, the wear rate of these hinges is not related to movement of the parts. It is IMHO caused 99% by vibration/flutter. The gear doors rattle so much during taxi that one can hear them in the cockpit. They are adjusted so they are just-tight when the gear is up, of course. But one cannot prevent the elevator trim tab rattling around in the airflow and that is what wears the hinges; not the miniscule amount of movement during trim.
But I posted it here not just to have a moan 
but in case somebody knows why it was done that way.
That Commander bolt issue should be really obvious to anyone who watches the clearances when the gear is operated by hand with the plane jacked up.
Peter wrote:
I have no issue with which side the pin is. What I don’t get is using screws for the part which doesn’t wear and using rivets for the part which does wear.
Maybe there is a reason for this. I don’t know, but it could be.
How often did you need to replace this? When I did a lot of B1 work I always used to replace both halfs and the pin as an assy of this kind of constructions. when they are lubricated they should last a long time. I would expect this to be the first renewal on your aircraft? Not to bad then is it?
Peter wrote:
That Commander bolt issue should be really obvious to anyone who watches the clearances when the gear is operated by hand with the plane jacked up.
That’s to easy sitting behind a keyboard. “A mechanic / monkey” however you would like to call us, will have a hard time to see this, due to the nature of the construction. One usually don’t take the wheels of during this kind of inspection.
There are numerous of other issues where something might be done differently than what seems logical, for a good reason.
A totally different example
Once seen this issue with a old Cessna fuel selector
One fuel tank had more fuel in it then the other tank. Someone selecter 20 degrees to the left of both, thinking the engine would be fed with more fuel from LH then RH. It is too easy to blame someone for this IMHO. Sure he could have known from POH that it has only four positions. However I can also see where someone got this drift, having lines between LH and both and both and RH (and no lines between LH/RH and OFF) suggesting that these are all positions.
Some things some logical and are not, other seems unlogical but are logic when you figure out why.
How often did you need to replace this? When I did a lot of B1 work I always used to replace both halfs and the pin as an assy of this kind of constructions. when they are lubricated they should last a long time. I would expect this to be the first renewal on your aircraft? Not to bad then is it?
Mine (TB GT) are the later teflon coated version. No lube is allowed; apparently because it attracts dust/dirt which grinds off the teflon nicely.
You are right that at 15 years they are only just coming up to needing doing, but that’s because half the play comes from the trim linkage (and I changed the bushings in that a good number of times) and another 1/4 of the play can be fixed by changing the outermost two of the teflon coated pins (I changed those a few times too).
So by being reasonably pro-active about it, one can make the actual hinges last a long time. But no shop I know of does that. They determine the play exceeds Xmm and they change the lot. I reckon the current price is about €2000; more with labour. The shop won’t sign it off unless they do that. But this reduces the play by only about 50%, because the system is quite sloppy.
The hinges also come unpainted, which adds yet more work/cost.
BTW one could easily recover the teflon coated pins to a brand new condition if one found the same teflon heatshrink which the mfg uses to sleeve them. Unfortunately all the heatshrink I have seen is way too thin.
Peter wrote:
So by being reasonably pro-active about it, one can make the actual hinges last a long time. But no shop I know of does that.
I think they do. You can see it at my answer, as I was expecting this kind lifetime from it. It seems @Michael has similair experiance for lifetime of these. So I don’t really see your point?
Peter wrote:
Mine (TB GT) are the later teflon coated version. No lube is allowed; apparently because it attracts dust/dirt which grinds off the teflon nicely.
