I have just got around match-drilling the replacement hinges and found they can’t be easily gripped in the turret mill to measure the hole positions – because they are crooked!
Maybe there is a reason for this. I don’t know, but it could be.
Exactly, Jesse, you don’t know! Lots of things “could be”.
This is a gear door hinge, not a Cessna fuel selector. It’s not even a B787 fuel selector, but I fully admit it “could be”. But it looks like a hinge to me…
It turns out that the hinges are all different sizes. They are hacked by some little guy sitting in a French garden shed and he trims them by hand. The old ones are on the left:
The most likely reason Socata rivet the half which wears and screw the half which doesn’t is this: They drill and screw the screwed half to the gear door, and then somebody holds the gear door in position while somebody else marks the other (to be riveted) half of the hinge (through holes which were hand-drilled in the airframe in fairly random positions). Then the riveted half gets riveted, after which it can’t be moved, obviously. Then, because the first guy was not able to hold the gear door exactly right, they loosen the screws on that and waggle it into position and re-tighten the screws! Or some version of that,
Whereas if they bought off the shelf, god forbid American, hinges, which are extruded and all the same size and don’t need trimming if you pick the right one, they could have jig drilled the lot and it would just fit together, with just the above mentioned gear door adjustment.
Actually the hinges are extruded and then this little man gets a file and trims them, so they come out all funny widths.
This doesn’t explain the elevator trim tab hinges which actually are extruded without subsequent trimming… maybe they just copied the procedures used for the gear doors.
Peter wrote:
The most likely reason Socata rivet the half which wears and screw the half which doesn’t is this: They drill and screw the screwed half to the gear door, and then somebody holds the gear door in position while somebody else marks the other (to be riveted) half of the hinge (through holes which were hand-drilled in the airframe in fairly random positions). Then the riveted half gets riveted, after which it can’t be moved, obviously. Then, because the first guy was not able to hold the gear door exactly right, they loosen the screws on that and waggle it into position and re-tighten the screws!
That could also be done if the other part was screwed into it, doesn’t make sense to me.
Peter wrote:
Whereas if they bought off the shelf, god forbid American, hinges, which are extruded and all the same size and don’t need trimming if you pick the right one, they could have jig drilled the lot and it would just fit together, with just the above mentioned gear door adjustment.
Not sure why you don’t fly an American airframe, if you hate these French ones?
On a Piper of Cessna you would have the same, these parts are often undrilled, and when they drilled with pilot holes, they often don’t align with the rest of the construction. So trimming / drilling / fitting / adjusting these parts is normal. It’s not produced as a car.
Again little difference between USA and Europe.
Not produced like a car, sure, but it isn’t a car. I think jig drilling was perfected long before Brunel.
I wonder if anyone who actually knows this business can shine any light on why these procedures exist?
I don’t hate French planes; no idea where you got that idea from. I paid 195k+VAT of my hard earned after-tax money on this one in 2002 and would buy the same again, for the same mission profile. Further information can be found here
Archie wrote:
It will have been built to last a x number of hours. Overdesigning an aircraft is terribly uneconomic.
Fixing bugs is not overdesigning.
Archie wrote:
No, it’s a fairly common practice across manufacturers
That may very well be. But piano hinges are designed to have the wire free floating.
I can see a lot of stuff is done “this way” because it has always been done this way. Aviation is not the only place that happens.
But not predrilling things like hinges adds hours to the installation. I don’t see any benefit – except to the maintenance company being able to charge a few hundred quid more.
A number of people have reported €1000 labour just changing the aileron trim tab hinges.
The other problem is that most maintenance companies will simply avoid doing that sort of work. A few are crooks, but most just want the plane out of the hangar and get the next one in. Consequently, most planes end up shagged, with wobbly trim tabs, gear door hinges, etc. I could never get anyone to properly lube my landing gear. The company doing the Annual always got out of it. So I took it elsewhere, for a few hundred quid, but even they never completely greased all the parts. These arcane procedures just lead to a lot of GA being decrepit.
Especially when the hinges are all different sizes so the holes cannot be referenced to the edge(es) of the hinge. They have to be referenced to the very ends (actually just one end with the Socata ones because they vary in length too!) and to the pin. I measured it on a turret mill with a DRO but it took me a while and most fitters won’t have that facility; they will do it visually and then file out the holes into ovals so they can get the rivets in 
And still nobody has posted ideas on why the side which wears more is the one which is riveted…
I am not sure about floating hinge pins. Socata bend over one end and secure it, so it can’t come out. It’s simple and effective.
Peter wrote:
I can see a lot of stuff is done “this way” because it has always been done this way. Aviation is not the only place that happens.
That is what I mean by it is not manufacturered like a car. It is done on to go, as each one is an individual product.
Peter wrote:
I don’t see any benefit – except to the maintenance company being able to charge a few hundred quid more.
For sure, the average designer, doesn’t take maintenance in account. The result might be that the maintenance company has to spend a lot of time, because (sometimes) poor design. Sometimes, something seems to be poor design, but is done for a reason, see my post at 14. Apperently there where a few who changed the design, which actually didn’t work out.
Peter wrote:
Especially when the hinges are all different sizes so the holes cannot be referenced to the edge(es) of the hinge. They have to be referenced to the very ends (actually just one end with the Socata ones because they vary in length too!) and to the pin. I measured it on a turret mill with a DRO but it took me a while and most fitters won’t have that facility; they will do it visually and then file out the holes into ovals so they can get the rivets in
Installers will actually have a tool to copy the holes very accurate, and the hinge won’t get near a drill press or mill. During installation they like “just” drill holes, as making all kinds of templates cost much time as well.
Peter wrote:
Whereas if they bought off the shelf, god forbid American, hinges, which are extruded and all the same size and don’t need trimming if you pick the right one, they could have jig drilled the lot and it would just fit together, with just the above mentioned gear door adjustment.
If you have close look your hinges are an MS P/N. It is normal that these parts are deliverd in long lengths, and cut to size. This also happens at Cessna / Piper for example. Again no difference here, and it is quite possible that the part has actually been manufactured in the USA and cut to size in France.
Then they would also have to make the door in a jig, and then they should al be the same. If you take a look at multiple aircraft, with gear testing, you will see that aircraft doors are not identical either. Actually nothing is, and that is the reason why most sheet metal is deliver either undrilled and to large, so you can trim it for best fit. It’s just not like with a car, where parts are much more equal for a better fit. Yet if you would ask a car body shop, they will also tell you that they have to adjust parts for the best fit.
