Airframe Fixtures

[Sparweb]

Subjects such as this one remind me that I am still a new arrival in the Transport Category aircraft world, having spent much of my early career working on rotorcraft modifications, on both Normal and Transport Category helicopters, and only a few fixed-wing types. This may have biased my opinion on the matter of using airframe fixtures for extensive structural repairs and mods. For a long time, I have taken for granted that repairs that involved removing numerous skin panels or primary structural members demanded the use of a fixture. This is the standard in the rotorcraft world, but maybe there is room for debate where large aircraft are concerned. I have seen many helicopters in fixtures. I can also think back to seeing some small fixed-wing aircraft being re-skinned, sitting in fixtures to keep them straight. I can also think of others that were not. When is fixturing an airframe necessary, and when is it not?

Most facilities that do major repairs to helicopter airframes have fixtures that they use to align the major interfaces to known datum references as they disassemble the damage, and reassemble the new structures on an airframe held true. The OEM's even participate in this matter by certifying the fixtures when a repair station builds one. In the rotorcraft maintenance world, this is so common, that you can tell what airframes they are equipped to maintain by the fixtures you can see in their facility. Yes, this does have something to do with the tendency of helicopters to get "bent" more frequently than fixed wing aircraft...

The rotorcraft fixtures that I've seen are very similar: welded tubular frames, employing thick machined plates to bolt onto engine mounts, landing gear fittings, transmission lugs and the like. Most such fixtures are also anchored to concrete floors, and when the helicopter airframe is not inside you can often see alignment marks; probably used for checking the fixture before each use. The use of tubular structural steel seems obvious and the most suitable. When these members are placed properly the fixture does not inhibit workers from moving panels into place and assembling them, yet provide enough support that weight/clamping/fastening do not result in distortion.

However-
To scale up such a fixture for a transport category aircraft would be daunting. Before carrying out a major structural modification to a large fixed-wing aircraft, like re-skinning a portion of the belly, or enlarging a door, one would probably want options better than a gigantic steel spider. Your modern airliner also offers fewer "hardpoints" than a tyipcal heli. The nose gear/jacking point is a long way from the main gear, and you can't easily pull the wings off like a light aircraft will allow you to. The answer is not as obvious.

Do general guidelines exist for the sizing, construction and alignment of these sorts of fixtures (heli or fixed-wing)?

When one is considering a major repair or structural modification, what criteria should drive you to use or not use a fixture to support the airframe?

Should one expect an OEM (fixed-wing, transport category) to provide the necessary data to a repair facility, the way the rotorcraft industry does? I do not believe AMM Chapter 6 Dimension data alone is sufficiently detailed for this purpose. I believe a data licensing agreement could be in order.

During my (too few) visits to OEM factories, the fixtures they use are not appropriate for use in the field. They are either sub-assembly specific, or they simply cannot leave the factory. Can anyone provide an example of a fixture that would be appropriate for a large or medium aircraft, allowing a large number of skin panels to be removed?

STF

 

[LiftDivergence]

SparWeb,

I wish I could answer your question better, but here is my input. I have worked many repairs and modifications for aircraft where extensive skin work was done. This includes multiple skin panel replacements. Other types of repairs I have been involved with are several aircraft lap joint modifications (lap joint cut out and repair splice) for 737-300s according to Boeing SB 737-53A1177 R7 (at least, we worked repairs associated with any discrepancies encountered when working the SB). The 737 classics are a good example for aircraft where extensive skin rework may be required over the life of the airframe. There are several service bulletins that deal with skin modifications, and potential replacement of several skin panels.

I don't have extensive experience spending time at MROs (although I have been on site during 737 cargo modification work). I have never seen a fixture in the sense you are talking about (where the metallic framework completely encompasses and surrounds a helicopter) applied to a transport aircraft airframe.

That being said, there are guidelines and limitations for jacking / shoring / alignment which are very important.

I have seen a repair which was submitted to an OEM where precautions were not taken during reskinning, and differences in installation on other side of a butt joint were causing local wrinkling in the splice plate.

First off, there are guidelines for the number of cradles used and the locations where the cradles and jacks must be placed. The aircraft SRM typcially contains this type of data. For example, for the 737, SRM 51-50 details symmetry check and support of the aircraft for repair. This outlines support locations, the type of cradles and jig to be used, incidence and alignment check points, etc.

I would also look at some relevant service bulletins if you can get a hold of them. I don't know what type of aircraft you are specifically dealing with, but Boeing SBs like 737-53A1177 or some kind of equivalent for another OEM model will either explicitly outline all of the support procedures required for extensive skin removal, or provide a reference to other manuals which do so. It is the purpose of those SBs to give operators all the data they need. Usually, there are limits on how many skin panels can be removed at one time. These should not be adjacent to one another. The SB instructions provide an order, if I remember correctly.

I know the data I referenced is all OEM material, but if it is available, it might give you a better idea of the support procedures.

Keep em' Flying
//Fight Corrosion!

 

[Sparweb]

Thank you LD, that is an avenue I will pursue.

The current level of information currently available to me (medium commercial passenger jet) is in the SRM, which is a 1-page task with only the most general statements available. For instance:

...When large repairs are required, it is necessary to give support to the rest of the aircraft structure
before and during the operations. These include the removal of large areas of skin, plating,
frames, longerons, struts etc. Support must be sufficient to release the load on the damaged area
while keeping the position of the adjacent structure and components. You usually shore the
aircraft when it needs large structural repairs to specified areas and to prevent damage to other
parts of the aircraft. To shore the aircraft, you install contour boards below the lower surfaces of
the wings and fuselage or use steel support equipment. These are welded or bolted together to
support the aircraft. Use correct cushioning materials applied to those parts that will come in
contact with the aircraft....

I am in full agreement with this, but it is light on the specifics. Probably because no 2 repairs are the same.
The only cross-references in this particular task is to leveling and to symmetry checks. Necessary, but do not cover the meaty middle.

I do not have ready access to service bulletins on this aircraft, but I may be able to find someone who does, and/or other aircraft types that are more likely to have had a major skin repair.
While I am prepared to gain proper access to all documents I reference, these days it seems like my only contact with this OEM is with a "librarian" type. They can only provide a document if I can ask for the right title and number, and if it can be found. Open ended inquiries about what the documents contain go unanswered.


STF