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.
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?