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Originally Posted by madcorgi
It's pretty important when wings are being screwed onto big airplanes. Mr. Higgins would know this better than me, so he can chime in.
When I worked on the 777 program, one of the sections I procured was the wing root area (section 44 and the keel beam, for Boeing folks), which were mated to Boeing-built wings, where tiny variations at the mating surfaces could put the wings pretty far off "even," which was a bad thing for a lot of reasons. This was the first digitally designed airplane, and there were targets outboard at some distance and at the tip that were laser measured within some really tight tolerance. I watched the first wing-to-body join happen on 001 and watched them hit the targets perfectly. Mighty impressive for parts designed and built 9000 miles apart by several different companies.
The key was that everybody measured things from common reference points, all called out from a level nominal point. Also--everybody used the same units of measure, which is another pretty important constant.
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When I first started there, we used simple surveyors' instruments - optical levels and transits - for this kind of thing. By the time I retired, we were using laser trackers. One of my responsibilities on any major airframe repair, as the tooling engineer, was to "shoot" the airplane periodically as it was torn apart, and again as it was being reconstructed, to ensure straightness and level.
There are locations of know value for station, waterline, and buttline that are marked with "golden rivets" that we "shoot" to get out values. I would have to figure jacking loads to twist the airframe back into shape as we removed and replaced large body panels, wing panels, or whatnot. Fun stuff.