Lesser US carriers had for many moons used FAA Foreign Repair Stations for Heavy Maintenance - see weird ex-PanAm 707s at ATEL/Stansted, 1970s. But in February 1986 PanAm's Pacific routes were sold. By some measures the world's largest airline, UA, launch Customer/design-influencer of DC-10/10, took over 6xL1011/500. Director E&M said what the...is this? and declined to let them into his shed. (OK: he actually said he had no intention of setting up for a chicken-s---t fleet likely to fly on his watch for months, not years.) But his mechanics were represented by the IAM, a hefty US entity, tussling for turf with the Teamsters.
The airline put their L10s, lock, stock, limey engine, and butterfly fasteners, into the Cathay/Gulf/Air Lanka pool, thus accessing a mass of structural life data just as FAA was taking a great interest in the "ageing aircraft" issue. By no engineering measure did UA's Fleet suffer - Engineering Dispatch Reliability, or average carried allowable defects, for example. (Great fun defining such things - does Appearance Engineering count? or exist?) Labour cost was not an issue - indeed the visible man-hour rate and parts mark-up % were higher than they were themselves charging for their own (low-volume) 3rd. Party work, or were building into the Business Case for a big new MRO site at Indianapolis. The overall deal made sense - example: less revenue-blocked downtime p.a on L10 than on UA/MRO DC-10.
The issue outlived UA's L10. IAM brought it to the point of a Transatlantic Trade War, and IIRC, it was only the advent of Gulf War 1 which prevented exactly that.
What Euro carriers did - SR, LH, BA - was to export oddball jobs - like one-off structural mods, and to commit to their Engineering Unions that no redundancies would be caused by outsourcing. What we now have in EU is, precisely, retirement of in-house MRO capacity - huck's point: But the skilled workers just aren't there. We go overseas out of necessity. New entrants (fogies will remember when "Apprentice" was not a TV game show) can't/won't get their hands dirty. Quality is constant, in-, or out-of-house, because that's the rule of the RFP: no bean-counter gets a look at the bid until the Continuing Airworthiness team qualifies it. If politicos. now try to embargo US carriers from placing their MRO at the site best suited - cash+time - to the operation, they will accelerate the demise of the industry.