To Make, or to Buy hangar maintenance: the key is that very word “line” of mtce. 539 (!) a/c operated to a repetitive schedule, minor seasonal fluctuation, throw up time-due inspections wholly predictably at a volume where it makes sense for the Operator to “own” labour/hangar/backshops. “Ownership” of steady-state” labour/land might be dearer than a market offer, but control of downtime, and flexibility of inputs give Operator payback. Ryanair put everything out to market in its early days, then brought selected work in house when volume moved from “pulse” to “line”.

In logic what Southwest puts out to C.America are labour-intensive, but downtime-erratic teardowns with unknown rectification workload, with risk of “waiting for parts” (in my youth, the 2nd. or 3rd. D-check). Such findings would disrupt a “line”. Sane management, in harmony with sane Unions, dump such dog-inputs: Singapore Airlines did the first D-check cycle on its early 747s, but sold them off before the 2nd. became due, not to avoid cost, but to avoid hangar scheduling disruption.