Yet DL has a lower TOTAL maintenance outsourcing rate than all other US airlines except for AA, as of this year. Based on the plans which AA has announced and is putting in place, DL will have the lowest rate of total maintenance outsourcing among large US airlines.
If you factor in the revenue that DL gains from insourcing to calculate DL's NET rate of outsourcing (and you can do the same thing w/ any airline if you want), DL's rate of outsourcing is just 15%, better than 1/3 the total rate of outsourcing of WN and US which have about the same percentage of outsourcing.
Note that US has an almost identical rate of outsourcing as WN, a carrier that built its business model FROM THE BEGINNING based on outsourcing of a large part of its maintenance operation.
There was a time when US airlines owned flight kitchens and their external (travel agency) res systems - but that has all changed too. I'm not sure why it is so difficult to grasp the concept that the industry is changing. Thus, it is valid to ask what percentage of total work that an airline has retained to keep - be it total maintenance or percent of total flying (vs regional carriers) is done inhouse.
DL did a better job of keeping jobs of its employees during BK and has added to its workforce using inhouse capabilities since BK. That is probably why DL will have the largest maintenance operation in the Americas by the time AA is thru w/ their cuts and DL continues to bring flying done by regional carriers back to DL employees.