Private equity firms are increasingly targeting the little-known companies that fix planes for commercial airlines — preying on an industry with already weakened oversight and further stoking aviation safety advocates’ fears.

In the past, commercial airlines in the United States did the critical work of inspecting and repairing their fleets in-house, employing teams of mechanics to conduct airplane maintenance at their facilities across the country. But over the last two decades, repairs have increasingly been outsourced to third parties, many of which operate overseas.