In April 2018, a KC-135 Stratotanker aircraft at Tinker Air Force Base (AFB), Oklahoma, USA was being prepared for return to its unit following programmed depot maintenance when a mechanic in the 76th Aircraft Maintenance Group was in the wheel well—a space the size of a small closet—and noticed ~1.5 in (38 mm) of corrosion on the keel beam. The beam runs along 5 to 6 ft (1.6 to 1.8 m) of the body of the aircraft and supports its floor.
The keel beam, mostly hidden by other structural components, is standard aircraft aluminum with holes in it for fasteners, but also is comprised of complicated angles. Some aircraft parts are not intended to be removed, and the keel beam is one of them.
Following the initial discovery, the other KC-135 Stratotankers located at Tinker AFB were inspected, and some were in the same corroded condition. The discovery was alarming because corrosion of the beam had never been discovered or worked on previously, and failure of the beam would be catastrophic.
A one-time inspection of the KC-135 fleet (396 aircraft) was initiated, and affected aircraft not already at Tinker AFB were immediately grounded. Planning began immediately for them to arrive at the base one at a time for repair. Members of the 553rd Commodities Maintenance Squadron and KC-135 System Program Office became involved to coordinate the removal, manufacture, and replacement of the keel beam, none of which had been done before.
Originally, replacement of the beam in all 15 affected aircraft was expected to take a year, but the work and coordination of several units at Tinker AFB led to the project being completed in less than six months. The maintenance squadron found the material they needed in a thicker form that could be machined down to the correct dimensions, which cut the material delivery time from 12 weeks to two, then several shops worked together to create a prototype beam in just two weeks.
Source: Daisy Grant, Air Force Sustainment Center, www.afsc.af.mil.