As part of a continuing project on aging aircraft and the replacement-or-repair decision, the authors develop a parsimonious model of the decision and apply it to the US Air Force's C-21A transport and KC-135 tanker aircraft.

They find that, for the C-21A, it probably would be appropriate to undertake a 20,000 flight hour system and component replacement schedule-prescribed renovation in the 2012 timeframe, but the aircraft should be retired around 2020.