One-liner: When is it more cost efficient to replace an aircraft rather than continue to maintain it? 450-character abstract: The report presents a model of the repair-replace decision for aging aircraft and data describing the C-5A cargo aircraft. The authors examine the decision whether to modify or retire the C-5A fleet. They also extend the modeling approach to evaluate prospective investment in additional depot-level capacity. The study’s preliminary conclusion is that the C-5A’s planned reliability enhancement and re-engining program modification is worthwhile if it occurs soon.