PAX or freight ops? Does the aircraft fly a time-critical schedule to interface with a daily central package-sort schedule? If so, component reliability might be a major issue, so research in this arena might reveal weak points it its systems.
ABF was flying rehabbed DC-9s in such a operation and had an instrument failure at an outstation. They had to hire a LearJet to deliver a replacement. This was the incentive for them to investigate retrofitting DC-9s with glass cockpit - lower DOC plus reliability.
Engine (maybe prop) upgrade could pay off too in fuel savings; Many carriers liked the CFM56 refit on the DC-8. The airframe had lots of structural life left, but needed modern engines.
Last edited by barit1; 3rd Feb 2013 at 02:44.