Here's what I'm noticing with my Fokkers and Tupolevs in 1968... I currently operate a fleet of Fokker F27-200s, Fokker F27-400s, Tupolev Tu-104As, Tupolev Tu-114s, and Tupolev Tu-124s (replacing Fokker aircraft).
I have just begun to replace my F27-200s, F27-400s, and Tu-104As and I have noticed that after 1.5 years of service, the appraised value of my aircraft is roughly 80% of the price of a new aircraft. Go figure.
From what I've heard as well, it seems like Airline Mogul may also factor in commonality into the game -- my costs have been relatively low compared to some of my rivals in my private world... at one point, with my Fokkers and Tupolev Tu-104As and with double the airplanes of my rival CominAir, my maintenance was half of his. His aircraft weren't too old (average age was 0.7 years), but he operated Helio H600B Stallions, a variety of Antonovs, Ilyushins, and Pilatus PC-6/350 aircraft. It appears that the commonality killed him. Since you operate both McDonnell Douglas and Boeing aircraft, there is virtually no commonality, despite Boeing having acquired McDonnell Douglas...
@Stephen: In the game, 10 years for an aircraft = old, but in real life, 10 years is pretty good... in fact, if the average fleet age is around 10 years, at least among the U.S. carriers, that's great. Just take a look at American Airlines' average fleet age... ;-)
