Is there a maximum altitude that the FS9 AI planes can fly to? I have most of the heavies flying at about FL 360 or 370 but for my F-18's, Concorde, and high altitude military planes (B-52, etc) I'd like to go higher. Appreciate any feedback I can get so I can set more realistic altitudes.

It is not so much how high they can fly - but how they can descend and land in about 50-70 nm.The flight dynamics of the individual aircraft will control whether or not the aircraft can maintain altitude after it is generated in cruise flight.John Murchinson's Concorde seems to be able to hold FL480-500 pretty good.But if the cruise altitude is set that high - I've had trouble getting them to land. In general FS likes the cruise altitude to be no more than 35,000 or 36,000 higher than the landing airport altitude so that the descent routines work out.Most AI aircraft can survive descent rates of about 7,000 FPM, but some models start failing when they exceed that rate.The descent starts when the aircraft is generated about 100 nm from your aircraft if the landing airport is in your active area. Initially the AI aircraft start to descend at about 2,000 fpm.About 70-80 nm from the landing airport - the AI aircraft is assigned an intermediate waypoint with an altitude target. This target is usually aobout 10-20 nm from the IAF point and the target altitude is about 5,000-7,000 ft AGL.So your can end up with an AI aircraft at FL430 assigned to get down to FL050 in 50 nm. And they will try to make it. Some will, some will not.I'm not considering the steep dives of AI aircraft in the first few minutes of a flight session which are generated too close too high in the same catagory.You just have to tweak your flight plans to get an altitude and a landing behavior you can be happy with.