As others have said, the film viewer program comes automatically as a component of all recent versions of Aces High. I think it is named "ahfilm.exe". Look for it in your filesystem, in the same folder where ah.exe (the main Aces High simulator program) is stored. Double-click on the name and it will start. From there, you can use the conventional "File" drop-down menu to find and load a film file whose name ends in ".ahf".
HOWEVER, there are two additional, complicating factors as follows:
1 of 2: Every film was recorded in a specific Aces High "terrain". If you don't have that terrain loaded into your "AHIITERR" folder, then the .ahf film will still run, but you won't see any land features, airfields, cities, etc. It will look like the flight takes place in the middle of the ocean somewhere. Often this doesn't matter, but if you want to watch takeoff, landing, or low-level combat dodging mountains and trees, you won't get the full effect.
2 of 2: Every film was recorded using a specific version of "Aces High". The Aces High flight simulator has been evolving for a long time, and dozens of different versions have become popular and then been retired over the years. Newer versions can store information into film files that older versions didn't know how to interpret. Older versions of Aces High stored film information in ways that newer versions of Aces High will misinterpret. Accordingly, if you want to look at an old film that somebody else has posted in these forums or on a different Internet web site, you may have trouble viewing it unless you have the corresponding, old version of the Aces High software. If you want to build and consult a comprehensive collection of Aces High films, you will probably find it to be a frustrating experience. I've done it, but I also have to maintain a large collection of old versions of Aces High. I hope this helps.
