The Apache is simply Black Mesa’s version of the Gunship from Half-Life 2. As such, you can probably set it up the same way following one of the numerous Gunship tutorials. Check the Valve Developer Community for those.
From what I remember, it involves setting up a series of path_track_sky object and linking them together, similar to the path_tracks you would use for NPC patrols.
Sorry I can’t be more help. In truth, I’ve always avoided using both the Gunship and the Apache because of how finicky they can be. Hope some of this helps though.