I've replayed memory OA many times on Expert, applying extreme concentration and considerable A2 CO fighting skills, and still have not yet completed the CAS task. I have nailed the first technical many times with hydras, my miniguns sweeping up any remaining soldiers, who always run away to the north after the technical is blown. This first technical is always blown well north of the crash site, on the north side of the E-W drainage just north of the site. I then have gone further north and engaged the other technical and taken it out also. There is an empty BTR and a squad of troops up there at the "enemy position" marked on the map, but I have never had time to wipe them out before CAS fails*. Regardless of what I do, the CAS task fails, and that is most frustrating. Often, just AFTER the task fails, I get radio that several opfor units are approaching my position, even after I have destroyed several of them. Perhaps there are other unseen opfor infiltrating the crash site from the south? Don't know.
My impression is that, since the memories are "more challenging and optional" extra missions, one is not supposed to worry too much about completing this CAS task and just move on, or that there is a design problem with the CAS task completion trigger (perhaps relative to difficulty settings).
Once, out of frustration, after I had wiped out both technicals and after the CAS task failed, I landed the chopper up north, entered the BTR, and used it to wipe out the squad of opfor troops up there.