I was just recently an Aide to a 3 star and from experience I can tell you that mil-air for a 3 star and below is not guaranteed. There has to be a reason that comm air is not acceptable and then it has to be vetted through the JAG and in the case of an OPNAV Flag, DNS has to give the final approval. 4 stars and their equivalent have to fly on mil air for other reasons. That being said, it is not hard to justify mil air travel for a 3 star in CONUS and they almost always get mil air once overseas. With my last Boss, we always flew over to the desert comm air and then used mil air once in theater. The use of business class or first class for comm air travel is a whole other ball of wax that requires certain criteria in order to get and it is a pain in the ass. Most Flags, non 4 stars who have a requirement, actually like comm air because they want the mileage. It really depends on how senior the Flag is. We were usually the only Navy 3 star flying out of Andrews on one of the C-21's. The AF on the other hand seemed to work the C-21 VIP schedule a little bit better. 3 stars almost always get C-21's or C-35's for CONUS travel. FWIW, the Marine C-35's were the nicest planes to fly on by far. Once, JOSAC sent a C-9 to pick the 2 of us up without any other pax's. It all depends on JOSAC at that point and their plane availability.