I was wrong, it was SAM 27000. Not sure where I got "MAC" from, but SAM means Special Air Mission. That the tail number of the plane. When the President wasnt on board its call sign would be its tail number. Or at least thats the way it happened when nixon was officially out of office yet onboard AF1.


Actually any aircraft that has POTUS on board is referred to by the branch of the military and then "One" (Air Force 1, Army 1, Marine 1, Navy 1, Coast Gaurd 1)