Really no limit of altitude to start the Tac Decent as long as you have enough sky between you and the ground. Here is why the C-17 has the capability. Say your are flying over un-friendly ground forces that might have shoulder launched capability to shoot at aircraft. You want to stay as high as possible for as long as possible till you reach the objective. Flying at 37,000 feet and being directly over the landing airfield, the aircraft can descend at 20,000 feet per minute and be at 7,000 in 120 seconds. Returning to normal flight takes 2,000 feet of altitude now your at 5,000 feet and can stay in a circling approach directly over the airfield and away from the bad guys and perform a max effort landing like this video demonstrates.Wow....what's the starting altitude?