The upgrades of the inherited Strategic Air Command KC-135 Stratotanker to E, R, RT and T models have extended their lifetimes to 36,000 (E) and 39,000 flying hours (R, RT and T), respectively. The last KC-135E was retired in 2009 and all remaining operational USAF KC-135 aircraft are of the KC-135R, KC-135RT or KC-135T series. Acquired by SAC in the late 1950s, according to the Air Force, only a few KC-135s would reach these limits before 2040; but at that time, some of the aircraft would be about 80 years old. The Air Force estimates that their current fleet of KC-135s have between 12,000 and 14,000 flying hours on them...only 33 percent of the lifetime flying hour limit...and none will meet the limit until 2040. Therefore, the USAF has decided to replace the KC-135 fleet. However, since there were originally over 500 KC-135s with the since-retired KC-135E included, these aircraft will be replaced gradually, with the first batch of about 100 aircraft to be replaced in the current buy. The effort to replace the KC-135 has been marked by intense controversy.