In September 2006 the Air Force's KC-135 Stratotanker fleet celebrated 50 years of enhancing fighter, bomber and cargo aircraft missions. In 2009, the last KC-135E retired from the inventory. The aging tanker has received several upgrades over the years to the point it is almost a completely different aircraft than when it first flew in the 1950s.
The KC-135 was purchased in the late 1950s and early 1960s, as an interim while the Air Force developed a dedicated tanker to escort the bombers on their strategic missions. The B-29's aging airframe and limited fuel offload capability definitely made it an interim tanker (although the last of the B-29 derivatives - the KB-50J - didn't retire until November 1957). In the late 1940s Boeing had always considered the KB-29P an interim tanker and soon enough was working on a tanker version of the C-97 Stratofreighter with more fuel carrying capability. However, even the new KC-97 operated with several limitations. While a single KC-97 could adequately refuel a B-47, it took two or more to refuel a B-52.
The Boeing Military Airplane Company's model 367-80 was the basic design for the commercial 707 passenger plane as well as the KC-135A Stratotanker. In 1954 the Air Force purchased the first 29 of its future fleet of 732. The first of these aircraft left the assembly line at Boeing Airplane Company, Renton, Washington, July 18, 1956, and flew for the first time August 31, 1956. The Air Force received its first KC-135s at Castle Air Force Base, Calif., Jun 28, 1957. The first aircraft flew in August 1956 and the initial-production Stratotanker was delivered to Castle Air Force Base, Calif., in June 1957. The last KC-135A was delivered to the Air Force in 1965.
SAC maintained nearly a one-to-one ratio of KC-135s to B-52s. Often, they set on alert status together, with the tanker providing fuel in a "buddy" fashion. On the other hand, forward basing the tankers to rendezvous with the bombers offered a significant advantage: forward-based tankers had more fuel to give the bombers since they did not have to travel such a great distance.
In addition, Boeing built 88 aircraft in over 30 different models for other Air Force uses, such as flying command posts, pure transports, electronic reconnaissance and photo mapping. The last of these special-purpose aircraft was delivered in late 1966. Responsibility for the KC-135 was transferred to Boeing in Wichita in 1969.
In Southeast Asia, KC-135 Stratotankers made the air war different from all previous aerial conflicts. Mid-air refueling brought far-flung bombing targets within reach. Combat aircraft, no longer limited by fuel supplies, were able to spend more time in target areas.
Structurally, the KC-135 is similar but not identical to the Boeing 707 commercial airliner. It is a swept-wing, long range, high altitude, high speed jet transport. The KC-135 can haul either 83,000 pounds of cargo, airlift up to 80 passengers or carry 202,800 pounds of JP-4 jet fuel, most of which is transferable for global refueling missions.
The primary mission of the KC-135 is the refueling of strategic long-range bombers. It also provides air refueling support to Air Force, Navy and Marine Corps aircraft as well as aircraft of allied nations. The KC-135 is equipped with a flying boom for fuel transfer; a special drogue can be attached to the boom on the ground so it can refuel probe-equipped aircraft. During air refueling, the large flyable boom attached to the airplane's belly can offload fuel at 6,500 pounds per minute. This is enough fuel in one minute to operate an average family car for one year.
Normally during inflight refueling the boom operator is in radio contact with the receiver aircraft. The hook-up is made by directions given to the receiver aircraft through a system of lights located on the belly of the aircraft just behind the nose gear. The KC-97 used the same system. The fuel cells in the tanker are made of nylon fabric less than one-sixteenth of an inch thick. A fuel cell weighing 80 pounds will hold seven tons of fuel.
A major program to replace lower wing surfaces on the aircraft was completed in 1988, with a total of 746 C/KC-135 aircraft -- most of them tankers -- modified over a 13-year period. The work involved replacing about 1,500 square feet of aluminum on the underside of the wings -- which carry most of the wing load in flight -- with an improved aluminum alloy.
The original wing surface consisted of a type of aluminum more susceptible to fatigue. Skin panels were milled, machined and contoured at Boeing. The wing then went into a rivet assembly jig where stiffeners and skins were joined. The bottom section of the old wing was cut away and replaced by the new sections. In addition to the skin panels, engine strut fittings were also replaced. Each unit required a total of 564 parts, 32,200 steel fasteners and 19,500 aluminum rivets.
Remaining in-service KC-135A's have been modified with new CFM-56 engines produced by CFM-International. The re-engined tanker, designated the KC-135R, can offload 50 percent more fuel, is 25 percent cheaper to operate and is 96 percent quieter than the KC-135A.
On 20 February 2000 officials from Air Mobility Command announced the stand down of 198 out of a fleet of 546 C/KC-135 "Stratotanker" air refueling aircraft. The stand down of one-third of the Stratotanker fleet was the result of a suspected defective part that was being used to repair the aircraft's flight controls. The Stab-Actuator, roughly a large, metal, motorized nut is located on a jackscrew assembly in the aircraft tail that allows aircrew to trim the stabilizer in flight. Officials stated the stand down measure was taken as a precautionary measure to ensure flying safety and was in no way connected with the recent crash of the Alaska Airlines ME-83. The Air Force had been in the process of replacing, what was in some cases, an original factory part in this 44-year old aircraft fleet wide when this problem surfaced. However, many KC-135s had done a lot of sitting on alert with the then Strategic Air Command, and did't have as much total flying time on them as some of the other aircraft did.
The KC-135 Reduced Vertical Separation Minimums (RVSM) modification provides increased accuracy in measuring the aircraft's altitude. The KC-135 System Program Office at Oklahoma City Air Logistics Center administered the modification, making the fleet certified for operation in RVSM airspace. Air Mobility Command issued RVSM certification for KC-135E/R tankers in January 2002. As of early April, more than 190 KC-135 aircraft had been released to operate in RVSM designated airspace. RVSM-compliant aircraft are allowed to operate at altitudes where aircraft vertical separation has been reduced to accommodate increased aircraft traffic. The RVSM designated altitudes provide optimum aircraft cruise conditions and minimize fuel consumption. Aircraft that do not comply with RVSM requirements must fly above or, primarily, below the RVSM designated altitudes and will experience increased fuel consumption and typically slower cruise speeds.
As of May 2002, the Air Force had 545 KC-135 Tankers, 134 E Models and 411 R Models. In order to replace the aging E Models and thereby save maintenance costs, the Air Force proposed leasing 100 Boeing 767 Tanker/Transport aircraft to replace 127 E Models of KC-135. This plan would be completed by 2009, with seed money for the project first appearing in the FY05 budget.
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