Still 'passing gas' at fifty
Gary Parsons looks at the KC-135 on its fiftieth birthday
Incredible as it may seem, the USAF's venerable KC-135 celebrated its fiftieth anniversary in August, a remarkable achievement for one of Boeing's finest designs. Known as the 'Stratotanker', the KC-135's primary mission is to provide aerial refueling support to United States Air Force, Navy, Marine Corps and allied aircraft.
Born from Boeing's radical Model 367-80 jetliner (above left) of 1954, it is testament to the design of the aircraft that it still looks the part against today's modern types such as the 767 and 777. Nick-named the 'Dash 80' and intended as a replacement for the KC-97, the 367-80 was a world away from the propliner with its swept-back wings and four Pratt & Whitney JT3 turbojet engines, each producing 10,000 pounds of thrust. Because the prototype was constructed to sell first as a military-tanker transport, designated the original '717' by Boeing, it had few windows, no seats and two large cargo doors. Rolled out under the watchful gaze of company founder William Boeing on 14 May 1954, the 367-80 first flew on 15 July in the skies over Seattle. A remarkable $16 million private venture by Boeing, no orders had been placed for either the military of 707 civilian variant at this time.
But the aircraft's dazzling performance soon won admirers in the air force and a week later an order for twenty-nine aircraft, designated KC-135A, was placed with the prototype making that historic first flight on 31 August 1956. Many changes had been incorporated into the basic 367-80 design, and although the KC-135 resembles the commercial Boeing 707 and 720 aircraft, under the skin it's quite a different aircraft, being shorter and having a narrower fuselage. The first production aircraft was delivered to Castle Air Force Base in California on 28 June 1957. In all, 732 KC-135s were delivered by 1965, many of which continue to serve in updated form today. Air Mobility Command (AMC) manages more than 490 'Stratotankers', as the aircraft is officially known, of which the Air Force Reserve (AFR) and Air National Guard (ANG) fly approximately 250.
Substantial investment in a series of upgrade programs has happened during the last fifty years, including re-skinning of the lower wing surfaces, the installation of new engines and new avionics systems. Over a thirteen-year period, beginning in 1975, Boeing replaced the lower wing surfaces on all KC-135s with an improved aluminum-alloy skin, giving a projected service life beyond 2020.
Following this, the KC-135A's original engines of 1950s technology were replaced with French-built CFM56 engines, enhancing performance and dramatically improving fuel off-load capability; the resulting KC-135R variant can do the work of one-and-a-half KC-135As. Interestingly the original KC-135R was a short-lived reconnaissance version of the KC-135A, with today's re-engined variant initially termed the KC-135RE.
So, what happened to the 'B, 'C etc. variants? The KC-135B was ordered in 1962 with TF33 engines instead of the J57 turbojets and equipped as airborne command posts - these were later re-designated EC-135C. Following this the KC-135D was four RC-135As converted to tanker use, only being withdrawn from service a couple of years ago. More familiar is the KC-135E, a programme to re-engine all KC-135As with refurbished JT3D engines taken from used, commercial 707 airliners. This upgrade, like the KC-135R program, boosts performance while decreasing noise and smoke pollution levels. The modified KC-135E provides thirty percent more power with a noise reduction of eighty-five percent. Otherwise the system of variant allocation has been a little haphazard - the KC-135Q was the variant modified to carry the JP-7 fuel necessary for the SR-71 Blackbird, segregating the JP-7 from the KC-135's own fuel supply, and the KC-135T is a receiver-capable tanker, used for pilot training and operational refueling missions. The KC-135 is also in service with the air forces of France (eleven aircraft), Turkey (seven) and Singapore (four).
Some aircraft have been adapted under the 'Multi-Point Refueling System Program' in an effort to enhance the efficiency and flexibility of fleet. Forty-five KC-135Rs have been fitted with the Mark 32B hose-and-drogue and air refueling pods, supplied by Flight Refuelling Ltd., for refueling NATO and US Navy aircraft. This allows refueling of two probe-and-drogue aircraft at the same time.
Since the completion of the KC-135R programme, other upgrades have been introduced to keep the aircraft a fully integrated and flexible asset within the air force. In May 1995 the started to upgrade all aircraft under programme PACER CRAG (Compass, Radar and Global Positioning System), the upgraded avionics suite comprising of cockpit enhancements including an integrated flight management system, liquid crystal flat-panel multifunction flight display, and forward-looking predictive wind-shear weather radar. The flight management system has been integrated with a traffic alert and collision avoidance system (TCAS) and an enhanced ground proximity warning system (EGPWS). Subsequent to PACER CRAG came the Global Air Traffic Management (GATM) initiative, focusing primarily on upgrading the aircraft's communication and navigation systems to free operation in civil airspace.
The latest in the long line of upgrades is ROBE, the 'Roll-On Beyond Line-of-Sight Enhancement' Spiral 2 programme. ROBE will allow allied forces in battle zones near-real-time communications with any headquarters across the globe, providing theatre commanders a better picture of the battlespace. To keep communications flowing in near real-time, ROBE Spiral 2 is able to take information from many different sources, combine them into one stream and upload to satellites, eliminating line-of-sight limitations.
A familiar sight to many enthusiasts in the UK, managing KC-135 operations in Europe is the 100th Air Refueling Wing, based at RAF Mildenhall in Suffolk, England. The wing's annual flying programme totals more than 6,000 hours, offloading almost 11 million gallons of fuel to receiver aircraft. The 100th Operations Group manages all air refuelling support for US Air Forces in Europe's theatre of operations - it provides trained crews, mission ready aircraft, and planning and coordination for all mission activities supporting flying operations. The Group comprises of the 100th Operations Support Squadron, Operations Squadron and the 351st Air Refueling Squadron, which flies fifteen KC-135R Stratotankers, including half-a-dozen or so multi-point hose and drogue aircraft for NATO operations. The 351st has a rich heritage - part of the 100th Bomb Wing (nicknamed the 'Bloody Hundredth due to its alarming casualty rate), one of the many Suffolk-based B-17 units during the Second World War, it operated from nearby Thorpe Abbotts, near Diss. As a salute, the squadron's KC-135s all carry the 100th BW's 'Square D' insignia on the tail, and is the only USAF unit approved to do so in peacetime.
The 351st flies approximately 7,000 hours a year, supporting NATO right across Europe as far as Turkey. Most of its KC-135Rs have only amassed around 16,000 hours each in their forty-odd years of service, nothing compared to a commercial Boeing 707 that would have been operated around-the-clock and accumulated that sort of mileage in four to five years. Most of the 351st's trade are the F-15 Eagles of the 48th Fighter Wing, based at nearby RAF Lakenheath, but all USAFE fighter units make use of the 100th ARW on a regular basis. Mildenhall is ideally situated to enable the 100th to routinely support USAFE and also Trans-Atlantic 'Coronet' flights of fighters detaching from the mainland USA. The 351st is seen as a good posting for KC-135 pilots due to the tempo of operations - as they like to claim, "No-one kicks ass without tanker gas!" During Operation 'Enduring Freedom' in the Middle East the squadron has passed more than twenty million pounds of gas since 2001, and still supports operations today.
With an average age of forty-five years, the current KC-135 fleet, with projected modifications, will fly well into the middle of the century, being expected to remain in service well into the 2020s and possibly as far as 2040; some airframes could be approaching eighty years old by retirement!
However, aircraft corrosion presents a significant challenge - it is presently difficult, if not impossible, to model this major life-limiting factor over long periods of time. At current use rates, the KC-135 structure should remain sound - calculations using a predicted structural service life of 60-70,000 hours and based on current annual flight hours reveal that the structural life could extend into the twenty-second century! But these numbers taken alone are misleading, as they do not include the effects of corrosion.
The KC-135R's maintenance capability and reliability rates are among the highest of any weapon system AMC operates, and its operating cost is the lowest. The E-model economic service life is markedly different because of the difference in age and technology of some of its major components, most notably the engines. The basic airframe should, in theory, last as long as the R-model, but the age of the engines points to the likelihood that upkeep could become expensive in terms of parts and maintenance man-hours.
The Air Force has recently accepted that its Economic Service Life Study (ESLS), published in February 2001 and stating that the KC-135 fleet would be "structurally viable until 2040", was conceptually flawed because it did not adequately predict fatigue and corrosion problems. During 2002 AMC conducted a further study on the expected lifetime of the KC-135 tanker fleet through the 'Aging Aircraft Study' (also called the 'KC-135 Corrosion and Service Life Report'). The study's findings led the Air Force to conclude that corrosion, not the number of flight hours flown or the mission assigned to the KC-135, had the most significant impact on the expected life span of each airframe. The study found that as airframes advance in age, the diagnosis and treatment of airframe fatigue and corrosion in the KC-135 demanded greater man-hours during the aircraft's heavy, or depot-level, maintenance. Only complete re-manufacture of the aircraft can eliminate corrosion, and that airframe corrosion in the KC-135E is "significant, pervasive, and represents an unacceptable risk" in terms of airframe life. The study stated that "Although corrosion susceptibility is understood, there are no accurate growth rate models." An Air Force assertion is that "One of our greatest concerns is a potential fleet-wide ground event that could emerge with little or no warning because of the unforeseeable effects of corrosion on the aircraft."
On 1 May 2003 the Air Force published its KC-135 Business Case Analysis (BCA) that combined information from the ESLS, updated depot maintenance cost data, and the Aging Aircraft Study. The Air Force concluded that corrosion, increased operating costs, and other operating uncertainties justified a decision to retire forty-four KC-135Es in Fiscal Year 2004. Between 1991 and 2003, KC-135E corrosion-related depot maintenance increased more than three times, thirty percent of the tanker's heavy maintenance man-hours now devoted to the mitigation of corrosion damage. The Air Force Fleet Viability Board (FVB) analysis estimated that availability of the fleet would drop below sixty-five percent starting as early as 2023 - it recommended retiring the 'E' models as soon as possible, and completing recapitalisation of the entire KC-135 fleet no later than 2030. The KC-135E models are the oldest and least capable tankers in the inventory, now used primarily for stateside training, homeland defence, and air bridge activities. The Air Force has requested to retire seventy-eight 'Es in FY 07 and the remaining thirty-six in FY08.
In January 2006 Secretary of Defense Donald Rumsfeld announced the cancellation of the KC-767 programme intended to replace some of the KC-135 fleet. The tanker replacement programme was based on a competitive procurement of a commercial derivative tanker with a notional contract award in FY07 and first delivery in FY10. A new request has now been issued as the KC-X programme, but the timescale for implementation will now be several years 'to the right'. Thus the Air Force will continue to depend upon its Stratotankers for a good few years, if not decades, to come - a lot of gas is still to be passed!