As of 2000 the Air Force had about 550 KC-135 tanker aircraft. The average age as of 2001 was 41 years; 100 of the 500 planes are in the depot for repair at any one time. These aircraft spend over 300 days on average in depot, though some have been in the depot for repair as long as 600 days. KC-135 depot maintenance time is significantly greater than that experienced by commercial airline fleets during their periodic heavy maintenance work, referred to as D checks, which most closely represents Air Force depot maintenance. The Air Force's goal to complete KC-135 depot overhauls is 153 calendar days. However, the actual time that KC-135s spent in the depot for maintenance increased significantly. In fiscal year 1991, KC-135s spent an average of 158 days in the depot; by fiscal year 1995, that average had increased over 50 percent, to 245 days.
With projected modifications, the KC-135 will fly and refuel into the 21st century. A new aluminum-alloy skin grafted to the underside of the wings would add 27,000 flying hours to the aircraft. Aircraft corrosion presents a significant challenge to AMC. It is presently difficult if not impossible to model this major life limiting factor over long periods of time. Technologies required to deal with corrosion have not evolved, leaving AMC with a deficiency that of not knowing exactly how long its older aircraft will operate economically. At current use rates, the KC-135 aircraft structure should remain sound. The fleet is projected to be in the Air Force service well into the new century.
Corrosion is impacting the ability of the Air Force to accurately predict the KC-135 service life to allow timely force decisions. If possible, Air Mobility Command will retain the KC-135 through at least a 56 year service life, and its replacement, the KC-X Advanced Mobility Aircraft, should be ready to enter the inventory by FY2013. The KC-X Advanced Mobility Aircraft is a proposed new tanker aircraft.
In fact, calculations using a predicted structural service life of 70,000 hours (structural data only) and based on current annual flight hours reveal that the structural life could extend into the twenty-second century. However, these numbers taken alone are misleading as they do not include the effects of corrosion.
Most experts agree that the R-model and T-model will continue to operate economically well into the new century. The R-models 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 TF-33 (E-model) engines were previously used but refurbished to an expected 6,000 hour service life. At current use rates, the TF-33 began to need another major overhaul around the turn of the century. Additionally, since the TF-33 does not meet FAA Stage III noise requirements for the year 2000, more time and money must be expended to ensure compliance. The Boeing 767 leasing proposal was intended to allow the rapid retirement and replacement of the KC-135Es.
The March 2004 Defense Science Board Task Force Report on Aerial Refueling Requirements found that "Usage, which induces material fatigue, is not the driving problem. Total flying hours are relatively low for the KC-135s: the current airframe average is about 17,000 hours. Fatigue life is estimated to be 36,000 hours for the E, 39,000 hours for the R. Cycles are commensurately low on average (3800 for the R and 4500 for the E). Thus, the airframes should be capable to the year 2040 based on current usage rates."
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