The TF33 is used on the B-52H, C-141's and many of the C/KC-135 models. The TF33, an adaptation of the older J57, employed technology that was essentially 15 years old. The TF33 power plants on the Joint STARS ground surveillance aircraft are 1950s and 1960s technology. A number of limitations are traceable to the old turbofan, including operational restrictions, increased maintenance costs and reduced reliability.
The TF33 is a derivative of the commercial JT3D engine, which powered the commercial Boeing 707, 720, and DC-8 aircraft. Both engines are manufactured by Pratt and Whitney, and they have a high degree of similar parts-between 40 percent and 70 percent, depending on the model or version of the engine. The TF33 engine is repaired by the Oklahoma City Air Logistics Center.
The original manufacturer, Pratt and Whitney, and two independent repair activities-Aviall and Greenwich Air Services-reported more than double the reserve capacity needed to handle the total TF33 engine workload as of 1997. However, only Greenwich Air Services is currently repairing the JT3D. Although Pratt and Whitney and Aviall previously repaired the JT3D, they closed these repair operations because of the declining commercial market and because the JT3D represents older technology. They did, however, express interest in the TF33 military workload.
DOD had an inventory of 3,600 TF33 engines as of 1997 - the largest of any military turbofan engine with a commercial counterpart. DOD had 60 percent of the 5,800 engine population as of 1997. The TF33 also has the largest workload of commercially derived engines in DOD's depots, with an expected workload of 0.9 million direct labor hours for fiscal year 1997.
The degree of similarity between military and commercial engines can range from 30 percent to 100 percent. For example, the interchangeability of parts between the TF33 and its commercial counterpart can range from 40 to 70 percent, depending on the model being compared. These engine types have a high degree of commonality in their engineering design and require the same repair processes, equipment, and skills to overhaul.
Three different engines have powered the KC-135 tanker aircraft. The first KC-135s were fitted with the J57 engine, which was later replaced with the TF33 engine. By the mid-1990s the Air Force was replacing most of these engines with the F108. The F108 engine, with an unscheduled removal rate per 1,000 flying hours of 0.10, has 91 percent fewer unscheduled engine removals than the J57, which has an unscheduled engine removal rate of 1.16, and 79 percent fewer than the TF33, which has an unscheduled removal rate of 0.48. The KC-135A's original engines, like those of the B-52H, were also 1950s vintage TF33-103s and didn't meet modern standards for fuel efficiency, pollution or noise levels. By installing new, CFM56 engines, the Air Force enhanced performance and fuel off-load capability dramatically. The modified airplane is designated a KC-135R. Two-re-engined KC-135Rs can do the work of three KC-135As.
By 2004 the actual depot price for TF33-103 maintenance was well over triple previous estimates; and the cost continued to rise. Previous Air Force studies, using Air Force Material Command figures, estimated the depot overhaul cost of a TF33-103 engine to be $257K (FY96$). Further, previous studies assumed cost would remain relatively constant over the remainder of the weapon system's life (until about 2037) and would never exceed $300K. This estimate is no longer valid (see page ES9). Between 1996 and 2001, the per-engine overhaul price rose from $257K to $539K. By FY03 the price had risen to $710K--an increase of 176% since 1996; and in FY04 price increased another 17% to over $832K. Boeing and the B-52H System Program Office estimated that an annual real cost growth of 2% over the remaining economic life of the airframe would be a reasonable planning assumption, but the Air Force program office increased this rate to 5% in light of the rapid price escalation since 1996. Based on the current depot price and the new cost growth assumption, the per-engine depot overhaul price at the estimated end of the B-52 airframe life, 2037, would be $7.5M in then year dollars.
Long-term experience with TF33-103 engine reliability indicates that approximately 87 engines will be removed from the current B-52H fleet each year for depot overhaul. On the other hand, today's new commercial turbofan engines are so reliable that the task force estimates that in a re-engined B-52H fleet, no engine removals would be necessary for the remainder of the B-52H's economic life unless caused by unforeseen failures, such as severe foreign object damage (FOD), combat damage, or equipment defects.
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