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F108 / CFM56 Engine

The F108/CFM56 is manufactured by CFM International, a consortium of General Electric and the French company, Snecma. The F108/CFM56 powers the Air Force KC-135 tanker and Navy E-6 airborne communications aircraft. The first flight of a CFM International CFM56 two-shaft turbofan engine was made on Feb 16, 1977, installed in the first McDonnell Douglas YC-15 as part of the AMST development program.

The CFM56-2 engine is a high bypass ratio, separate flow turbofan with a sea level static thrust rating of 22,000 lb, nominal bypass ratio of 6:1, an overall pressure ratio of 28:1, and a maximum exhaust gas temperature (EGT) of 1598 F (870 C). The single-stage fan and three-stage booster compressor is driven by a four-stage low pressure turbine (LPT); only the first-stage LPT nozzle is cooled, using fifth-stage high pressure compressor (HPC) bleed air. A single-stage cooled high pressure turbine (HPT) drives the nine-stage high pressure compressor. The HPT blades and vanes feature air cooling through nose holes, gill holes, and trailing edge slots, using cooling air from the HPC discharge.

The CFM56 has three general configurations: the CFM56-2, CFM56-3, and CFM56-5. The Air Force and the Navy aircraft use the CFM56-2 configuration, which also powers re-engined DC-8 aircraft used by the commercial aviation industry. The CFM56-3 and CFM56-5 configurations power the Boeing 737 and various models of the European Airbus. DOD has 25 percent of the 6,300 engine population. The military is by far the largest user of the CFM56-2 engine configuration. However, General Electric engineers consider the CFM56-2 and the CFM56-3 to have 80 percent commonality, excluding the fan module. The military owned 1,600 CFM56-2 engines as of 1997, while the private sector had 3,944 CFM56-2 and CFM56-3 engines. This engine is repaired by the Air Force at the Oklahoma City Air Logistics Center.

The quieter, more fuel-efficient CFM56 engines are manufactured by CFM International, a company owned jointly by SNECMA of France, and General Electric of the United States. Because the KC-135R uses as much as 27 percent less fuel than the KC-135A, the U.S. Air Force can expect huge fuel savings by re-engining its fleet of KC-135s - about $1.7 billion over 15 years of operation. That's enough to fill the gas tanks of some 7.7 million American cars each year for 15 years. Annual savings are estimated to be about 2.3 to 3.2 million barrels of fuel, about three to four percent of the Air Force's annual fuel use. This equals the fuel needed to provide electrical power for 145 days to a city of 350,000 to 400,000 people.

Re-engining with the CFM56 engines also results in significant noise reductions. Areas surrounding airports exposed to decibel noise levels are reduced from over 240 square miles to about three square miles. This results in a reduction in the noise-impacted area of more than 98 percent. Maximum take-off decibel levels drop from 126 to 99 decibels. Boeing has delivered approximately 400 re-engined KC-135Rs and is under contract for about 432 re-engine kits. Each kit includes struts, nacelles, 12.2 miles of wiring, and other system modification components.

DOD considers the F108 to be part of a family of engines whose combined in-house support provides important synergy of expertise, equipment, and facilities for the DOD core capability. Thus, in the past DOD has concluded that the F108 should be maintained in DOD facilities. According to the DOD report to Congress on commercial counterpart engines, the F108 is most like a commercial model that does not have a large inventory. However, according to General Electric engineers, CFM56-2 and CFM56-3 engines are very similar and the F108 shares the same repair process as some other engines. Four private repair activities, including the manufacturer - General Electric, were interested in repairing the military CFM56 engine, and they had enough reserve capacity to perform four times the military's projected fiscal year 1997 workload.




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