CFM56-7B Offers Reliability, Life Extension for Boeing 707 Platforms

October 29, 2020

In January 1980, the United States Air Force (USAF) awarded a design and development contract to integrate CFM56 engines from a fledging CFM International with the KC-135. The USAF publicly cited higher performance, lower life-cycle costs, lower noise, and easier support as the reasons for its selection.

Today, the KC-135 re-engining program remains one of the most successful modernization programs in USAF history. With approximately 2,000 engines in its arsenal, the USAF remains CFM’s largest customer. The F108, which is ideally matched to KC-135 and RC-135 missions, has more than delivered on the promised economic benefits in terms of reliability, fuel efficiency, fleet operations, commonality, and maintenance costs.

As the USAF continues to develop its Advanced Battle Management System (ABMS), extending the life of current Boeing 707 platform (E-3A AWACS, E-8C JSTARS) could prove to be essential. More than 90 derivatives are still operating with the USAF, U.S. Navy, NATO, and international fleets; upgrading these aircraft with advanced CFM56 engines would provide significant performance, fuel savings, life cycle cost savings, obsolescence avoidance, and improved overall warfighter readiness.

Although avionics, communications, and radar aircraft updates can keep the E-3A AWACS and E-8C JSTARS platforms operating for decades, the USAF has deemed the current powerplants to be unstainable beyond 2030 and no re-engining plan currently exists. Upgrading these aircraft with the commercial CFM56-7B engine would enable these platforms to fly well into the future as ABMS evolves.
Compared to the current engines, the CFM56-7B engine is capable of providing higher thrust capability with the possibility of a de-rate and 25-30 percent better fuel burn, depending on the application. The CFM56-7B also meets all environmental requirements, including ICAO* Stage 4 noise and CAEP/6 emissions.

Additionally, a CFM56-7B-powered 707 would:
− improve critical field length and hot day performance;
− increase cruise altitude for better radar coverage/survivability;
− enable longer time on station and mission radius;
− reduce the number of aircraft required to cover orbits;
− lower the burden on refueling tankers; and
− provide industry-leading reliability and maintenance costs.

The CFM56-7B gearbox (configured for the P-8A) has been proven to handle a 180 kVA IDG, exceeding the electrical power requirements for all 707 applications. Additionally, the engine could be installed without a thrust reverser; this would save weight and provide simplicity.

CFM56-7B engines, which power military Boeing 737 applications including the P-8A, E-7A and C-40A/B/C, are still in production with an active supply chain and worldwide MRO support network. More than 15,000 engines have been delivered to operators around the globe and the fleet had logged about 440 million flight hours and 427 million flight cycles through September 2020.


*International Civil Aviation Organization

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Jamie Jewell,

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Jens Martesen,

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