The U.S. Navy and Air Force, working with the CFM International joint venture, have developed an engine modification that increases the efficiency and extends the on-wing time of CFM56-2 (F108) turbofans powering the Navy’s E-6Bs and Air Force KC-135Rs.
The two services expect to save $2 billion in reduced fuel and maintenance costs over the life of the engine, according to the Naval Air Systems Command (Navair), which manages the E-6B Mercury airborne command post, a Boeing 707 derivative. The modification restores exhaust gas temperature (EGT) margins, improving fuel economy, and extends the engine’s TBO from 10 to 15 years.
Gerry Cronkrite, the Navy’s E-6B CFM56-2A engine manager, said the modification increases EGT margins–the difference between an engine’s redline and peak EGT during takeoff–by 16 to 20 degrees. The EGT margin shrinks as an engine ages and burns more fuel, resulting in higher exhaust temperature. “That’s how we achieve those cost savings,” Cronkrite explains in a video. “Because of the increased EGT margin, the maintainers aren’t going to spend as much time troubleshooting EGT margin issues, which is a big part of their task right now.”
The Navy and the Air Force have both struggled to reclaim lost engine efficiency in aircraft powered by older CFM56-2 engines, Navair said. “In our case, only half of the engine life was being regained after the first overhaul,” said Andy Noble, Navy CFM56 propulsion engineer. “We could not gain back the performance we saw with the original engine build.”
The Navy’s CFM56 engine team approached CFM about four years ago to develop a solution, Navair said. CFM submitted a proposal in 2009 to incorporate design improvements used in newer CFM56 variants. The Navy and the Air Force coordinated efforts to share the cost of the engine modification, including flight and ground testing and gathering the data required for certification of the modification by the FAA.
The services plan to incorporate the engine design improvements during depot-level overhauls at the Oklahoma City Air Logistics Center in Fiscal Year 2013, Navair said.