PARIS—CFM International has unveiled details of an upgrade to the Leap 1 which has been developed to fix the long-running coking issue around the fuel nozzle that has affected the engine on the Airbus A320neo and Boeing 737 MAX fleets.
The coking problem was traced to temperature-induced evaporation of unburned fuel around the nozzles which inject fuel into the combustor. This led to to hard deposits of solid carbon being gradually laid down in some parts of the engine, eventually shortening time-on-wing between overhauls and in some extreme cases even causing in-flight shutdowns.
CFM joint-venture partners GE Aerospace and Safran say the initial investigations showed that coking was triggered by residual heat in the engine core hardware “soaking back” after shutdown, which caused the temperature of the fuel nozzles to go above the coke-formation threshold temperature. “This is designed to remove the heat after shutdown”, says Karl Sheldon, GE Aerospace’s executive vice president for CFM.
Speaking on the eve of the Paris Air Show, Sheldon says the solution is an add-on retrofittable external device called the reverse bleed system (RBS), which consists of a valve that opens after engine shutdown and a blower which turns on to blow air in the reverse direction through the existing engine bleed duct system. This provides cooling air into the flow path and prevents the residual heat from pushing the fuel-nozzle temperature above the coke-formation threshold.
CFM says the system automatically turns on after engine shutdown and runs for a specified amount of time before automatically shutting off. The RBS is expected to be introduced into production engines and available for retrofit in 2024.
Individual components of the RBS and the overall system have been tested on the ground as well as during a series of flight tests using GE’s 747-400 flying testbed at Victorville, California, as well as on an A320neo and is currently flying on one of the two A321XLR development aircraft. “Some of this testing also includes exposure to intentional high levels of vibration, to ensure robustness,” CFM says.
“This testing is a mix of certification tests, as well as additional testing that CFM has chosen to do to ensure the system is mature and robust before introducing it. Various tests have also been done to ensure it provides the intended cooling effectiveness across a wide range of environmental conditions,” CFM adds.
In other areas, CFM is also tackling vibration issues that have impacted starter air valve reliability and premature wear in bearings. The starter air valve has caused problems, particularly on the Leap 1A, and will be solved by isolating the unit with a revised housing on the fan case. A fix for this and a non-synchronous vibrations wear coupling issue on the bearing housing will be introduced later this year. “The good news is that we have managed that through the last several months using data and analytics from the sensor suite on the engine to help manage and predict issues ahead of time,” Sheldon says.
Improvements to the high-pressure turbine designed to enable CFM56-like durability have also been validated in a series of dust-ingestion tests now getting underway at GE Aerospace’s facility in Evendale, Ohio, and will be introduced into the fleet later in 2024.
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