An Airworthiness Directive (AD) is set to be released by the United States (US) Federal Aviation Administration (FAA) to prevent potential issues with the CFM International LEAP-1B engines. The directive is aimed at stopping any failures of the engines when running engine anti-ice (EAI) systems in dry air. The FAA has stated that the AD was prompted by a report from June 2023 which found that using the EAI system in dry air for over five minutes during certain environmental and operational conditions can cause overheating of the engine inlet inner barrel beyond the material design limit. This can result in the failure of the engine inlet inner barrel and severe engine inlet cowl damage.
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During the incident, which occurred during flight testing and analysis, it was discovered that the usage of the EAI system in dry air for over five minutes in certain combinations of altitude, total air temperature, and N1 settings can result in engine inlet cowl temperatures exceeding design limits when not in visible moisture. Subsequently, the excessive heat buildup can cause the engine’s inlet inner barrel to overheat beyond the designed limits, resulting in the LEAP-1B’s inlet inner barrel failure and subsequent inlet cowl damage.
To comply with the directive, airlines operating the CFM International LEAP-1B engines will be required to revise their current airplane flight manuals (AFM) to limit the usage of EAI and make changes to the minimum equipment list (MEL) to prohibit dispatch under a certain item. The FAA has stated that there have not been any in-service failures associated with the condition, but if it is not addressed, it could result in the departure of the inlet and potential fan cowl failure and departure from the airplane. Subsequently, the fuselage or the aircraft’s windows could be damaged, potentially resulting in decompression and hazard to window-seated passengers aft of the wing and/or impact damage to the wing, flight control surfaces, and/or empennage, which could result in loss of control of the airplane.
Another danger is that after the loss of the inlet, the increased aerodynamic drag and asymmetric lift could result in the added risk of the aircraft running out of fuel, resulting in a forced off-airport landing and injury to passengers. The CFM International LEAP-1B engines are used exclusively to power the Boeing 737 MAX aircraft family, which includes the 737 MAX-7, MAX-8, MAX-8200, MAX-9, and MAX-10. The FAA estimates that the AD will affect 402 aircraft registered in the US, with the cost of compliance with the directive being $85 per aircraft as it only requires a change in the AFM/MEL. However, the regulator has noted that the current AD is only interim, with CFM International currently developing a modification that will address the unsafe condition identified in this AD. After the finalization of the modification, the FAA could follow up with additional action. The AD will be published by the FAA on August 10, 2023, and will be effective 15 days after publication. Stakeholders will have 45 days to comment on the directive after its publication.