The US Federal Aviation Administration (FAA) has flagged potential engine inlet overheating issues that could cause cowl temperatures to rise beyond recommended levels on certain B737 MAX aircraft. The agency says the issue affects 1,187 aircraft worldwide, including 402 aircraft registered in the United States.

According to Airworthiness Directive (AD) 2023-15-05 issued on August 10, the FAA says they received a report in June 2023 indicating that flight testing and analysis revealed that the use of engine anti-ice in dry air for more than five minutes during particular combinations of altitude, total air temperature, and N1 settings could result in engine inlet cowl temperatures exceeding design limits when not in visible moisture.

"Excessive heat buildup can cause overheat of the engine inlet inner barrel beyond the material design limit, resulting in failure of the engine inlet inner barrel and severe engine inlet cowl damage," reads the AD. "There have been no reports of in-service failures of the engine inlet inner barrel to date."

The FAA warns that if not addressed, it could result in "the departure of the inlet" and potential fan cowl failure and detachment from the plane. "The departure of the inlet may cause fuselage and/or window damage, 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. Inlet loss also causes significantly increased aerodynamic drag and asymmetric lift due to wing blanking, which risks fuel exhaustion on certain flights, resulting in a forced off-airport landing and injury to passengers," the AD says. "The FAA is issuing this AD to address the unsafe condition on these products."

"The FAA issued an immediately effective Airworthiness Directive for Boeing 737 airplanes equipped with CFM Model LEAP-1B engines," an agency spokesman told ch-aviation. "The AD addresses the use of the engine anti-ice system after receiving a report that it can cause the engine inlet inner barrel to overheat when used in certain operational and environmental conditions."

Boeing acknowledged that in "a very specific combination of weather and operational conditions," the engine anti-ice system could potentially create a situation where the portion of the inlet structure exceeded its maximum design temperature range. Boeing says that while the affected parts are manufactured in-house, the problem was only recently identified "as part of our design analysis." The risk was described as a "theoretical possibility" and was unlikely to impact deliveries. The manufacturer said they have developed mitigations and is working with impacted airlines to implement them while awaiting Airplane Flight Manual operational guidance and a more permanent solution. ch-aviation has contacted Boeing for comment.