Boeing is redesigning a 737 MAX engine nacelle component after discovering that operating the anti-ice system in certain conditions can damage the part and operators will be required to adhere to certain limits until the fix is complete.

The issue involves using engine anti-ice (EAI) in dry conditions, says an FAA airworthiness directive (AD) set for publication Aug. 10. Recent “flight testing and analysis” revealed that running EAI in dry air for at least 5 min. combined with certain altitude, total air temperature, and engine N1 settings can push inlet temperatures beyond design limits.

“Excessive heat buildup can cause overheating 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,” the directive says. That, in turn, could lead to airframe damage caused by pieces of the nacelle breaking away. No in-service incidents have been linked to the problem, the FAA says.

The “direct final rule” AD, meaning it was deemed important enough for the agency to bypass inviting public input on a draft version, will require operators of U.S.-registered 737 MAXs to update flight manuals within 15 days. Under the update, the use of EAI will be prohibited “when not in actual or anticipated icing conditions,” the FAA says.

Also prohibited will be dispatch of any 737 MAX with an EAI valve locked open. The current master minimum equipment list (MMEL) permits dispatch with this condition, but the FAA says an updated MMEL will mean dispatch will be prohibited until more is learned about the failure condition.

“Further analysis of this item is necessary to determine whether continued use will cause failure of the engine inlet inner barrel,” the FAA says.

Similar directives likely are to be adopted by other regulators.

Meanwhile, Boeing is developing a modification to address the issue. Boeing designed and produces 737 MAX inlets at its North Charleston, South Carolina, engine propulsion facility. The AD does not provide details on the modification or its timeline and Boeing declined to answer specific questions.

“Boeing has identified measures to mitigate the potential issue and are working with our customers to deploy those measures while a permanent fix is developed,” the company said. “We will continue coordinating closely with the FAA.”

The risk of nacelle damage is being scrutinized carefully following a series of occurrences involving 737s and 777s in recent years, including one fatal accident. Boeing is redesigning the 737 Next Generation nacelle as a result—a process that began in 2019. The company recently told the FAA it expects to need another seven years to validate the required changes and provide enough time for operators to retrofit their fleets.