WASHINGTON — The US Federal Aviation Administration (FAA) has issued a final Airworthiness Directive (AD) to address high-pressure compressor (HPC) stalls on the CFM LEAP-1B engine, utilized exclusively by Boeing 737 MAX aircraft. 

The directive comes after multiple aborted takeoffs and air turn-backs (ATBs) caused by high levels of non-synchronous vibration (NSV) were reported.

An investigation by CFM International, a collaboration between GE Aviation and Safran Aircraft Engines, found that wear on the No. 3 bearing spring finger housing could lead to elevated NSV levels. Airlines using the affected LEAP-1B engine versions on their Boeing 737 MAXs must now conduct repetitive calculations of oil filter delta pressure (OFDP) data and replace the No. 3 bearing spring finger housing depending on calculation outcomes. The directive also forbids the installation of engines with affected housings on aircraft that already have defective housings.

The FAA warned that the unsafe condition could result in engine power loss during critical flight phases, loss of thrust control, reduced airplane controllability, and even aircraft loss. The directive becomes effective on June 13, 2023. Airlines must calculate OFDP data before the engine reaches 125 flight cycles (FC) and recalculate at intervals not exceeding 100 FCs. If OFDP data surpasses limits, operators must replace the No. 3 bearing spring finger housing within 25 FCs.

The FAA estimates that the directive will impact eight engines currently installed on US-registered aircraft. OFDP data calculations will cost $85 per work hour, while No. 3 bearing spring finger housing replacements will cost $66,035 per aircraft engine. American Airlines, Lynx Air, an anonymous commenter, and the Air Line Pilots Association, International (ALPA) provided feedback on the AD, including requests to remove installation prohibitions of engines with affected housings and revisions of estimated costs.

The FAA has adopted the AD as proposed in the Notice of Proposed Rulemaking (NPRM), with minor editorial changes and no additional economic burden on operators.