Reports of unapproved repairs has led the Federal Aviation Administration to require airlines replace high-pressure turbine cases in some CFM International Leap-1B engines, which power Boeing 737 Max.
An airworthiness directive made public on 29 November targets only some derivatives of Leap-1B turbofans and applies to just 13 engines worldwide, including two engines in the US fleet, according to CFM and the FAA.
Airlines must replace so-called "stator cases", which surround the high-pressure turbine, within 200 cycles following 15 December, when the directive takes effect, the order says.
The issue has caused no engine failures and will not cause widespread fleet disruption nor slow CFM's deliveries of Leap-1Bs to Boeing, CFM stresses.
Stator cases on four of the 13 affected engines have already been removed, it adds.
"Our customer teams are working with the impacted customers to minimise any disruption," CFM says.
The FAA issued the directive due to a "quality escape" at CFM supplier AECC Aero Science and Technology, a Chinese company. AECC performed unapproved "welds" on Leap-1B stator cases "to correct errors introduced in their manufacturing process", the directive says.
Stator cases are "life limited" parts, meaning they must be replaced at set intervals.
"The unapproved repairs reduced the material capability of these cases, which requires their removal prior to reaching their" life limit, says the directive. "This condition, if not addressed, could result in failure of the [high-pressure turbine] case, engine fire and damage to the airplane."
CFM addressed the stator issue in a 5 November service bulletin, the FAA says.
The directive does not relate to a previous issue involving coatings on ceramic composite components in Leap high-pressure turbines, CFM says.
Jointly owned by GE Aviation and Safran Aircraft Engines, CFM has accelerated Leap engine deliveries in recent quarters, signaling a recovery from engine delivery delays.
Those delays resulted from several technical issues and broader supply chain shortages, and came as Boeing and Airbus ramped up 737 and A320 production.