The US Federal Aviation Administration is proposing that airlines be required to inspect and replace seals in high-pressure compressors of CFM International CFM56 turbofans to prevent the risk of uncontained seal failures.

A proposed airworthiness directive (AD) made public on 19 September requires airlines to inspect and replace the “high-pressure turbine inner stationary seal” in CFM56-5B and CFM56-7B engines.

CFM56-5Bs power Airbus A320-family aircraft and CFM56-7Bs power Boeing 737NGs. The order applies to 210 engines on US aircraft, the FAA says.

CFM International says it already addressed the issue by publishing a service bulletin to operators in April. The number of affected engines represent just 1% of the CFM56 in-service fleet, and many of those engines “are already in compliance with the service bulletin”, CFM adds.

Safety investigators and regulators in the US and elsewhere have been studying turbofan failures following several incidents involving engines made by different manufacturers. In 2018, a CFM56 on a Southwest Airlines 737 failed; debris from that engine impacted the engine’s cowling and inlet, which fragmented and shot outward, breaking a cabin window and killing one passenger who was sucked partway out of the aircraft. CFM does not make the CFM56’s cowling or inlet.

The FAA’s directive says it received reports of two cracks found in the high-pressure turbine front seal of a CFM56-5B.

The seal “may not have received the correct braze heat treat cycle at the time of the honeycomb replacement”, says the directive.

The condition could affect the life of the seal and possibly “result in an uncontained release of the seal, which could damage the engine or airplane”.

Aircraft operators must remove inner seals during the engine’s next shop visit and inspect them for “honeycomb separation”. If separation is found, airlines must replace a “front seal” and inspect the high-pressure turbine’s rotor blade for metal debris.

The FAA is accepting comments about its proposed rule for 45 days.

Story updated on 26 September to clarify that not all recent engine events under investigation involved uncontained engine failures, which the National Transportation Safety Board (NTSB) defines as an event involving “fragments of rotating engine parts penetrating and exiting through the engine case”. The 2018 failure of a Southwest CFM56 did not rupture that engine’s case. Rather, the engine failure caused a failure of the engine’s cowling and inlet, which impacted the aircraft, causing cabin decompression. says the NTSB. The distinction between “uncontained” engine failures and explosive failures of cowlings and inlets is important due to regulatory distinctions and because cowlings and inlets are often not made by engine manufacturers.