NTSB issues urgent recommendations for CF6 engines

Washington DC
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Cargo airlines are likely to be the worst hit by urgent safety recommendations issued by the US National Transportation Safety Board affecting older General Electric CF6 series engines after a spate of uncontained engine failures, which could have a major impact on operations.

The NTSB is recommending that the US Federal Aviation Administration issues an airworthiness directive requiring GE to immediately redesign the engine's low-pressure turbine stage-three rotor disk, and for operators to carry out borescope inspections every 15 flight cycles until the new design can be installed.

The NTSB says the LPT S3 disk "can fail unexpectedly when excited by high-pressure turbine rotor unbalance".

The move follows four incidents involving aircraft powered by CF6-45 and -50 engines. Those affected are the Airbus A300B2/B4, Boeing 747-200/300 and McDonnell Douglas DC-10-15/30. In excess of 100 aircraft are thought to be impacted, the bulk of which are operated by cargo airlines.

The latest of four in-flight incidents occurred in April when an ACT Cargo A300B4 was forced to reject its take-off from Bahrain after suffering an engine failure which activated its fire suppression system.

The first event, in July 2008, involved a Saudi Arabian Airlines 747-300 which experienced an engine failure on climb-out from Jeddah. Then in March 2009 an Arrow Cargo DC-10 freighter in Manaus, Brazil experienced a loss of oil pressure in one engine, prompting an in-flight shut-down. Nine months later the crew of a Jett8 Cargo 747-200F heard a muffled explosion and lost pressure in one engine, prompting a landing at Singapore.

"Engine cases are not designed to contain failed turbine disks," says the NTSB. "Instead, the risk of an uncontained disk failure is mitigated by designating disks as safety-critical parts, defined as the parts of an engine whose failure is likely to present a direct hazard to the aircraft."