Aircraft

Operators flying a small number of CFM56-5B turbofan engines will be required to replace one of the aircraft's two powerplants if both are shown to have an exhaust gas temperature (EGT) margin of less than 80 deg C, according to an emergency airworthiness directive (AD) issued by EASA today.

The action follows a 15 December incident in which a CFM56-5B-powered Airbus A321 experienced "stalls on both engines during the same flight," states EASA. "The event was caused by high pressure compressor (HPC) deterioration."

EGT margin refers to the buffer between an engine's normal operating EGT temperature and its maximum EGT, the temperature at which it must be overhauled or replaced. A higher EGT is an indication of the HPC wear that can cause a compressor stall.

CFM had attempted to address the problem with the release of new engine control unit (ECU) software, version 5BQ, in January 2007 which features "two degrees additional Variable Stator Vanes closure in the low power region to increase stall margin," according to the AD.

"The 5QB software introduction has reduced the frequency of stalls, however since April 2008, six different engines have experienced stalls at three different operators with the 5QB software," the AD continues.

EASA says the stalls on both engines during flight could cause dual in-flight engine shutdowns.

A CFM spokeswoman says about a dozen engines are covered by the AD, all with more than 14,000h operating life. She says the incidents involved a loss of thrust rather than a surge in the engines, adding that a new software version will be released for the affected engines late in January.

The emergency AD calls for operators to identify aircraft in which both engines have greater than 80 deg C EGT margin deterioration and to replace one of the engines with a powerplant featuring greater margin.