“At the time of publishing this report, the engine manufacturer was continuing work to better understand the conditions that can lead to HPT distress and to further improve the durability of the HPT components,” says the ATSB.
As a result of the Emirates incident and the findings on other engines, Engine Alliance called for recurring borescope inspections of the nozzles on the engines and the FAA later issued an airworthiness directive for the inspections.
Contributing to the failure of the Emirates engine—which did not have the upgraded nozzles—were thresholds in the engine monitoring software in the General Electric and Pratt & Whitney-built engine.
“The threshold limits for the engine trend monitoring program were not set at a level that provided sufficient opportunity for inspection of the engine before failure could occur from the effects of HPT stage-two nozzle degradation,” says the ATSB.
The modified trend monitoring system is designed to give operators more advanced notice of a potential failure. The ATSB says under the new process, the change in exhaust gas temperature experienced by the Emirates engine would have triggered an “Urgent–Prior to Next Flight” notice two flights before the incident flight.
The ATSB says the combination of new nozzle guide vanes, inspections and tighter trend monitoring thresholds “adequately addresses the safety issue.”