The Australian Transport Safety Board (ATSB) says new hardware, recurrent inspections and changes to engine monitoring “adequately address” its concerns about failures of high-pressure turbine nozzles in the Engine Alliance GP7200 turbofan engine for the .
Investigators determined that higher-than-expected metal surface temperatures had caused the failure of high-pressure turbine (HPT) second-stage nozzle guide vanes in aturbofan on an A380 in November 2012. The resulting damage caused the engine to shut down in flight.
The aircraft had departed Sydney Airport and was climbing through 9,000 ft. when the crew heard a “loud bang” and received an exhaust gas temperature over-limit warning for engine number three. The pilots dumped fuel and returned for a non-eventful landing.
Investigators later determined that the shutdown was the result of “significant internal damage” that began with the high-pressure turbine (HPT) second-stage nozzle failure, most likely caused by hotter-than-expected operating temperatures, according to a final report on the Emirates engine failure.
The issue is not related to theA380 engine incident that occurred in 2010. In that case, a engine experienced an uncontained failure initiated by a cracked oil feed pipe in the high-pressure/intermediate pressure hub assembly. Debris from the engine caused multiple aircraft systems to fail.
In the Emirates incident, the ATSB says debris from the failure—consisting of nozzles, shrouds and shroud hangars—was ejected from the rear of the engine, with no threat to passengers or the aircraft’s structure.
During the aircraft’s previous flight, the engine health and trend monitoring system had identified shifts in number three engine parameters. An “urgent remote diagnostic notice” was sent to Emirates, notifying the carrier of potential deterioration of the aircraft’s number three engine high-pressure compressor efficiency. Emirates had scheduled an inspection when the aircraft returned to its home base in Dubai.
“At the time of the failure event, the engine’s exhaust gas temperature and high-pressure compressor trend monitoring criteria were not yet at a level that would have required an immediate inspection of the engine,” says the ATSB.
The ATSB notes that four other engines had experienced HPT second-stage nozzle “distress” in the four weeks preceding the Emirates incident, one of which already had received new nozzles that the Engine Alliance started offering in 2011, after discovering thermal distress in the components in a test engine.
“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 thelater 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 theand 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.”