The failure of a high pressure turbine (HPT) nozzle has been identified as the cause of an engine failure and uncommanded shutdown on board an Engine Alliance GP7270-powered Emirates Airbus A380 aircraft in 2012.
The aircraft, registration A6-EDA, was on the Sydney-Dubai route when the incident happened on 11 November 2012. The crew heard a loud bang shortly after take-off at approximately 9,000ft (2,740m). This was followed by an engine no.3 exhaust gas temperature over-limit warning and an uncommanded engine shutdown thereafter.
An investigation by the Australian Transport Safety Bureau (ATSB) found the incident to be the result of the break-up and dislodgement of some stage 2 HPT nozzles, producing substantial downstream damage, and allowing a breach within the turbine casing walls.
"The nozzle failure stemmed from the effects of cumulative oxidation and distress across the airfoil surfaces," adds the ATSB.
The engine manufacturer said that a review of the failed engine suggests that hotter-than-expected metal surface temperatures at the HPT stage 2 nozzle forward attachments led to the accelerated hardware distress. Factors including the engine combustion gas temperature profile, a degraded nozzle cooling capability and the engine operating regime have been identified as contributors to the higher metal surface temperatures.
ATSB found that the design cooling characteristics of the stage 2 nozzle components were inadequate, as they allowed higher-than-expected metal surface temperatures during operation. This made the nozzles susceptible to distress, premature degradation and failure.
Prior to the Emirates incident, the engine manufacturer had made design changes to the HPT stage 2 nozzle design to improve durability, although similar nozzle distress was also discovered on an engine with the new design nozzles.
Before this incident, four other engines aboard A380s were also identified with HPT stage 2 nozzle distress.
In December 2012, Engine Alliance - a General Electric and Pratt & Whitney joint venture - issued a service bulletin which provided on-wing inspection instructions for susceptible HPT stage 2 nozzles. It continues to study the cooling characteristics of the nozzles for further design improvements.
In early 2013, the US Federal Aviation Administration issued an airworthiness directive requiring airlines to perform initial and repetitive borescope inspections. It said that the discovery of one or more holes in a nozzle would require removal from service.
Emirates has since installed new nozzles on its fleet of A380s. The new stage 2 nozzles have also been installed across the majority of the engines within the worldwide fleet.
The engine manufacturer has also since enhanced the trend monitoring system to receive alerts earlier should there be a change in exhaust gas temperature.
Source: Air Transport Intelligence news
