The failure of some Stage-2 high-pressure turbine (HPT) nozzles triggered the uncommanded shutdown of an Engine Alliance GP7200 turbofan on an Emirates Airline A380 in November last year, according to the findings of an investigation by the Australian Transport Safety Bureau (ATSB).
The aircraft was climbing through 9,000 feet shortly after taking off from Sydney for Dubai when the crew heard a loud bang, followed quickly by an excessive exhaust gas temperature warning on the number-three engine. The engine shut itself down a few moments later as the crew began jettisoning fuel for what was eventually an uneventful return landing at Sydney.
The investigation determined that the engine initially shed metal that landed on the roofs of homes beneath the flight path. It caused no significant damage.
The ATSB believes the break up and dislodgement of the HPT nozzles produced substantial downstream damage and allowed a breach to develop within the turbine casing walls. The nozzle failure stemmed from the effects of cumulative oxidation and distress across the airfoil surfaces. According to Engine Alliance (a joint venture between GE and Pratt & Whitney), hotter-than-expected metal surface temperatures at the HPT stage-2 nozzle forward attachments led to the accelerated hardware distress. The engine had been scheduled for inspection once it landed at Dubai after a number of unusual trend indications were noted on a previous trip. Engine Alliance reported that HPT stage-2 nozzle distress had been observed on engines with more than 1,800 operational cycles. This engine had flown 1,876 cycles.