The Australian Transport Safety Bureau (ATSB) says corroded turbine blades caused by chemical residue that was not removed after cleaning led to an engine failure on a Qantas Airbus A380 flying from Los Angeles to Melbourne.
Qantas A380 VH-OQG, operating QF94 on May 20 2017, was two hours out from Los Angeles and climbing from 32,000ft to 34,000ft when crew on the flightdeck “heard a loud bang and felt a sudden and unusual vibration of the aircraft”.
Some of the 484 passengers and 20 crew on board also reported seeing flames and sparks from the right outboard or Number Four engine. Qantas’s 12 A380s are powered by Rolls-Royce Trent 900 engines.
The ATSB final report on the incident published on Thursday said flight data showed that as the aircraft passed 32,500ft, the no 4 engine intermediate pressure turbine experienced an overspeed and its intermediate pressure (IP) shaft speed, or N2, increased from 92 per cent to the redline limit of 98.5 per cent over the next two seconds.
The pilots received a number of ECAM (electronic centralised aircraft monitoring) messages following the bang and vibration, including an engine fire warning on the no 4 engine.
They shut down the engine and discharged fire retardant, and decided to return to Los Angeles.
After jettisoning fuel to reduce the landing weight, the aircraft landed safely.
The ATSB said an initial engineering inspection of the engine after the incident found damage to the low-pressure turbine blades.
There was no visible indication of fire and no breach of the engine casing. There was also minor damage to the right flap and flap fairing from debris exiting the rear of the engine.
The ATSB said Rolls-Royce’s investigation into the incident found “internally-corroded low-pressure turbine stage 2 (LPT2) blades” that led to fatigue cracking and subsequent release of blade shroud debris, resulting in significant downstream engine damage.
“The blade corrosion resulted from chemical residue associated with the cleaning procedure used during the last engine service,” the ATSB said.
The ATSB report said the last service took place in July 2015.
As a result of this incident, the ATSB said Rolls-Royce had revised its blade overhaul cleaning operation instructions with additional detail to incorporate best practice with respect to removal of process solutions and chemical residues.
“This included modifying the orientation and support of the blades during the cleaning process and pressurised water flushing of aerofoil cavities after cleaning to ensure removal of residual cleaning compounds,” the ATSB said.
“The rationalised best practice has been applied at all facilities that conduct cleaning of Trent 900 Stage 2 low-pressure turbine blades.
Further, the ATSB said Rolls-Royce had identified 12 other engines that were potentially affected by the cleaning process. Of those five had been removed from service.
Also, the European Aviation Safety Agency (EASA) released an airworthiness directive “relating to the potential for blade corrosion due to residual cleaning contaminants”, which mandated the replacement of the affected blades in accordance with the Rolls-Royce non-modification service bulletin (NMSB).
ATSB report highlights effective crew resource management
The ATSB said the incident highlighted the importance of reviewing maintenance processes to ensure best practice is followed.
Moreover, it was “also an example of effective crew resource management techniques”.
“The flightcrew reported that their actions and response, from the initial engine issue when only two members of the flightcrew were in the cockpit to the approach and landing with all four crewmembers on the flightdeck, flowed very smoothly,” the ATSB said.
“They also felt that their collaborative decision-making was excellent and that the highly experienced second officers provided valuable support to the flying pilots.”