Andrew Heasley April 16, 2012
An old turbine blade in a Rolls-Royce engine fitted to a Qantas jumbo, carrying 213 passengers and 18 crew, broke off and set in train a sequence of events that led to an uncontained engine explosion at 25,000 feet, safety investigators have found.
The explosion led to an emergency landing after the Sydney-bound jet took off from San Francisco on August 30, 2010, after pilots were forced to off shut down the damaged engine and dump 70,000 litres of jet fuel.
High-speed engine shrapnel tore a hole in the engine's casing and hit the jumbo jet's wing.
Footage shot by passengers showed showers of sparks shooting from the engine across a pitch-black night sky.
The pilot calmly announced to passengers: "You must be assured we are trained for this situation. Normally we do it on a simulator of course."
Relying on a technical analysis by Roll-Royce's own engineers, investigators from the Australian Transport Safety Bureau concluded the engine had an older-design housing holding the engine bearings, which failed when the spinning engine rotors became unbalanced, severing blades and shafts and allowing internal components to thrash about.
Investigators found the last time the engine was overhauled in May 2009, the turbine bearing had accumulated nearly 72,000 hours of operation, or more than 9000 take-offs and landings.
Although maintained by the book, the turbine blades had accumulated about 80,000 hours of of operation or about 10,000 take-offs and landings.
When one blade broke off, it caused other blades to shear, creating an imbalance, overloading the turbine roller bearing, which failed, allowing one turbine shaft to ''orbit'', striking another shaft that snapped.
As the engine started to destroy itself, the heavy turbine disc spun too fast and out of centre, ''losing its axial and radial location'', investigators said.
The spinning disc and blades crashed into components, ultimately leading the turbine casing to rupture as shrapnel that flew out, striking the wing.
Rolls-Royce told investigators the blade that broke was the only one of its kind to do so in the history of the RB211 engine type, totalling 40 million hours of service over 23 years of global operation.
But investigators found ''high-service time'' blades of the type that broke ''were susceptible to a reduction in fatigue endurance as a result of vibratory stresses sustained during operation at speeds close to maximum''.
As a result, investigators recommend airlines replace high-service blades and retrofit a newer, more robustly designed bearing housing during ''the next maintenance visit''.
A Qantas spokesman said 28 of its 72 of its Rolls-Royce RB211 engines had been updated so far, and all remaining in service would be completed in the next 12 months.