Air safety investigators are still no closer to understanding what caused a turbine blade in a Qantas jumbo's Rolls-Royce engine to break off, showering white hot sparks across Bangkok skies last year.

Qantas flight QF2 had just departed Bangkok for Sydney on October 16, carrying 358 passengers and 18 crew, when eight minutes into the flight at 13,000 feet crew and passengers heard a loud bang and the plane shuddered with vibrations.

The condition of the blade fragments was such that identification and physical analysis of the blade release mechanism was not possible. 

Fumes entered the cabin for several minutes afterwards.

The pilots shut down engine number 3, and broadcast an emergency "pan" message (one level down from the highest level of emergency, a mayday).

The plane then circled for almost an hour as it dumped 55 tonnes of jet fuel, before making a safe landing back at Bangkok.

Investigators from the Australian Transport Safety Bureau released an updated report today into the incident.

Investigators said when the engine was examined, a single compressor blade located in the heart of the engine had snapped off, causing "significant damage" to other engine components it struck on its way to being ejected out the back of the engine.

"The condition of the blade fragments was such that identification and physical analysis of the blade release mechanism was not possible," investigators said.

Investigators established that the Qantas engine had accrued almost 20,000 hours and 2000 take-offs and landings after it was overhauled and fitted to the Qantas Boeing 747.

But they're no closer to understanding why the blade snapped off, something being examined by Rolls-Royce.

"The engine manufacturer was continuing their internal investigation into the occurrence," investigators said.

Rolls-Royce reported to investigators that in 2005 a blade was found to be cracked at its root, while in 2007 another blade was found to have have moved backwards in an engine.

The company maintains the Bangkok incident was the first in which a blade had let go inside an engine in more than 40 million hours of this type of engine flying.

The RB211 is a broad family of Rolls-Royce engines, one variant of which is the more powerful Trent 900, the type of engine that exploded under the wing of the Qantas flagship A380 operating as QF32.

That explosion was traced to a badly manufactured oil pipe that cracked, starting an oil fire.