The explosion tore through the plane’s second engine around 15 minutes after the Sydney-bound aircraft took off from Singapore’s Changi Airport in November 2010.
The report by the Australian Transport Safety Bureau (ATSB), which was published today, said the pipe caused an oil fire, starting a “sequence of events” that led to engine failure.
It said: “That defect resulted in fatigue cracking in the pipe, so that oil sprayed into an engine cavity where it ignited because of the high air temperature.”
The oil fire weakened a turbine disk in the second engine, which broke into several parts, which penetrated the left wing and other parts of the plane, causing major structural damage.
The engine manufacturer Rolls-Royce said it was working with the ATSB to make sure all the issues were addressed.
The report sais that Rolls-Royce had already revised manufacturing procedures and risk assessment and that the investigation would monitor this.
The pilot was able to land the plane safely back in Singapore airport, after raining debris on populated island, Batam in Indonesia.
None of the 433 passengers or 26 crew members were injured, nor any people on the ground.
Qantas reached a settlement with Rolls Royce worth AUS $95 million (£64 million) in June 2011 over the incident and the aircraft is still grounded in Singapore.
A final ATSB report on the incident will be released in May.