Europe's safety regulator has extended the inspection threshold and interval requirements for Rolls-Royce Trent 900 engines with certain modification standards.
The European Aviation Safety Agency issued two emergency airworthiness directives after the uncontained failure of a Trent 900 engine on a Qantas Airbus A380 on 4 November.
Both mandated operators to inspect the high- and intermediate-pressure (HP/IP) support structure within 10 flight cycles and to repeat these inspections at maximum intervals of 20 cycles.
The agency has revised these requirements and extended the initial threshold to 100 cycles and interval period to 200 cycles for engines with a specified HP/IP support structure, detailed by Rolls-Royce in a bulletin to operators earlier this week.
Qantas disclosed in an affidavit on 2 December that there are three modification standards - designated A, B and C - for the HP/IP support structure on the Trent 900 series. The document specified the failed engine as an 'A' modification powerplant.
According to Rolls-Royce's engine manual, the life of the 'A' support structure was limited to 2,000 cycles, said the airline. The 'B' modification was issued in December 2007 and its life limit extended to 14,800 cycles. The latest 'C' modification standard was introduced in April 2009 and has unlimited life, says Qantas.
The Australian Transport Safety Bureau declared its investigation was concentrating on 'A' and 'B' standard engines.
Powerplants with the 'C' modification status are not affected, because they do not have the same oil feed stub pipe installation, which is considered central to the engine failure chain.
Investigators believe that the stub pipe developed a fracture in a section with a small wall thickness, which led to an oil leak and fire in the HP/IP structure cavity. This thin wall section has been confirmed as a result of the manufacturing process.
EASA issued the revision of the second emergency directive this week, stating: "Manufacturing and inspection data, and stress analysis performed by Rolls-Royce, now confirm that oil feed tubes with a defined minimum thin wall section feature a higher life and lower risk of fracture." This would allow longer service periods for respective engines before the inspections.