The European Aviation Safety Agency (EASA) yesterday withdrew requirements for repetitive Rolls-Royce Trent 900 inspections it mandated following the uncontained failure of a Qantas Airbus A380 engine last November. The relevant airworthiness directive (AD 2010-0242R1), which applies to all examples of seven engine variants, follows “further assessment of manufacturing data and additional stress analysis.”
Continued investigations support an initial “failure scenario” that an oil fire in the high-pressure/intermediate-pressure (HP/IP) structure air-buffer cavity led to rupture of the IP turbine-disc drive arm and subsequent disc over-speed and burst. The AD required inspections of the low-pressure turbine case drain, HP/IP cavity and oil service tubes to detect abnormal oil leakage. EASA noted that part of the reason for cancelling the requirements centered on its assessment that the “intrusive” inspections risked possible maintenance errors.
The “most probable” primary failure involved the fracture of an oil-feed tube with a “thin wall section” that originated during manufacture. "All HP/IP structures containing an oil-feed tube with a wall section below a certain [thickness], and therefore having a potential risk of fracture, have been removed from service,” according to EASA. Manufacturing and inspection data, as well as Rolls-Royce stress analysis, confirmed that tubes with a defined minimum wall thickness feature a higher life and lower fracture risk. Inspections under this and related ADs have “not identified any engine with a risk to develop a [similar] failure scenario.”