European safety regulators have permitted Rolls-Royce a temporary deviation from requirements to comply fully with certification criteria, while it continues investigating blade cracking on the Trent 1000.
The European Aviation Safety Agency has set out conditions for the deviation – which relates to high-cycle fatigue endurance – including requiring a proposed timescale for restoring full compliance with the certification standard.
EASA states that several occurrences have been observed in which high-cycle fatigue led to compressor blade cracking, and cracking in the compressor drum.
While inspection regimes have been implemented to ensure airworthiness, EASA says the initiation of cracking is “normally not in line” with the engine certification criteria which demand high-cycle fatigue life.
Rolls-Royce has proposed to EASA, however, that it can meet an equivalent level of safety if it can demonstrate that the crack initiation and propagation mechanisms are sufficiently understood to claim that they will not result in component failure.
This demonstration will be achieved through implementing repeat inspections, backed by its experience in engine testing, analytical modelling, and fractography.
Rolls-Royce has informed EASA that these capabilities provide a level of understanding adequate to “underwrite a conservative inspection programme”.
EASA has responded that Rolls-Royce must use its capabilities to show it fully understands sequences of multiple failure modes, including crack initiation and propagation, and use “sufficient conservatism” to prevent any in-service component failure.
It has detailed the measures on which the temporary deviation depends, adding that the manufacturer will substantiate that – despite the “expected” initiation of cracks in the compressor blades and drum during service – an “adequate and fully justified” service-management plan will ensure that none of these cracks will result in failure.
EASA's temporary deviation, contained in a 6 December revision of the Trent 1000 certification document, covers several models of the powerplant but excludes the Trent 7000.