Rolls-Royce plc

Rolls-Royce Plc recommends that the FAA retain the calender (date) format as specified in the referenced RR NMSB for compliance rather than converting to cycles for the inspection threshold for the sub-population of fan sets, for the following reasons: 1. At the request of the NTSB, Rolls-Royce Plc analysed the modification and installation data for each fan set using both hours and cycles. For some Operators, the highest risk value was based on hours and for others it was cycles. Whichever gave the highest risk value, together with the average utilisation, was then used to determine the dates at which the blades need to have their initial inspection. Therefore, converting to cycles may not be correct for some Operators. 2. Rolls-Royce will monitor N1 speed usage. A higher N1 speed usage, could result in the risk values being affected and NMSB72-AG244 being revised and re-isssued. Any change to the NMSB would necessitate changing the FAA AD. By retaining the date format and the FAA AD referencing the NMSB then any future changes to the dates in the Appendices of NMSB will not affect the AD. 3. The NMSB is clear and simple, making it easy for the Operators to monitor their affected fan blades. Monitoring a number of fan blades using cycles would make the monitoring more difficult for the Operator. The second point above will cause will add to the difficulty for the Operator.