Supplementary memorandum from AgustaWestland
1. The reliability of aerospace equipment
is typically quoted as a mean time between faults expressed in
flying hours. For a helicopter main rotor gearbox two values should
be considered of most significance.
2. Firstly, Mean Time Between Faults (MTBF)
which is the overall time between faults, including those that
can be corrected with the gearbox installed in the helicopter.
Secondly, Mean Time Between Unscheduled Removal (MTBUR) which
is the time between faults which require the gearbox to be removed
from the helicopter and returned to a depot for deeper level repair.
3. For a helicopter the most important measure
is the MTBUR as too many arisings will cause the aircraft to be
out of service for longer than planned as the gearbox has to be
removed which causes additional work. For a Merlin helicopter
using data available to AgustaWestland from the MoD's logistic
systems covering the period 2005 to 2008 the MTBUR for
Merlin was 1546 hours. If a comparison is made to the Sea
King for the same period the MTBUR for this aircraft was 1726 hours.
4. AgustaWestland has designed and qualified
a series of 20 modifications to improve the reliability of
the Merlin Main Rotor Gearbox and to also extend the time between
overhauls from 2000 to 3000 hours. These modifications
are now being installed in gearboxes as they are returned for
repair and overhaul.
1 June 2009
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