All helicopter engines were analysed but the data-set was skinny; up to that time there was no requirement to collect usage data, and report failure - except when a reportable incident or accident occurred. (The reliability figure was for each engine/helicopter combination.)
No engine/helicopter combination was excluded from assessment. The system of reporting (both of usage and failure) was defined and signed-up-to by the Manufacturers (and they have been reporting reliability rates, for every engine/helicopter combination, ever since).
The safety targets for acceptance into the 'Exposure' regime were realistically set - the first for helidecks (where a problem existed) and then for elevated heliports. The safety target established for these operations was 5 x 10**-8; which permited 'Exposure' of 9 second for a twin and 18 seconds for a single . That target followed an analysis based upon profiles and data supplied by the Manufacturers.
Having set a pragmatic safety target, there were no immediate assessment failures (although the S61 came close to it later in its life, as did the S92 in its early traumas). A notable failure in the system did occur fairly recently with the S76 (the fatal accident in Myanmar) when a known bearing failure was not flagged for an AD because the regulators, unknowingly, left them out of the population for early rectification. In their view, they 'always flew in accordance with the Category A requirements' - i.e. with engine-failure accountability.
JimL
Last edited by JimL; 6th Nov 2018 at 05:11.
Reason: Correction of safety target for helideck operations.