G,
The operators would have their own MTBF for only their fleet, its representative of how they operate and maintain their fleet, which may be better than or worse than the average worldwide.
The regulator will look more at a worldwide MTBF figure to benchmark an operator when considering granting approvals. A manufacturer will look at a worldwide MTBF, but will classify the results according to overall usage, and defined stage lengths, areas of operation etc.
The MTBF number derived at design phase normally comes from the reliability engineers who look at results of wear oil consumption etc from engine strip back during development testing. They come up with a plethora of numbers that are used for maintenance planning, parts replacement, overhaul periods etc.
Some snippets from NPRM AC 121-03(0): ETOPS Approval which may help ...
To establish whether a particular airframe-engine combination has satisfied the propulsion systems reliability requirements for extended range operation, an assessment will be made by CASA, using all pertinent propulsion system data. To accomplish the assessment, CASA will need world fleet data, and data from various sources (the operator, the engine manufacturer and the aeroplane manufacturer) which should be extensive enough and of sufficient maturity to enable CASA to assess with a high level of confidence, using engineering and operational judgement and standard statistical methods where appropriate, that the risk of total power loss from independent causes is sufficiently low. CASA will state whether or not the current propulsion system reliability of a particular airframe-engine combination satisfies the relevant criteria. Included in the statement, if the operation is approved, will be the engine build standard, propulsion system configuration, operating condition and limitations required to qualify the propulsion system as suitable for extended range operation.
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There is justification for the view that modern propulsion systems achieve a stable reliability level by 100,000 hours for new types and 50,000 hours for derivatives. 3000 to 4000 hours is considered to be the necessary time in service for a specific unit to indicate problem areas.
Normally, the service experience will be:
(1) For new propulsion systems: 100,000 hours and 12 months service. Where experience on another aeroplane is applicable, a significant portion of the 100,000 hours should normally be obtained on the candidate aeroplane. On a case-by-case, relevant test and design experience, and maximum diversion time
requested, could be taken into account when arriving at the in-service experience required.
(2) For derivative propulsion systems: 50,000 hours and 12 months service. These values may vary according to the degree of commonality. To this end in determining the derivative status of a propulsion system, consideration should be given to technical criteria referring to the commonality with previous ETOPS-rated engines.
Prime areas of concern include:
a) Turbomachinery
b) Controls and accessories and control logic
c) Configuration hardware (piping, cables etc.)
d) Aircraft to engine interfaces and interaction (fire, thrust reverser, avionics etc.)
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Considerations would be made on a case-by-case basis and would need to provide a demonstrated level of propulsion system reliability in terms of in flight shut down IFSD rate of the order of 0.05 per 1000-hours, as is necessary also for new propulsion systems.
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When considering safety targets, an accepted practice is to allocate appropriate portions of the total to the various potential contributing factors. By applying this practice to the overall target of 0.3 x 10-6 per flying hour, in the proportions previously considered appropriate, the probability of a catastrophic accident due to complete loss of thrust from independent causes must be no worse than 0.3 x 10-6 per flying hour.
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A family of ETOPS products with a high degree of similarity is considered as mature once:
(a) the product family has accumulated at least 250,000 flight hours for an aircraft family or 500,000 operating hours for an engine family;
(b) the product family has accumulated service experience covering a comprehensive spectrum of operating conditions (e.g. cold, hot, humid,);
(c) each ETOPS approved model or variant in the family has achieved the reliability objectives for ETOPS and has remained stable at or below the objectives fleet-wide for at least two years. New models or significant design changes may not be considered mature until they have individually satisfied the condition of sub-section (a).
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An analysis will be made on a case-by-case basis, of all significant failures, defects and malfunctions experienced in service (or during testing) for the particular airframe/ engine combination. Significant failures are principally those causing or resulting in in-flight shutdown or flameout of the engine(s), but may also include unusual ground failures and/or unscheduled removal of engines. In making the assessment, consideration will be given to the following:
a) the type of propulsion system, previous experience, whether the power-unit is new or a derivative of an existing model, and the operating thrust level to be used after one engine shutdown.
b) the trends in the cumulative twelve month rolling average, updated quarterly, of inflight shutdown rates versus propulsion system flight hours and cycles.
c) the demonstrated effect of corrective modifications, maintenance, etc. on the possible future reliability of the propulsion system.
d) maintenance actions recommended and performance and their effect on propulsion system and APU failure rates.
e) the accumulation of operational experience which covers the range of environmental conditions likely to be encountered.
f) intended maximum flight duration, and maximum diversion in the ETOPS segment, used in the extended range operation under consideration.
(3) Engineering judgement will be used in the analysis of the above such that the potential improvement in reliability, following the introduction of corrective actions identified during the analysis, can be quantified.
(4) The resultant predicted reliability level and the criteria developed in accordance with section 1.c will together be used to determine the maximum diversion time for which the particular airframe-engine combination qualifies.
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Statistical indicators (MTBF/MTBUR) and engineering judgement applied to the individual events must be used to evaluate the maturity and the reliability of all ETOPS significant systems.