A.4Propulsion System Reliability Objective

A.4.1Type Design

A determination will be made that the type design of the propulsion system achieves the desired level of reliability. Transport Canada will determine if the probability of total/unacceptable thrust loss due to design related and/or independent causes meet the criteria of this section.


A.4.2.1A determination will be made of the propulsion system's ability to achieve the desired level of operational reliability in ETOPS. Transport Canada will determine if the probability of total/unacceptable thrust loss for all independent causes meets the criteria of this section.

A.4.2.2The propulsion system reliability objective will ensure that the propulsion system achieves at least the minimum reliability criteria required of other critical aircraft systems, i.e., navigation, flight control, communications, etc.

Considering the complexity of the entire powerplant system, the approach to determine the reliability has been to use in-service data. This data therefore, not only considers design related failures (Airworthiness rates

The events to be considered are to include those occurring from the beginning of the take-off roll to the end of the landing phase, though items confirmed as not ETOPS significant will be discounted. Failures considered are, engine in-flight shutdowns (IFSD), any other significant power loss or loss of engine control. The reliability objective used by Transport Canada relates diversion time to the probability of a loss of thrust which precludes continued safe flight.

The target is expressed by the following formula:



Pe = probability of an engine failure (per hour)
t = diversion time (hours)
(109) represents the life of an entire aircraft fleet (hours)

Transport Canada believes some tolerance is required to account for verified corrective actions and precautionary shutdowns and also to provide for the expected variance over time in propulsion system reliability statistics. Reported occurrences beyond the tolerance will be grounds for withdrawal of ETOPS approval, or reduction in allowed diversion time. The maximum criteria is defined by the following formula:


Figure 1

Propulsion System Reliability Objective

Diversion Time (minutes) (t)

Reliability Table (Engine Failures per 1000hours)

Diversion Time (t) Target Criteria Minimum Criteria
60 minutes .032 .063
75 minutes .028 .056
90 minutes .026 .052
120 minutes .022 .044
138 minutes .021 .042
180 minutes .018 .036
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