Further to your coverage of the Aviation Rulemaking Advisory Committee (ARAC) ETOPS Working Group ("Redundant Rules?" Flight International, 31 December-6 January), the European Joint Aviation Authorities working group has not yet finalised its recommendations and will not until perhaps April. It is premature to indicate that there are differences between what ARAC recommended and what the JAA may adopt.
As to proposed new requirements for three- and four-engined aircraft, ARAC chose performance-based criteria. It is not so much a matter of equivalency among different aircraft as a matter of ensuring that all aircraft, regardless of the number of engines, have the same performance standard, ie the occurrence of any failure, or combinations of failures within a system that would prevent continued safe flight and landing must be extremely improbable.
To meet the performance standard, twin-engined aircraft will require a level of engine reliability many times more stringent than aircraft with more engines. The ARAC proposal notes that three independent electrical generation sources -- not generators - will meet the performance standard, while the currently drafted JAA guidance notes that one way to meet the standard (but not the only way) is to provide four generators. So the two bodies' pronouncements on this subject are not incompatible.
The proposed new definition of ETOPS - extended operations - applies to all aircraft, and is based on a preclude-and-protect philosophy. Through design enhancements, maintenance practices and operational requirements, the effort is made to preclude diversions in all operations, not just those of twins.
Tim Gallagher Chair, ARAC ETOPS Working Group
Source: Flight International
