All multi-engined aircraft have a critical engine, although the design and airworthiness team may not trouble to mention it to service aircrew. They don't really need to - the procedures and speeds are written around worst case, stick to them and things should work out okay, it's just that failure of the critical engine is likely to then give the lowest remaining margin of safety.
In a 4-engined jet, it may well be that the outboard engines are equally critical, but the selected critical engine during certification trials will be that into any crosswind on the day. Similarly on a twin.
But there are other issues, which get a little more complex. Not every system on an aircraft is powered by the same engines - for example on the C130 the critical engine generator happens to also power the galley (says a lot about Herc aircrew I've always thought). This is an undesirable feature of the need to keep system weight and complexity to a minimum, but can mean that control over the aircraft is, at-least initially, (until buses can be switched returning full power to whatever control was degraded by loss of the powering engine) more critical for one engine than another.
FARs are Federal Aviation Requirements. Some of them were originally based upon BCARs (British Civil Airworthiness Requirements), although in practice the two have run (almost) hand-in-hand for more years than anybody can remember. BCARs aren't used much now, having largely been superceded by European JARs (Joint Airworthiness Requirements).
Aren't all of the Pprune forums safety forums?
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