Several letters have recently been published in Flight International discussing the pros and cons of two- and four-engine layouts. The following is a simple approach basically demonstrating that initial cost is the ultimate criterion and that technology and the views of passengers are of secondary consideration. The discussion presumably concerns under-wing mounted engines.
With two engines, when one fails, 100% of the thrust is on one wing, and thus there is a large imbalance. However, with four engines, when one fails, you are left with a 67:33 wing load distribution, thus less imbalance. Imbalance is therefore easier to counteract with a one engine failure on a four engine aircraft.
The failure of two engines on a two-engine aircraft means big problems. The failure of two engines on a four-engine aircraft is not necessarily catastrophic.
Purely from a safety aspect, therefore, logic suggests four engines
Also, assuming that the four-engine arrangement uses smaller engines than the two-engine equivalent, it should be easier and cheaper to change and maintain a smaller engine than a larger one. There should therefore be a faster turnaround for the airlines should they experience an engine failure.
Also, I recall that the Avro RJX and predecessors were designed with four small engines instead of two larger ones to help reduce noise levels and increase the short take-off and landing characteristics of the aircraft, which makes accessible many previously inaccessible airstrips - for example Mount Cook in New Zealand and London City Airport? Or could the explanation simply be that one big engine is cheaper than two smaller ones?
Derek Gamble
Germany
Source: Flight International
