2 Engines and 4 Engines

[DirectAnywhere]

Actually Capt. J, it's not really that stupid a question. Boeing and Airbus can't agree which is the better way to go ie. A340-500/600 vs. 777-200/300ER.

ETOPS (Extended Twin Ops) is a very complex field and there's no easy answer to your question. Some of the things to consider:

*ETOPS aircraft have to demonstrate greater reliabilty than 4-eng aircraft with respect to engine and critical systems failures. Theoretically, this makes them as safe as four engine aeroplanes. Whether it does or not is open to debate. Things to consider are not only the number of engines but the availability of hydraulic and pneumatic systems, electrical power etc.

* Twin engine aircraft have to have more powerful engines than four engine aeroplanes. On takeoff an engine failure on a 4-eng aeroplane - lose 1/4 of your thrust, on a two engine aeroplane - lose 1/2. That two (now one) engine aeroplane still has to meet the minimum required performance gradients. This requires more powerful and thus heavier engines. This tends to impact negatively on range.

Conversely, because the engines are more powerful when both engines are working the twin engine aircraft, generally speaking, has bucketloads of excess thrust and thus performance, often better than 4-engine aeroplanes.

4 engine aeroplanes, however, require greater maintenance and this increases costs so it's a tradeoff.

* Control problems tend to be minimised on two engine aeroplanes as the thrust lines of the engines tend to be closer to the longitudinal axis of the aeroplane. This generally requires smaller rudder and vertical stabiliser assemblies than on four engine aircraft - tending to reduce weight.

*Certainly, if two engines fail, you're in a better position in a four engine aeroplane than a two. The aircraft is quite controllable. Two engine ceiling is up to about FL220 depending on weight. Performance leaves a lot to be desired but the aircraft will fly and can be handled quite safely. Lots of considerations but not too bad.