Originally Posted by
Ziggy
Most operators would plan routes to stay within an acceptable range of adequate airfield in case of emergencies. The most crucial emergencies dealing with degraded aircraft performance. So far everyone has been quick to point out just the loss of power in an OEI situation. But what about performance. Basic multi engine doctrine taught that with an engine failure on a twin aircraft. Not only did you lose half the power, but suffered an 80% degradation in aircraft performance. This being because now the failed engine is dead weight creating only drag. Most twin jets have nacelles and engines that have bigger surface area than the competing 3 or more aircraft.
I'd add that in addition to drag from the dead engine, you also have your centerline of thrust way out to the side at the nacelle with the working engine. So you have to waste a lot of energy to fly straight. Trijet, if you lose the center engine you still have centered thrust. If you lose an outside engine, the centerline of thrust is still only half way to the nacelle, so you should have to waste less energy to fly straight.
Originally Posted by ziggy
So when you lose an engine on a trijet sure you've only lost 1/3 the power, but performance wise. Smaller engine, less weight, less drag. This probably means you can continue to cruise at higher altitudes and speeds for the given weights, temperatures, and etc.
Maybe, but what if when people are designing airplanes, when they have a twinjet versus a trijet they know that they are going to be able to have 20% more thrust so they are able to build a bigger wing for the plane. And when an engine goes out they'll have somewhat less thrust to weight than the trijet, but they'll have more wing to weight. Which is safer?