Would having 4 engines actually increase the risk of having something like a catastrophic fan blade / disk failure? Also, I’d much rather have a proven engine like the CFM56, than a Trent 1000 or PW engine that’s having issues on the A321neo.
Yes. Whether this is enough to offset the lower likelihood of every engine failing in a contained manner depends on the specific failure rates of the engines in question, but there is such a point where that becomes the case.
Consider the case where a given engine has an uncontained, catastrophic failure rate of one every twenty hours, and a contained, non-catastrophic failure rate of one every two hours. If you mount two of those engines on a plane, you would expect a catastrophic failure of a single
engine once every ten hours, and a contained failure of all
engines once every four hours. If you mount three, you'd expect a catastrophic failure of a single engine once every 6 2/3 hours, and a contained failure of all engines once every eight hours. With four, it's a single catastrophic failure expected every 5 hours, and a contained all-engine failure every 16 hours. With 5, it's single catastrophic every 4 hours, and contained all-engine every 32.
If we translate this to probability of failure per hour, then:
With two engines, the probability of an uncontained failure in a given hour is 0.1, and the probability of all engines experiencing a contained failure in a given hour is 0.25, or a 35% chance of a fatal failure of some sort per hour. With three, it's 0.15 for single uncontained, and 0.125 for all uncontained, so 27.5% chance. With four, it's 0.2 for single uncontained, and 0.0625 for all uncontained, so a 26.25% chance of a failure of any sort. Finally, with five engines, it's 0.25 for single catastrophic, and 0.03125 for all uncontained, so 28.125% chance of failure every hour--so five engines is actually less
safe than four.
Obviously, this is a simplified example--as you add more engines, you're probably going to be using a different type which will have different failure rates, for one, and also not all uncontained engine failure scenarios are equally fatal, but the point is that there is a point where adding more engines makes the plane less safe.
Last edited by BerenErchamion
on Sun Apr 29, 2018 9:14 pm, edited 1 time in total.