Quote Originally Posted by Macca View Post
chance of both engines failing is so low that you'd never leave the house if that level of chance event was an issue.

According to the FAA the probability of just one (jet) engine failing is one event per 375,000 flight hours.

Someone better at maths than me can work out the chance of both going down.
Depends if events are independent or not. If they are - just square the probabilty - but often one failure may trigger another. They may not be closely interlinked, but often there is some linkage - for example if fuel has to be moved from one side to the other - etc. I don't know how any analysis of non-independent events is done. Another linking might simply be via the environment - flying a plane on just one engine may impose a higher load on the remaining engine, thus increasing the probability that it fails. I think the ETOPS rules require some duplication of parts which might contribute to one or other engine failing - for example fuel pumps, but there's probably more to it than that.