(xx)quad vs (xx)twin and "the next failure"
This value of xx should be less when there are ETOPS ratings and procedures involved.
Either environmental or common-servicing-error reasons could also come into play to make two failures on a quad more likely than just xx-squared.
When discussing this it might be better to consider not the number of accidents due to 3/4 or 1/2 of the engines working, or 2/4-or-2/2 failing, but rather to the incidences of these failures. From that, it might be clear whether we're overdue for the first accident to occur due to engine failures taking place a long way from safety in a twin.
Very roughly, with ~1000 ETOPS flights per day, and a few reported failures per year, it seems that ~1:100,000 flights have failures, and so a double failure might only be expected every ~10,000 years. If this is valid, then the current practice should be safe enough.
Last edited by awblain; 4th Jan 2014 at 19:58.