Originally Posted by
suninmyeyes
In the last 6 years I can think of 3 accidents in twin engined aircraft caused by total engine failure in flight, a 777, An Airbus A320 and a Boeing 737.
Since 4 engine commercial jet airliners started flying in the 1960s I cannot think of one accident on a 4 engined jet airliner due to multiple engine failure resulting in insufficient thrust to remain airborne. I am not counting fuel exhaustion as those were not mechanical failures of the engine.
Engines were not so reliable back in those days and 707's and the early 747's did sometimes lose one or two engines. There is a big difference between an uncontained engine failure with adjacent damage which is the scenario some people seem to be thinking about on this forum and an engine that has been shut down by the flight crew due to low oil pressure, high vibration, high temperature etc.
In the past before the days of the internet a lot of 4 engined aircraft would have carried on to destination after an engine failure with their passengers blissfully ignorant and the general public unaware. The pilots would have used their skill and judgement to decide whether to continue or not and unqualified people on forums who have never flown jet airliners would not be thinking they knew better than the pilots.
Do not be too sure that such common mode failures cannot happen. Your engines are reliable now with a lot of software support. If something happens that is outside what the system designer assumed likely then you may suddenly find that you have a common mode software failure
"Incident: Airbridge Cargo B748 near Hong Kong on Jul 31st 2013, both left hand engines surged at same time, one right hand engine damaged too"
Incident: Airbridge Cargo B748 near Hong Kong on Jul 31st 2013, both left hand engines surged at same time, one right hand engine damaged too
This poorly handled icing software glitch could have occurred in the ITCZ over mid-Atlantic then the story may have had a different outcome.
This is called a common mode failure. ALL aircraft with software controls that have been built on prior assumptions - even if the software is dual designed for resilience - can suffer these failures. Yes they are rare but when a software failure like this occurs ALL your software controlled super reliable engines are likely to go. Are even your engineers on telemetry to your engines aware of the wrong assumptions made by the analysts, software designers, programmers and verification testers?
And yes this affects aircraft with all numbers of engines - but in this case having more engines does not necessarily save you. You could be flying aircraft with 4 glass jawed engines and be totally unaware until the 'wrong' sequence of events hit them and the software makes the wrong decision.
As I said earlier Fault tolerance, resilience and reliability are interesting fields and basic frequentist statistics bear NO relation to probabilities of failure in complex fault trees.