Sure its a risk/consequence calculation where correct use of statistics are a tool - but you know what they say about statistics !
You say
If we exclude failure from (common) external factors
- That is TRUE - BUT we can't - we can pretend/reduce/try/hope etc but we cannot render POWER delivery independent - Especially if they share a common drive chain - where the risk of combiner gearbox etc etc must be factored in.
We expect the engine to deliver a POWER LEVEL (OEI30secs or OEI 2:30) which it has not regularly been tested to - it is being expected to do that because one engine has failed. An engine that it has
probably shared a history with (same manufacturing batch, same unaccounted for running on rocket fuel in Bongo-Bongoland? etc..)
Pilot's close down the wrong engine - extinguish the good one etc - many factors which come in at significantly greater than a compounded 1x10^-5,
If it really were 1x10^-10 then we would never see the examples we do - so we can see it is not correct 'by inspection'.
We might want to but we cannot fairly just compound 1x10^-5 and use that figure - other than very hypothetically.
Also the INCREASED risks you refer to:
As you have said, there are other reliability issues that have to be addressed; these are dealt with in certification and build standards, and component lifing; or, with redundancy
Well it's really not fair to hide them out of the equation - they are the price paid for the (hypothetical) 1x10^-10
And many of those standards (dual hydraulics) - chickens in tail rotors(?) etc are equally applicable to singles....
The negating of Hostility by mitigation looks like it has merit especially wrt ditching survivability - any controlled arrival into any water state should/could be rendered survivable - surely? (is that true?)
If the probability of arriving in the water were truly only 1x10^-10 (which it is not) then that is SO small that carriage of floatation equipment would not be required/safer, the risk could possibly be higher....
It is surely flawed to make risk from one specific cause (engine) better without including the increased risk from other causes - (like the tail boom falling off etc etc ...)
Nick Lappos has/had a really excellent arguement about 'over-engined twins' wasting payload on an almost irrelevant slice of 'exposure' during the Take-Off/Landing Phase - he wrote an excellent article on it - (somewhere?)
I am not against TWINs - and agree with Jim that it:
It all comes down to the old adage, horses for courses.
So it's perfectly fair and reasonable that manufacturers should be free to supply the free market with whatever hair brained scheme their engineering ingenuity comes up with to deliver the safety outcome the market demands - even if it means fitting two engines - or floating saftey cell cabins - to make the safest product ... dubious maths should not be enshrined in law to dictate particular engineering solutions.
Although I suppose it helps push the manufactures along....
Customers / oil companies / police forces should be free to demonstrate their commitment to safety by procuring helicopters which their consultants believe are safer...
it's a very strong driver.
Do you really want to be up an Alp in an EC145 in preference to a B3(e?) ?
Power / capability / reliability / margin or accountability .... ?
One saves pilots/people the other saves ..... accountable managers.
Patrick Goudou (EASA) doesn't have time to consider this topic:
"However, within the timeframe available, stakeholders and the Agency were not able to gather the appropriate data and establish a safety case that would show the need to reconsider the basic underlying principle of JAR-OPS 3, which was to transition fleets to multi-engined helicopters, given subsequent technological advances and increased reliability of single-engined helicopters."
Opinion04-2011
Jim has done a great job of forcing some rational considerations up the flag pole and into law - but isn't it time we 'upped the game' and lent a bit of support in the same way to
other solutions...
are there any aircraft designers out there who can clarify the mathematics?