JimL, thanks for the sincere answer, great respect to you for that, I hope these are useful points.
Originally Posted by
JimL
AnFI, if you intend to quote someone, perhaps you should provide a reference or justify your statement:
Jim
Hi JimL; it's a paraphrse rather than a quote. it's a mathematically tongue-in-cheek implication of what you say about probability of catestropic events, you say should be 10^-9 (10^-6 would already be pretty darn good), this 'catastrophy' is a conflation of your product of probability of risk factor and probability of fatal outcome (catestrophic consequence). If you hold that the consequence IS 100% fatal (which you do) and If the risk x consequence is < 10^-9 (you use), 'given' that the risk becomes a certainty then if the consequence is actually survival (for an event), then it must have been very very very lucky to survive, something like 10^-9. Of course the truth is these consequences are being wrongly identified as 100% fatal.
If you had a 99:1 chance of survival, and the acceptability of the event were 'reasonable' ie 1 per million hours, then the aircraft
would only need 10^-4 probability of forced landing.
Originally Posted by
JimL
You need to explain why redundancy with pilots, does not add unacceptable complexity!
Jim
Good point. In a complex aircraft, the reality is that a single pilot can easily be swamped by work load and confusion, his 'flight secretary' can reduce the work load to a manageable degree. The pilot error rate for a complex single pilot machines opperated by the most basically qualified pilot would be ludicrously high. It's an error rate that can justify the extra weight of another pilot.
It's a weight / utility / complexity calculation:
A spare engine is
only doing BAD STUFF for you, ALL THE TIME, it's introducing extra freewheel, extra explosion risk, extra gear box complication and payload consumed etc etc
It has to make up for it by being useful. The only time it gets to do that is ONCE per
million hours.
Really obviously not worth it.
A Spare Pilot is useful
more than once per
100,000 hours when the other guy has a heart attack.
He is COMPLEMENTARY, that is he actually improves the overall performance of the piloting. This is the highest yeilding safety return.
So to summarise:
a spare engine is only bad until it's useful
and
a spare pilot is only useful until he's bad
(and then the spare pilot concept kicks in)
The Glasgow incident would probably be averted by a spare pilot, but was enhanced by a spare engine.
Originally Posted by
JimL
Redundancy is only required when a component (or system) doesn't meet a specified reliability target and the consequences of failure is not acceptable - to some criterion.
Jim
ALoSP: You need an expected death rate for people on the ground per 100,000hrs to determine whether it is an 'acceptable rate'.
The principle that you can't have any death at any cost is not realistic. If that were the case everything would have to be banned.
The HSE provide clear guidance about the value of economic activities incurring mortal costs and is clear that one can expect a degree of cost.
They suggest figures like 10^-3 per year. What is the probability of a given person on the ground being exposed to death from over flight of a single engined helicopter vs a multi engined helicopter? Something like 1 per 1000 billion hours?
What differentiation in that rate occurs by engine type? Anything measurable? How much is it? How unacceptable is it?
"you have to measure it to regulate it" -
QUOTE?
Is someone actually computing the ALoSP - rather than some
random application of an inappropriate criterion for component reliability.
10^-9 was set for critical components. A Tail Rotor, a Gearbox, a RotorHead, Control Runs are made from an assembly of many components where 10^-9 is applied in manufacture to those component parts. These are the component reliabilities that deliver between approx 0.1 and 2 fatalities per 100,000hrs.
Originally Posted by
JimL
Protection of third parties is the highest priority with aviation Authorities; the next is CAT passengers.
Jim
What outcome risk level is the criterion? There does not appear to be a 'target'. There is no case made for there being a greater or lesser risk by mandating engine redundancy.
Originally Posted by
JimL
Protection of third parties is the highest priority with aviation Authorities; the next is CAT passengers.
To my knowledge, there is no regulation that prevents a pilot, of other than a CAT flight, from flying over a sea area where a failure of the engine or another system would result in a hazardous or catastrophic outcome. Rules of Air kick in only when third parties are exposed.
Jim
Not only is it debatable that engine redundancy delivers reduced risk to these 3rd parties.
ALSO
as you say the priority is to 3rd parties. For people on the ground that would entail you restricting (banning) PRIVATE HELICOPTERS from overflight of hosile congested areas. (and under your definitions hostile congested areas include playing fields, parks, rivers, car parks etc etc)
Is it justified? By what criteria in deaths per hour, say?
Did you look at post #97, how do you account for that, most death is not engines.
Crab, v personal again, some heli TR have hydraulics not does not equal MUST have hydraulics. The 'why' i answered.