Your assumption of 1 failure of alternator or regulater to be 1 on 1000
hours can't be realistic.
Back to cars, in the 40 years i have been driving, 3.500.000 km, about
500.000 hours, i never had an alternator, regulator nor battery failure.
Maybe i've been lucky, but still, car alternators are pretty reliable.
I can think of only one reason why airplane alternators would be less
reliable and that is lightning, or better the electromagnetic pulse from
it, in a non shielded, non metal airplane.
Back to odds:
In calculation the odds of failure one important view is usually
neglected. Every component, every wire, and every contact between them
has a failure rate. By adding components for safety you will obviously
end up with more components. At first sight adding a completely
independent system will only double the failure rate of the total.
But there will be extra components, to make switching between the
systems possible, inherently adding failing components. The result is
that the total system, the aircraft will be more error prone.
Really, simple is beautiful!
In case of bad pulse from lighning every semiconductor in the plane
could be destroyed. You don't even need a direct hit for that! So if, in
a sophisticated double system bus isolation or relay protection is by
diodes, we will have a complete failure. The best suggestion i saw was a
set of alkaline batteries to switch to. Mechanical :-)
Jos Okhuijsen
Fred Fillinger wrote:
>
>david joyce wrote:
> > ...It is entirely reasonable to allow for one unlikely event, but
> > the odds of two unlikely independent events happening simultaneously
> > are such that I am happy to use gliding for the back up mode.
>
>Agree but ever play with the actual math? [I think fwg is correct!]
>
>Consider a case, familiar with automobiles, where the battery starts the
>engine, but for the trip back home it's dead. Assume the odds of this
>are 200/1, to include possible poor system design, fabrication, or
>maintenance, but whether accurate will be shortly seen as irrelevant.
>Assume also the average flight is 1 hour for simpler math. If the odds
>of either alternator or regulator failure are one in 1,000 hours, then
>the odds of both charging system and battery failure are one in 200,000
>for each flight. Sounds good but...
>
>Over say 500 hours of airframe time for one's flying the plane, we're
>down to 400/1 for the chances of being caught someday, some year not
>knowing how long further flight is possible on the 914 if it's running
>at all. Reflecting also the odds of all other system failures causing a
>serious in-flight problem, the overall odds grow shorter. This
>phenomenon is evident in accident data, where builders too often do
>things for critical systems the FAA would not approve, and system
>failures occur at very low airframe times.
>
>Consider now two independent alternator/battery systems. Or that the
>odds of an emergency battery pack failing is also 200,000/1. The odds
>of both systems failing becomes 40 billion/1, making it irrelevant
>whether 200/1 odds on battery failure may be too low, because over the
>assumed years of flying it's still 80 million/1. A far cry from 400/1.
> For this reason, seems FAA may not make failure probabilities
>controlling for approval if full redundancy is provided (per AC 23.1309-1C).
>
>Regards,
>Fred F.
>
>
>
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