Jos Okhuijsen wrote:
> 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.
> 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.
I can think of many reasons. Auto alternators run at a fraction of
capacity, for satisfactory operation in city driving. The Rotax
alternator likely will be running close to full-tilt with any decent
instrument panel, generating considerable heat in any already hot
operating environment. Though it should be more robust, as a simple PM
type "dynamo," it was once recalled by Rotax. Maintenance-free and
low-maintenance batteries are improved technologies over the 1800's
technology flooded-cells, which are what A/C batteries are.
Amateur-built A/C are used much less frequently, shortening the life of
these batteries. The reliability of recombinant gas batteries not
approved by FAA is so far anecdotal. Auto alternators are
temperature-compensated for always optimum charge voltage. The Mickey
Mouse Rotax regulator isn't even putting out the correct voltage at the
low end of its spec!
The Teledyne-Gill battery in my other plane failed recently and
suddenly. In the annual inspection a couple months prior, it passed the
hydrometer and standing voltage test. The plane is also on it 2nd
FAA-approved alternator in 1100 hours.
> adding a completely independent system will only double the failure
> rate of the total.
Correct, as is your observation of further probability that the switch
might fail. However, for fail-danger analysis it's their individual
failure probability squared. If the systems have 10% chance of failing,
both failing is 1/10 x 1/10, or 1 over 10 squared.
Regards,
Fred F.
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