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It's called secondary damage
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 4 Apr 2008 07:26:11 Share Post by Email
In Reply to: why reliability really matters, steve, Wed, 2 Apr 2008 11:06:41
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Secondary damage - A common term for those folks that work in the reliability/maintainability and fault detection part of the world. 'Cascading Failures' is another.

It is controlled (not eliminated) by good design and good maintenance. A good design will limit the cascade of failures - the short circuit in the radio won't take out the electrical system because of proper wiring practices and fuse sizing. Good maintenance, by repairing problems when they are small. A classic example is the oil change. Change the oil frequently, so it doesn't develop sludge. Sludge isn't dangerous, not until it clogs the oil pickup. A clogged oil pickup means no oil pressure. No oil pressure leads to bearing failure. That's a cascade.

That's easily said. It's harder to do. One reason is that things break. Everything breaks. Rocks break (see: sand.) So you can't eliminate faults. You can minimize secondary damage and cascaded faults. But that requires some pretty extensive design analysis to find the faults, and then cost and weight to reduce the failure rates. You can make things very reliable, but they are very expensive and heavy. Or lighter, and very, very expensive. Or less expensive, but very heavy.

So cars are all about trade-offs. Yes, manufacturers can make things more reliable. But that adds cost and weight. So it's always a trade-off to make it 'reliable enough.' No car company will make money designing a car that will last a million miles if it costs a million dollars. And then add in that no matter how much analysis you do, there are always failures you didn't think about. That's because Physics never sleeps, and it never forgets its own laws. People do.

I'm basing this on my experience, which is 30 years in aerospace design and diagnostic systems - building things so they don't break, and then detecting the problems when they do before the faults get worse. We spend lots of money doing the analysis to show a jet engine will stay lit, because the downside is pretty far down and a sickening crunch at the end. I can bet that car companies are going to be spending their limited resources to make sure the brakes and steering work flawlessly, and less on the window regulator design.

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