Saturday, December 12, 2015

Double/Triple Contingencies

Marathon, FL

Things break.  We all know that.  We also know that almost everything breaks sooner or later.  But we are not typically prepared for multiple simultaneous failures.  In our minds, we consider the change of that happening to be so low that we disregard it.

Arriving in Marathon, it was time to take my bike ashore.  It had been riding on Tarwathie's forward deck.  Libby sensibly said, "Check the tire pressure before going in."  I did, and the pressures were low.  I got out the hand pump we keep on board, and pumped both tires up to the specified 55 psi.

The next day, I found that the rear tire was flat.  Oh no, I hate flat tires on bicycles.  But after removing the tube and putting it under water, it had no leaks.  I put the tube back, but then I couldn't pump it up.  I blamed the schrader valve on the tube.  So I walked up to KMart and bought a new tube.  That too wouldn't pump up.  WTF?

Weary from the long walks, I walked the whole bike up island to a gas station with an air pump.  That worked, and the tire now holds 55 psi.

Careful inspection of my hand pump shows that there is a small cone inside the fitting that is supposed to press the pin in the schrader valve.  The cone was made of soft plastic, and deformed, so it make the pump inoperative.  But the hard part to comprehend was that the pump must have failed after I pumped up the tires on board Tarwatthie, and before I tried to refill the tire on land the day after.  A tire when flat tire and the pump broke while trying to fix the flat.  How likely is that?

My Pump Looked Like This

By the way, engineers like me are fond of applying redundancy to problems of reliability.   If the car won't start, take the bus.  If the jib won't deploy, use the staysail.  If the GPS won't work, use the paper charts.

How many layers of redundancy are needed?  On our recent travels, (1) our GPS chart plotter failed, (2) my phone with backup GPS chartplotter apps failed, (3) our paper charts had been stored in a wet place, they were all turning to mush.  So even for something as simple as a sail, three levels of diverse redundancy were nearly inadequate.

Without giving it much thought, we use a simple mental formula: more redundancy means more reliability.  But we very often forget "common mode" failures.  What does that mean? It refers to cases where the redundant devices all fail at once because of a common reason.   In the Fukushima Nuclear Plant in Japan, multiple diesel-generators provide redundant critical backup power.   But after the tsunami, all the fuel tanks got contaminated by salt water.  No matter how many redundant diesels there were, they would all fail at once.  Buying fuel for those diesels from the same source, is another way to produce common mode failures.  Be on guard for common mode failures in your own life.

1 comment:

  1. I liked Fix-a-Flat in my bike tires. A lot cheaper than the "solid no flat" tires. Miracles of modern chemistry! Ken


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