I like airplanes, and given the amount I travel that is a good thing. Seeing these incredible machines–aluminum and composite monuments of human ingenuity–makes the atrocities of most American airports almost bearable. (My genetically tanned, ambiguously ethnic appearance must scream "al-Qaeda!" I get patted down more than Donald Trump's mane on a windy day.)

Modern airliners, as one author put it, are "the most complicated machines man has ever built." But they are still regarded as the safest form of transportation. There are over 20,000 commercial flights every day in the United States. If you were to drive rather than fly one of those routes, you would be 65 times more likely to be killed. Perhaps more surprising, since 1980 the number of airplanes, flights, and passengers has doubled, but accidents per year have been declining. Flying is five times safer now than 30 years ago.

How is that possible? There are many factors that contribute to air safety, but a significant one is what the ...