The incident of the Mars Climate Orbiter, described in the previous post, shows that it pays to check your units, and get them right. One spectacular miscalculation, which fortunately ended with no loss of life, was when Air Canada Flight 143 ran out of fuel on a routine flight from Montreal to Edmonton on July 23, 1983. There had been a problem with the fuel gauges, so the maintenance workers decided to calculate the amount of fuel that they needed to top up the tanks. To do this, they used the density figure (1.77) provided by the refueling company. Alarm bells should have started ringing when there were no units attached to the figure. As it turned out, the figure was in a peculiar mixture of Imperial and metric (pounds per liter), but the workers took it to be in kilograms per liter. One kilogram ≈ 2.2 pounds, so the plane took off with only half the fuel that it needed! Luckily one of the pilots was an experienced glider pilot, and when all the fuel ran out he was able to bring the plane down on a disused airstrip at a place called Gimli – hence the name “Gimli glider” that has been attached to this incident ever since.

ADDENDUM: SOME USEFUL MNEMONICS I grew up in an era when land areas were measures in acres. Then in came hectares, and I could never for the life of me remember whether it is 2.5 hectares to the acre or the other way round, until I found a simple trick. Hectares are actually bigger than acres (it’s approximately 2.5 acres to the hectare), and the fact that the word “hectare” is longer than the word “acre” works as a mnemonic.

Here’s another useful mnemonic to get a “feel” for a unit – in this case the unit of force called the Newton. Few people have much of a feel for what a Newton actually is, but it’s quite simple: a Newton is the force exerted by the Earth’s gravitational field on a smallish apple.

The unit of energy called the Joule is even less familiar to most people, but it’s easy to get a feel for it – a Joule is approximately the energy needed to raise a cup of coffee to your lips.

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