Hello and welcome to more or less on the BBC World Service. We are your guide to the

numbers in the news and in life, and I'm Tim Halford.

Comedy mathematician Matt Parker has a new book out, Humboldt Pie. That's Pie Without

a Knee. As you might guess, it's a celebration of the causes and consequences of mathematical,

engineering and programming errors. You might think of a mathematical error as taking place

in the realm of abstract thought, but an engineering error can be painfully visceral. Consider

the following maintenance issue from a BA or British Airways flight in the happy,

Heady Days of 1990. This was someone working on a BA aircraft, and they were placing a windscreen,

and they had took the bolts out and they had to put new bolts in, and they got the wrong

bolts when they put them back in again. I bought some of these bolts, so I bought a 211 7D bolts,

and a 2118 C bolts, 70 and the 8C. They're...

Dangerously close. The 7C, according to the specifications, is 0.026 inches wider,

about 0.66mm, whereas the 8C technically 0.1 inches longer, so that's about two and a half.

One slightly longer. The 8C is slightly longer. The 7D is slightly wider, but if you hold them

very hard to tell the difference, and so this guy took the bolts out of the aircraft to be

fair, he correctly identified what they were, but they didn't have enough, and so we had to drive

to a different store room, and all these incredible sozo things went wrong. Like him forgetting

his glasses that day, and later on they checked, and like over half of the compartments in the store

had the wrong parts in them. And there was a particular kind of wrench that he could have used.

Exactly. He would have helped him see that there was a problem, but that was missing.

So he had to use a different one, and that meant that he's hand was covering it, so he didn't see

that it'd fit properly. One tiny mistake after another tiny mistake.

...