Mark Vanhoenacker is a Boeing 747-400 pilot for British Airways who also happens to have a wonderful almost lyrical way with words. In this NY Times piece, Vanhoenacker gives an overview of how a London-to-Tokyo flight functions, from take-off & landings to what pilots see in the dark night skies to the determination of altitude.
Three altimeters in the cockpit — two bright digital readouts, and one old-school device with hands that turn like those of a clock — show 31,000 feet.
Yet we know that we are probably not as close to 31,000 feet as these altimeters suggest. We are somewhat lower; or perhaps we are higher. One thing is certain — it would be easy to find a dozen airliners flying over different parts of the world, all of whose altimeters displayed 31,000 feet, none of which are at the same altitude.
How is this possible?
Planes calculate their altitude by measuring air pressure. The air lies most heavily on places that are lowest, the places that have the most air piled above them. A barometric altimeter (baros, meaning weight) equates high air pressure — lots of air weighing down — with low altitude. As a plane climbs, there is less air above it. The altimeter senses less air weight and reports a higher altitude.
There’s a problem, however. Air pressure is not constant. It varies across the Earth. It also varies in each place as time and weather pass.
And I love this bit about the names of the geographic waypoints used to navigate the area around Boston:
Boston has etched a particularly rich constellation onto the heavens above New England. There is PLGRM, of course; CHWDH, LBSTA and CLAWW; GLOWB and HRALD for the city’s newspapers; while SSOXS, FENWY, BAWLL and OUTTT trace the fortunes of the city’s baseball team in long arcs across the stars. There’s a NIMOY waypoint; Leonard was born in Boston.
The piece is adapted from Vanhoenacker’s new book, Skyfaring: A Journey with a Pilot. My dad was a professional pilot for many years1 and I’ve always loved flying, so I’m definitely going to give this a read.