NewMexiKen is a fan of Salon’s feature Ask the pilot, despite the annoyance of having to watch an ad to get a free day pass. Though I knew something about altimeters and altitude, I still found this interesting:
A plane’s altimeters (typically there are three independent units) — and, in turn, all air traffic control instructions — are referenced to sea level, not to the ground. Parked at the gate at Mexico City, they will read about 7,500 feet. Thus, when the captain announces that you’re cruising at 35,000 feet, the surface of el mundo may or may not be 35,000 feet below you. Over the ocean, yes, but over a high plateau, mesa or mountain range, no. A sea level datum allows aircraft to be safely and evenly sequenced. You couldn’t have a sector full of planes, all at 15,000 feet, if 15,000 kept changing each time you passed above a hill or depression.
Altimeters have no idea where the ground is, strictly speaking. Essentially barometers, they are set by dialing or programming in the local barometric pressure, gauged in inches of mercury (or metric millibars) like the ones given out by TV meteorologists. The atmospheric standard for sea level is 29.92 inches (1013.2 mb), but actual pressures will vary from place to place, depending on the weather. If you enjoy tuning in to United’s Channel Nine, you’ll hear crews being advised of the local value. “Altimeter: two-niner eight-six,” for example.
En route, above certain heights — 18,000 feet in the U.S., but usually lower elsewhere — all aircraft change over to the single standard of 29.92 inches. At this point, cruising altitudes become known as “flight levels.” Thirty-five thousand “feet” is technically “flight level three-five-zero.”
There’s a separate cockpit unit called a radio altimeter that measures a plane’s position relative to the turf itself. Height above ground level, or AGL, is referenced when flying close to the surface — up to maybe 1,500 feet or so– such as during instrument approaches. Over uneven terrain, the radio altimeters, unlike the main altimeters, will be constantly changing.