Aileron

This past week was my first time on an airliner after starting flight lessons and what hit me was how fast these planes can climb, fancy jet engines and all; the ground just falls away. Traveling this week also leads to my sixth misconception dispelled:

An airliner is really flying at 32,000 ft when the captain says it's flying at 32,000 ft.

It's probably not and the captain doesn't really care.

To explain, a brief something about altimeters: As altitude increases, air pressure decreases. Altimeters are just sensitive barometers that read in feet above mean sea level (MSL) instead of in mmHg or Pa. But in addition to altitude, weather systems also affect air pressure so that, when flying, a pilot must regularly get reports of the air pressure on the ground and use that to re-zero his altimeter and keep it reading accurately.

Above 18,000 ft though, where airliners cruise, pilots kind of give up on accuracy. They simply set their altimeters to standard pressure (29.92 mmHg) and leave it there for the entire cruise regardless of what the actual sea level pressure is below them. This means that for most of that time, their altimeters read incorrectly. The reasoning is that at that altitude, the only thing you're concerned about hitting is other airplanes. As long as all airplanes are incorrect about their altitudes by the same amount, then the vertical spacing between airplanes will be correct. And you get the bonus of not having to adjust your altimeter every thirty to sixty minutes.

This standard pressure convention is the idea behind flight levels. Below 18,000 ft, airplanes are assigned and report altitudes in feet MSL, their actual height above mean sea level. Above 18,000 ft, they use flight levels, e.g. FL240 means whatever altitude I end up at if I set my altimeter to standard pressure and I make it read 24,000 ft. FL240 corresponds to the idea of 24,000 ft MSL, but they're not equivalent. In fact, an airliner maintaining a constant FL240 is always changing it's actual altitude MSL as it flies through different pressure systems. But that's okay because all of the other airplanes in that area are doing the same thing, so our plane at FL240 will always really be 2,000 ft below a plane at FL260.

You may point out that a handful of mountains exceed 18,000 ft, which is a very good thing to know if you're a pilot without accurate altitude. Airplanes tend to give such bits of terrain a wide berth both horizontally and vertically.