Flight Levels, altitude, and barometric pressure

In a previous post, I noted that a clearance might include an initial altitude of 7,000 feet and an assigned cruise altitude of Flight Level 390, which corresponds to 39,000 feet. Why don't we just say "39,000 feet"? Why do we use two different forms of language for talking about different types of altitude?

There are two reasons. One of which is related to communication, and the other to methods of determining altitude.

We use the phrase "Flight Level" when referring to altitudes including and above 18,000 (above sea level). At these altitudes, for safety's sake, we don't worry about intervals of less than a few hundred feet. For communications clarity, over sometimes crackly radios, saying "one niner thousand feet" (19,000) might be confused with "niner thousand feet" (9,000). It's a matter of clarity. Note that flight levels are always given as three digits, basically leaving off the last two digits of the altitude in thousands, so FL390 is 39,000 feet, and FL 200 is 20,000 feet.

More importantly, though, the transition from one system to another has to do with consistency in the measurement of altitude. When operating at lower levels, near airports, it's vitally important that your instruments reflect the correct altitude relative to the airport. If the altitude of the airport is 50' above sea level, then you certainly want your altimeter telling you that you're at 50' when you're sitting on the tarmac. Airplanes tell altitude in a number of ways (GPS, radar altimetry) but the standard method is using barometric pressure. There's a great article on pressure and altimeters here.

Barometers measure the pressure of the air at a given point; essentially a measure of the weight of the air column sitting on top of the barometer. In North America, this is typically measured in inches of mercury, and the standard sea level pressure is considered to be 29.92" of mercury. You know when the weather report talks about "highs" and "lows"? Those refer to the fact that weather patterns affect the density of the air in a given location, and the pressure is greater or lower. You all know how your ears pop when you're driving up a mountain or climbing in an airplane? That's because of the change of air pressure that occurs with altitude. The higher you go, the less air there is between the top of your head and the top of the atmosphere. Airplane altimeters measure this change, and convert it into altitude.

So, when operating near an airport, it is important to find out what the current barometric pressure is at the airport, and to adjust your altimeter accordingly. This calibrates your altimeter to the local altitude, regardless of what the weather is doing. Aviation weather reports always include the pressure for this purpose. However, at high altitudes, the air becomes much thinner, and aircraft are transiting the air from a wide variety of destinations. At a certain height, all pilots will recalibrate their altimeters to the standard reading of 29.92, which virtually guarantees that everybody is working from the same scale, and two planes indicating that they are at 29,000' above sea level will be at precisely the same height. In North America, the altitude at which this recalibration happens is called the transition altitude, and it happens at 18,000 which is the same as FL180. So Flight Levels refer to the uniform, consistent altitude based on the standard pressure of 29.92" of mercury.

See Michael Oxner's article on altitudes and flight levels.