FL210 versus 21,000'

Can someone help.
Can someone explain me what is the difference between FL210 and
21,000 feet?

-- Ignacio marino, February 6, 2006

Answers

21,000' is 21,000' above mean sea level. FL210 is wherever you happen to find yourself in space with your altimeter set to 29.92 (standard barometric pressure) and reading 21,000'. For planes that are very high up and flying long distance, it doesn't make sense for them to have to keep setting the altimeter so that they know what height above the ground they are. Above 18,000' within the Lower 48 states of the U.S., for example, there are no mountains to hit so you just have all airplanes set their altimeters to 29.92 and there will be a good vertical separation of airplanes.

-- Philip Greenspun, February 6, 2006

I want to add to Philip's response. When discussing the differences between FL and feet, invariably the topic of transition altitude and transition level arise. Simply, transition altitude is the altitude, when climbing, at which you reset your altimiter to 29.92" or 1013 hpa. In the US, this is 18,000" Transition level, is the FL at which, when descending, you reset your altimited to QNH, or the local altimeter setting. In the US, this is FL180. I have seen the transition altitude as low as 3,500' in some countries, and the transition level down around FL55. The US is the highest I have seen at 18,000 / FL180.

-- Bradrick Pretzer, February 7, 2006

I'd like to add one little thing about QNH.

QNH at a field is set to the settings that would get your altimeter to indicate the field altitude when you're on the ground.

Regional QNH, for example the QNH you get from Boston Approach, is the lowest QNH in the Boston area.

When you're below the transition altitude, there are two important things about the altimeter settings: That all planes will use the same settings (for traffic vertical separation) and that your actual altitude will be identical *or higher* to what your altimeter shows (for ground clearance). When operating at the flight levels, the ground clearance is no longer a factor, which is why you operate there by pressure altitude.

-- Tal Reichert, February 7, 2006

In my home area, which is Denmark, Northern Europe, the Transition Altitude is only 3000 feet in most of the country. This means that practically every PPL VFR pilot is quite familiar with flight levels. The ATIS of our airports include information about Transition Level, which is the lowest available Flight Level (often, but certainly not always FL 35). The layer from Transition Altitude up to Transition Level is called the transition layer, and this layer must have a positive "thickness". So Transition Level is not a constant, it depends on QNH. With a low QNH, Transition Level will be higher, moving in steps of 5.

However, the metropolitan area around Copenhagen has a higher Transition Altitude of 5000 feet. So when flying from Copenhagen to other parts of the country at, say, 4000 feet and level, at some time we are told to "climb Flight Level 40", which sometimes could be a climb of only 100 feet or less. Flying VFR, we may not always get the ATC prompt and has to remember changing the altimeter between QNH and "standard" setting, even when flying level.

As you might expect, altimeter setting errors happen on a frequent basis here, and rumours are that Northern Europe will get a common Transition Altitude of something like 7000 feet in a few years from now, in order to decrease workload especially during approach.

-- Henrik Vaeroe, February 7, 2006