FL (Flight level)

Flight level (FL) is a standard system used in aviation to indicate an aircraft's altitude relative to a standard datum plane, which is typically set at 1013.2 hPa (29.92 inHg) of atmospheric pressure, also known as "standard pressure." FL is measured in hundreds of feet and is commonly used in air traffic control (ATC) communications to ensure that aircraft maintain a safe vertical separation.

In this system, the altitude of an aircraft is expressed as a multiple of 100 feet above the standard datum plane. For example, an aircraft at FL300 is flying at an altitude of 30,000 feet above the standard datum plane. FL is always measured in hundreds of feet and is typically used for altitudes above 18,000 feet, where the use of the standard altimeter setting is mandatory.

The use of FL is important in aviation for a number of reasons. First and foremost, it provides a standardized way for aircraft to communicate their altitude to air traffic controllers and other pilots. This ensures that all parties are using the same reference point when discussing altitude, which is critical for maintaining safe separation between aircraft.

Additionally, using FL rather than actual altitude helps to simplify ATC communications, as it eliminates the need for pilots and controllers to convert between feet and hPa or inHg when communicating altitudes. This can help to reduce errors and misunderstandings in ATC communications, which is critical for maintaining safety in the skies.

The use of FL is also important for maintaining safe vertical separation between aircraft. In general, aircraft flying in the same direction and at the same altitude should maintain a separation of at least 1,000 feet. However, at altitudes above 29,000 feet, the minimum separation is increased to 2,000 feet. By using FL to indicate altitude, pilots and controllers can quickly and easily determine whether the required vertical separation is being maintained.

It is worth noting that while FL is a useful system for indicating altitude, it is not without its limitations. For example, because FL is based on a standard pressure, it does not take into account variations in atmospheric pressure that can occur due to weather conditions or other factors. This means that an aircraft flying at FL300 may not be at exactly 30,000 feet above mean sea level (MSL), which is the actual altitude above the Earth's surface.

To account for these variations, pilots and controllers will often use the term "altitude" in addition to FL when discussing an aircraft's vertical position. For example, a pilot might report their altitude as "FL300, actual altitude 31,000 feet." This provides additional information about the aircraft's true altitude, which can be important for ensuring safe vertical separation.

In addition to FL, there are a number of other altitude measurement systems used in aviation. For example, pressure altitude (PA) is the altitude above the standard datum plane, as measured by an altimeter set to the standard pressure setting. This is similar to FL, but is measured in feet rather than hundreds of feet.

Density altitude (DA) is another measurement used in aviation, which takes into account variations in atmospheric density due to factors such as temperature and humidity. DA is calculated by first determining pressure altitude, and then correcting for temperature and humidity to arrive at the true altitude above MSL.

In conclusion, flight level (FL) is a standard system used in aviation to indicate an aircraft's altitude relative to a standard datum plane. FL is measured in hundreds of feet and is commonly used in air traffic control communications to ensure that aircraft maintain a safe vertical separation. While FL is a useful system, it is not without its limitations, and pilots and controllers must take into account variations in atmospheric pressure and other factors when determining an aircraft's true altitude.