In aviation, there are many types of altitudes with their own meanings and uses. In its general term, altitude can be defined as an aircraft’s vertical distance above a fixed datum, typically the ground level or sea level. Flight Levels define altitude differently. Flight Levels refer to an aircraft’s vertical distance referenced to a standard air pressure datum instead of the real altitude above the ground or sea level.
Other types of altitudes are absolute altitude, indicated altitude, true altitude, density altitude and most importantly for Flight Levels – pressure altitude. Pressure altitude is the height above the standard datum air-pressure datum which is referenced to 29.92” Hg or 1013.2 millibars. This type of altitude is used as a transition level in the Flight Levels. Flight Level values are expressed in hundreds of feet so that if the pressure altitude is 25,000 feet, the Flight Level becomes FL250.
In the United States and Canada, Flight Levels are classified as Class A airspace and begin at FL180, (18,000 ft) and extend to FL600. The transition level between altitudes and Flight Levels differs by country, depending on the terrain and highest obstacles in that country. Pilots set their altimeters to 29.92”Hg or 1013.2 millibars when entering Class A airspace so that all aircraft are set to the same reference level which eliminates errors when flying over areas with different local air pressures. Pilots will keep this setting in the altimeter until they descend below the transition level to their destination when they will contact air traffic control to acquire the local air pressure and enter that value into the altimeter.
Flight in all airspace functions similarly to our roadways, and just like there are rules guiding how we drive, there are rules and principles guiding the use of airspace. Aircraft fly according to a principle referred to as the semicircular rule that aircraft must adhere to when flying at different Flight Levels. When an aircraft flies westbound, with a magnetic heading of between 180° and 359°, they fly even-numbered Flight Levels, such as FL180, FL200, FL220, etc. When they fly eastbound with a magnetic heading of between 000° to 179°, they fly at odd Flight Levels, FL190, FL210, FL230, etc. By using the semicircular rule, air traffic control can provide sufficient separation and prevent collisions from happening to flights coming from opposite directions.
Typically, aircraft flying in the Flight Levels are vertically separated by 2000 feet. However, Reduced Visibility Separation Minimums (RVSM) reduces this separation to 1000 feet between FL290 and FL410 in certain regions. RVSM airspace currently includes airspace over North America, Europe, parts of Asia and Africa and the Pacific and Atlantic oceans. This allows more aircraft to be stacked in the same airspace allowing for more flights while still retaining safety and efficiency. Pilots require additional training to fly in RVSM airspace. Additionally, the aircraft must be equipped for flight in RVSM airspace and the autopilot and altimeters must be specially certified to higher accuracies.
Class A airspace is ATC controlled, uses Instrument Flight Rules (IFR) and there are no minimums for visibility. The Flight Level a pilot can attain depends on the certification or the license they possess. To use Class A airspace a pilot would need to be instrument rated. Use of the Flight Levels is dependent on the aircraft and engine types and is typically only used for high-performance aircraft, business jets or commercial airliners. Flying at the higher Flight Levels offers tremendous benefits for those types of aircraft. There is less drag due to the thinner air, greater fuel efficiency, and they can avoid most of the severe weather.