Flight level is a term for describing the altitude of an aircraft in relationship to the average sea level. To differentiate between measurements using average sea level and true sea or ground level, aviators will refer to the “flight level” specifically. For high flight, flight level is the most crucial measurement; while at lower altitudes, the real altitude of the aircraft in relation to objects on earth is important as well. Aircraft have the capacity to measure both.
Altimeters to calculate the altitude of an aircraft rely on changes in air pressure as planes rise. To calculate flight level, they are calibrated with the average sea level as the baseline. To get a reading of true altitude, the altimeter needs to be recalibrated. Aircraft may have a second altimeter or a toggle to switch between flight level and altimeter setting to get both readings, depending on how the instrument panel is designed.
The flight level is given in units of hundreds of feet. A plane at flight level 20 is 2,000 feet above average sea level, for example. When pilots give readings, they will precede them with “flight level” so listeners know what frame of reference and baseline is being used. This is important for avoiding midair collisions. Two planes relying on local altimeter settings could end colliding even though their altimeters have different readings, while when both are measuring in flight levels, they can avoid each other by sticking to specific tracks while flying in the same area.
Airspace is controlled at various altitudes to determine what kinds of craft can be present, the rules they must follow, and how traffic will be routed and prioritized. Air traffic controllers use flight plans and other reports from pilots along with radar and similar tools to keep track of what is occurring inside their air space so they can make recommendations to pilots. These can include orders to climb or drop altitude to avoid collisions, as well as directions on how to approach airports and air strips. Flight controllers also maintain local altitude data for pilots who need to recalibrate their altimeters.
In many nations, air space is divided into tracks, with planes flying in a specific altitude range depending on the direction they are going in and the routes they are taking. This standardizes routine air traffic, making it safer to fly. It can also eliminate confusion in emergencies when a plane needs priority because of engine trouble or other issues necessitating a rapid landing.