Cardinal altitude is a common aviation term that is expressed in thousand-foot increments, known as flight levels. In order to maintain safe vertical separation between airliners, private planes and other aircraft flying in the same vicinity, air traffic controllers will assign specific flight levels. Traditionally, altitude is measured based on the local atmospheric air pressure using an altimeter. It works on the premise that, the higher you are away from the earth’s surface, the less air pressure there is. One of the challenges arising from this approach is that the localized air pressure can vary from region to region. As a result, pilots could find themselves flying at the same flight level if their altimeters aren’t calibrated to the same standard.
Establishing Standardized Flight Levels
To overcome this, aviation authorities introduced the concept of cardinal altitude. This enables pilots to fly at standardized flight levels based on air pressure at sea level. These flight levels are measured in hecto-feet. For example:
- FL250 is 25,000 feet
- An altitude of 17,500 feet would be designated as FL175
Private planes only use flight levels once they reach a certain altitude, known as a transition altitude. In the United States, this is typically 18,000 feet, or FL180. The reason for this is that aircraft traveling at lower altitudes are primarily concerned with potential collisions with the ground and other obstacles. As a result, they determine altitude using the local pressure readings. Once they reach the transition altitude, they switch to standardized flight levels. They do this in order to maintain accurate vertical separation between private planes.