For the purpose of stipulating geometric design standards for the various types of airports and the functions which they serve, letter and numerical codes and other descriptors have been adopted to classify airports.

For design purposes, airports are classified based on the aircraft they accommodate. While at any airport, a wide variety of aircraft, from small general aviation piston-engine aircraft to heavy air transport aircraft, will use the airfield, airports are designed based on a series of “critical” or “design” aircraft. These aircraft are selected from the fleet using the airport as those most critical to airfield design. The FAA defines the term critical aircraft as the aircraft most demanding on airport design that operates at least 500 annual itinerant operations at a given airport. In many cases, more than one critical aircraft will be selected at an airport for design purposes. For example, it is often the smallest aircraft that is critical to the orientation of runways, while the largest aircraft determines most of the other dimensional specifications of an airfield.

certain dimensional and performance characteristics of the critical aircraft determine the airport’s airport reference code. The airport reference code is a coding system used to relate the airport design criteria to the operational and physical characteristics of the aircraft intended to operate at the airport. It is based upon the aircraft approach category and the airplane
design group to which the aircraft is assigned. The aircraft approach category, as shown in below Table is determined by the aircraft approach speed, which is defined as 1.3 times the stall speed in the landing configuration of aircraft at maximum certified landing weigh

The airplane design group (ADG) is a grouping of aircraft based upon wingspan or tail height, as shown in below Table . An airplane design group for a particular aircraft is assigned based on the greater (higher Roman numeral) of that associated with the aircraft’s wingspan or tail height.