Calculating Airport Airspace Dimensions: A Comprehensive Guide

Airspace dimensions are classified by the Federal Aviation Administration (FAA) using letters, with each class serving a specific need, function, or level of control. The two main categories of airspace are regulatory and non-regulatory, and within these, there are controlled and uncontrolled airspaces. Controlled airspace consists of five tiers of restriction, from most restrictive to least restrictive: Class Alpha (A), Bravo (B), Charlie (C), Delta (D), and Echo (E). Uncontrolled airspace is known as Class Golf (G). Each class has its own weather minimums and rules regarding visibility and cloud clearance. For example, Class A airspace is for Instrument Flight Rules (IFR) only and has no minimum visibility requirements, while Class B airspace has a minimum visibility requirement of 3 statute miles.

Class A airspace

In the United States, airspace is categorized as regulatory and non-regulatory. Regulatory airspace includes controlled airspace (Class A, B, C, D, and E) and special use airspace, while non-regulatory airspace includes uncontrolled airspace (Class G) and other airspace.

Class B airspace

The criteria for considering an airport as a candidate for Class B airspace designation are based on the volume of aircraft at the airport and an assessment of the mid-air collision risk in the terminal area. The Federal Aviation Administration (FAA) has designed each Class B airspace to ensure traffic flows in and out of the airport safely.

The airspace typically centres around one major airport, the primary airport, and is defined by a series of circular "shelves" of airspace, each larger than the one beneath it. The exact shape varies from one Class B area to another, but it often resembles an inverted wedding cake. The airspace normally begins at the surface in the immediate area of the airport, with successive shelves of greater and greater radius beginning at higher and higher altitudes at greater distances from the airport. The upper limit of Class B airspace is usually 10,000 feet MSL, but there are exceptions. For example, due to its high elevation, Denver's Class B airspace extends up to 12,000 feet MSL to give airliners room to descend.

The boundaries of Class B airspace are depicted on charts with thick, solid blue lines. The lateral boundaries may be defined using latitude/longitude points, Fix/Radial/Distance references, NAVAIDs, alignment to coincide with prominent landmarks or terrain features, etc. The outer limits of the airspace should not extend beyond 30 NM from the primary airport.

The lowest part of the airspace is the area surrounding the airport, which goes from the ground up to the top of the Class B airspace and extends 10 NM laterally. The next shelf extends to around 20 NM from the airport, with a floor that is around 3,000 feet above airport elevation. The final shelf extends no more than 30 NM from the airport, with a floor between 5,000 and 6,000 feet above airport altitude.

To enter Class B airspace, a pilot must obtain specific clearance from Air Traffic Control (ATC) and use a unique transponder squawk code. ATC is required to provide separation within the airspace, although the pilot is ultimately responsible for collision avoidance.

VFR traffic needs explicit clearance to enter Class B airspace. Two-way radio contact with the approach controller is not enough. If flying under IFR, no explicit Bravo clearance is needed, as the IFR clearance authorises the pilot to fly their planned route.

All aircraft inside the Mode C Veil, a 30-nautical mile boundary around a Class B airport, must have an operating Mode C transponder and ADS-B Out. This allows controllers to monitor nearby traffic, even if they are not in communication with them.

Class C airspace

The shape of Class C airspace is often referred to as an upside-down wedding cake, with two primary parts: an inner core and an outer ring. The inner core extends within 5 nautical miles (nm) of the main airport from the surface to 4,000 feet above airport elevation (AAE). The outer ring stretches within a 10 nm radius, ranging from 1,200 to 4,000 feet AAE.

The Federal Aviation Administration (FAA) has set rules requiring pilots to have certain qualifications and equipment to enter Class C airspace and operate within it. While there is no specific certification required, student pilots can fly in Class C airspace only after receiving training and an endorsement from their Certified Flight Instructor (CFI).

To enter Class C airspace, pilots must establish two-way radio communication with the relevant ATC facility before entering the airspace boundaries. They must also obtain clearance from Air Traffic Control (ATC) before entering.

Aircraft need to have a two-way radio, transponder with Mode C, and Automatic Dependent Surveillance-Broadcast (ADS-B) Out equipment.

Pilots can fly VFR in Class C airspace under the following minimum weather conditions: 3 statute miles of visibility, 1,000 feet clearance above clouds, 500 feet clearance below clouds, and 2,000 feet horizontal clearance from clouds.

While in Class C airspace, pilots must abide by certain restrictions, including maintaining two-way radio communications, keeping their Mode-C transponder on, following the direction of ATC, and adhering to speed restrictions.

Class D airspace

The size of a Class D area is determined by ensuring it is sufficient to allow for the safe and efficient handling of operations and to contain specific types of operations. The standard radius of a Class D area is 4-5 nautical miles, but this can vary depending on the location. The vertical boundary is generally from the surface up to and including 2,500 feet above ground level (AGL).

The Class D airspace boundary is typically based on the airport reference point (ARP) or the geographic position (GP) of the primary airport. The ARP/GP is the centre of the airport expressed in coordinates and is incorporated into the surface area's legal description.

Weather observations and reporting services are provided to pilots, and maintaining orderly traffic flow requires pilots to observe certain restrictions when operating within Class D airspace.

To enter Class D airspace, pilots must establish two-way radio communication with the ATC facility providing ATC services before crossing the boundary. This involves contacting ATC and providing information such as their destination and intentions. Once the pilot hears their callsign in response, they have established two-way communication and can enter the airspace.

The speed limit within Class D airspace is 200 knots for aircraft within 4 nautical miles of the primary airport.

Class E airspace

Vertically, Class E airspace ranges from 700 or 1,200 feet up to 18,000 feet Mean Sea Level (MSL). At 18,000 feet, the airspace transitions into Class A. Class E also includes high-altitude airspace above 60,000 feet MSL. Laterally, Class E airspace is not standardised in shape or size. It covers large areas of most of the United States, and outside large metropolitan areas, it is the only controlled airspace present.

• Below 10,000 feet MSL: 3 Statute Miles (SM) visibility, 1000 feet above clouds, 500 feet below clouds, and 2000 feet horizontal from clouds.
• At or above 10,000 feet MSL: 1000 feet below clouds, 1000 feet above clouds, and 1 SM horizontal from clouds.

Frequently asked questions