National Airspace System
Table of Contents
National Airspace System
Figure 1 - classes of airspace within the United States & associated altitudes
The National Airspace System (NAS) is a complex network of airspace managed and regulated by the Federal Aviation Administration (FAA) in the United States. It encompasses various types of airspace, including controlled and uncontrolled airspace, which serve different purposes and have specific regulations governing their use. At the heart of the NAS are Air Route Traffic Control Centers (ARTCCs), also known as en-route centers, which are responsible for managing high-altitude air traffic across large geographic regions. ARTCC controllers monitor and control aircraft flying within their designated sectors, providing services such as traffic separation, routing, and coordination with adjacent centers to ensure the safe and efficient flow of air traffic. Air Traffic Control (ATC) facilities, including control towers at airports and terminal radar approach control (TRACON) facilities, play a crucial role in managing aircraft movements within terminal areas and around airports. ATC controllers are responsible for providing instructions to pilots during taxiing, takeoff, landing, and en-route phases of flight to maintain safe and orderly operations. Additionally, Flight Service Stations (FSS) provide flight planning, weather briefing, and other advisory services to pilots flying within the NAS. They offer valuable assistance to pilots before and during flights, helping them navigate airspace restrictions, weather hazards, and other potential challenges. The NAS includes various classes of airspace, each with specific rules and requirements governing aircraft operations.
Figure 2 - New York Class B
Class C airspace is typically established around airports with a moderate level of commercial and general aviation traffic. It extends from the surface to a specified altitude, typically up to 4,000 feet above ground level (AGL) and a radius of several nautical miles. Within Class C airspace, aircraft are required to establish two-way radio communication with air traffic control (ATC) and obtain clearance before entering. Class C airspace is designed to accommodate a mix of commercial, general aviation, and air carrier operations while providing enhanced safety and separation services from nearby Class B airspace.
Class D airspace is established around smaller airports with control towers and limited commercial activity. It typically extends from the surface to a specified altitude, often up to 2,500 feet above ground level (AGL), and within a radius of several nautical miles. Pilots are required to establish radio communication with the control tower before entering Class D airspace and follow ATC instructions for safe and orderly operations. Class D airspace facilitates the management of air traffic around smaller airports while providing separation services and traffic advisories to pilots operating within its boundaries.
Class A airspace, for example, extends from 18,000 feet above mean sea level (MSL) up to flight level 600 (60,000 feet MSL) and is typically reserved for high-altitude commercial air traffic operating under instrument flight rules (IFR). Class B airspace surrounds the busiest airports and is designed to manage the flow of air traffic in and out of these facilities. It typically extends from the surface up to several thousand feet above ground level and requires aircraft to obtain clearance from ATC before entering. These components and regulations of the NAS work together to ensure the safety and efficiency of air transportation in the United States.
Figure 3 - Burbank Class C
Figure 4 - Centennial Class D
Class E airspace encompasses controlled airspace that is not designated as Class A, B, C, or D airspace. It extends from either the surface or a specified altitude, depending on its location and purpose, up to the base of Class A airspace. Class E airspace is typically established to accommodate instrument flight rules (IFR) operations and provide controlled airspace for en-route traffic. It also includes designated transition areas, such as airspace overlying uncontrolled airports, where pilots must adhere to specific communication and clearance requirements to ensure safe operations.
Figure 5 - Fort Morgan Class E
Class G airspace, also known as uncontrolled airspace, is airspace that is not designated as controlled airspace (Class A, B, C, D, or E). It encompasses the airspace below 14,500 feet above sea level (MSL) within the contiguous United States and varies in size and configuration depending on the terrain and airspace structure. Class G airspace is typically found in rural or remote areas and is not subject to air traffic control services. Pilots operating in Class G airspace are responsible for their own navigation and separation from other aircraft, relying on visual flight rules (VFR) and see-and-avoid principles to maintain safety.
Figure 6 - text view of basic VFR Minimums for Class A, B, C, D, E, and G airspace
Figure 7 - entry requirements for airspace operations
Entry requirements for airspace in the United States National Airspace System (NAS) vary based on the classification of the airspace. These requirements are put in place to ensure safe and efficient air traffic management. In general, pilots must adhere to certain equipment and communication standards to enter specific airspace classes. The different classes of airspace in the NAS, ranging from Class A to G, each have distinct entry requirements and serve different purposes in terms of air traffic control and separation.
For instance, Class A airspace, which typically starts at 18,000 feet above mean sea level (MSL), requires pilots to be operating under Instrument Flight Rules (IFR) and maintain communication and altitude requirements specified by air traffic control (ATC). Additionally, aircraft operating in Class A airspace must be equipped with a Mode C transponder and IFR capabilities, which provides altitude information to ATC. In contrast, Class G airspace, which exists near the surface and is uncontrolled, may have minimal entry requirements, often only necessitating basic visual flight rules (VFR) equipment such as two-way radio communication if operating in certain areas. Equipment requirements vary across airspace classes. For instance, in Class B, C, and D airspace, which are typically found around busy airports, aircraft must be equipped with a Mode C transponder and establish two-way radio communication with the air traffic controller. This equipment allows for effective communication with ATC and provides critical altitude information to ensure safe separation between aircraft. Class B airspace additionally requires a clearance into the airspace from ARTCC with your callsign before you enter the airspace. Entry into Bravo airspace without the proper clearance can result in possible pilot deviations from ATC. Class C and D airspace require the same kind of two-way radio communication involving hearing your callsign from ATC, but do not require a clearance or have any minimum pilot certificate requirements.
Special Use Airspace
Special Use Airspace (SUA) in the United States is designated for specific purposes and has certain flight restrictions. The Federal Aviation Administration (FAA) oversees these areas to ensure safety and efficient use of airspace.
Prohibited Areas are defined airspace within which flight of aircraft is not allowed, usually due to security concerns. An example is P-56, which covers the White House and Capitol in Washington, D.C. Restricted Areas are areas where operations are hazardous to non-participating aircraft and contain airspace within which the flight of aircraft, while not wholly prohibited, is subject to restrictions. Activities within these areas must be confined because of their nature, or limitations may be imposed upon aircraft operations that are not a part of those activities.
National Security Areas (NSAs) are established at locations where there is a requirement for increased security and safety of ground facilities. Pilots are requested to voluntarily avoid flying through these areas. Warning Areas are similar to restricted areas; however, the United States government does not have sole jurisdiction over the airspace. A warning area is airspace of defined dimensions, extending from three nautical miles outward from the coast of the United States, containing activity that may be hazardous to nonparticipating aircraft.
Figure 8 - VFR Sectional depiction of Restricted Area R-5601B in Lawton, Oklahoma, USA
Military Operating Areas (MOAs) are established for the purpose of separating certain military training activities from IFR traffic. Whenever a MOA is being used, nonparticipating IFR traffic may be cleared through a MOA if IFR separation can be provided by ATC. Otherwise, ATC will reroute or restrict nonparticipating IFR traffic. Alert Areas are depicted on aeronautical charts to inform nonparticipating pilots of areas that may contain a high volume of pilot training or an unusual type of aerial activity.
Controlled Firing Areas (CFAs) are areas within which activities are conducted under conditions so controlled as to eliminate hazards to nonparticipating aircraft. The difference between CFAs and other special use airspace is that activities must be suspended when a spotter aircraft, radar, or ground lookout position indicates an aircraft might be approaching the area. The FAA publishes information about SUAs in the United States, which can be found in the FAA’s Aeronautical Information Manual (AIM), Sectional Aeronautical Charts, and the Chart Supplement U.S. (formerly the Airport/Facility Directory). These publications provide details about the location, vertical and lateral boundaries, and the times of operation of each SUA. They also provide information about the controlling agency and the chart on which the SUA is depicted.
Figure 9 - Sheppard 1 MOA & Restricted Areas R5601 on IFR Low En-Route Chart
Temporary Flight Restrictions
Temporary Flight Restrictions (TFRs) are defined areas of airspace where air travel is limited because of temporary hazardous conditions. These restrictions are coordinated between the Federal Aviation Administration (FAA) and the organization overseeing the event or situation that necessitates the TFR. The FAA issues a Notice to Airmen (NOTAM) to inform pilots of the TFR. TFRs can be issued for a variety of reasons. One common reason is for Presidential and VIP movement. When the President or other significant dignitaries travel, a TFR is typically issued around the area they are visiting for security reasons. This restricts all non-essential aircraft from entering the airspace for the duration of the visit.
Another common reason for TFRs is major sporting events or concerts. The FAA often issues TFRs over stadiums during major sporting events like the Super Bowl or World Series, or during large concerts. These restrictions are in place to ensure the safety of the people attending the event, and to prevent distractions or disruptions caused by low-flying aircraft. TFRs can also be issued for military activities such as live-fire exercises or missile tests. These are often referred to as “safe firing zones”. During these times, the airspace in the vicinity of the military activity is restricted to ensure the safety of civilian aircraft. The size and duration of these TFRs can vary depending on the nature of the military activity. It’s important for pilots to check for NOTAMs regularly to stay informed about any TFRs along their planned flight path.
Figure 10 - sample security TFR valid for 21 days
Clearance Procedures
Air Traffic Control (ATC) clearance procedures play a pivotal role in maintaining the safe and orderly flow of air traffic. These procedures are particularly crucial when transitioning aircraft through various types of airspace, including Class A, B, C, and D, under Instrument Flight Rules (IFR) or with Visual Flight Rules (VFR) flight following.
Under IFR, pilots are required to obtain ATC clearance before entering Class A airspace, which extends from 18,000 feet Mean Sea Level (MSL) up to and including Flight Level (FL) 600. For instance, a pilot might receive a clearance such as “Cleared for the John F Kennedy Localizer Runway 13 left approach via radar vectors.” Similarly, ATC clearance is mandatory for all aircraft intending to enter Class B airspace, which is designed to improve aviation safety by reducing the risk of midair collisions in the airspace surrounding airports with high-density traffic. A typical clearance might sound like “NWIK1, Cleared into the Bravo airspace, maintain 7,000, squawk 3248.”
When operating under VFR flight following, pilots communicate with ATC and receive traffic advisories, but they are responsible for their own navigation and separation from other aircraft. In Class C airspace, which generally extends from the surface to 4,000 feet above the airport elevation, communication with ATC and Mode C transponder capability is required for all aircraft. A typical takeoff clearance from a control tower might be, “Cleared for takeoff Runway 18, fly runway heading, climb and maintain 5,000.” In Class D airspace, which generally extends from the surface to 2,500 feet above the airport elevation, two-way communication with ATC is also required for all aircraft.
Figure 11 - all aircraft operating in Class A airspace are under an IFR flight plan with ATC and receive clearance to their assigned altitude block above FL180
VFR and IFR Procedures in Controlled Airspace
Figure 12 - pilots operating under VFR have the option to file a VFR flight plan with FSS before the flight or pick up VFR Flight Following, controller workload permitting
During Visual Flight Rules (VFR) en-route cross-country flights, pilots can take advantage of several services to enhance safety and efficiency. One such service is VFR Flight Following, a radar service provided by Air Traffic Control (ATC). This service is not mandatory but is highly recommended for cross-country flights. It provides traffic advisories and other useful information to the pilot. To request flight following, a pilot contacts the appropriate ATC facility, provides their aircraft type, location, altitude, and intended route of flight. ATC then provides radar traffic advisories and alerts the pilot to potential traffic conflicts during their flight.
When on the ground of an uncontrolled airfield, a necessary service is the use of Advisory Frequencies. These frequencies, often referred to as UNICOM or CTAF (Common Traffic Advisory Frequency), are used at airports without control towers or when the tower is not in operation. Pilots use these frequencies to announce their intentions and position to other pilots in the vicinity. This self-announce procedure helps maintain situational awareness and promotes safe and orderly operations within the airport environment. In addition to these, pilots can also use services like Automated Weather Observing Systems (AWOS), Automated Surface Observing Systems (ASOS), and Flight Service Stations (FSS). AWOS and ASOS provide real-time weather information, which is crucial for VFR flights. FSS provides weather briefings, enroute communications, and assists in search and rescue operations. Utilizing these services effectively can greatly enhance the safety and efficiency of VFR cross-country flights.
Figure 13 - flights operating under IFR must receive and IFR clearance from ATC, which may clear them through multiple classes of airspace
Instrument Flight Rules (IFR) procedures are used by pilots to fly an aircraft by relying primarily on instruments, rather than visual references. These procedures are critical when flying in various types of airspace, especially in conditions of reduced visibility or when precision is required for navigation and separation from other aircraft. In Class A airspace, which extends from 18,000 feet Mean Sea Level (MSL) up to and including Flight Level (FL) 600, all operations must be conducted under IFR. Pilots must receive an ATC clearance before entering this airspace. The clearance will typically include the route of flight, altitude, and speed. For example, a pilot might receive a clearance such as “Cleared to the John F Kennedy airport via J70, climb and maintain FL230, expect FL350 ten minutes after departure.” In Class B, C, and D airspace, IFR procedures may also apply. Pilots must establish two-way communication with ATC and receive an explicit clearance before entering. This includes providing traffic advisories, vectoring aircraft for sequencing and separation, and facilitating transitions between different airspace classes. It’s important to note that while ATC provides these services, the pilot is still responsible for the safe operation of the aircraft and must comply with all ATC instructions and clearances.