It may come as a surprise to you but not all airspace in the United States is controlled directly by the FAA at certain altitudes. There are many different classes of users of the national airspace system such as hot air balloons, gliders, ultra-lights, small piston-engine aircraft, multi-engine turboprops, business jets, heavy wide-body commercial airliners, UAVs, and even high-speed military fighters. It is the FAA's job to ensure that all these aircraft can safely coexist in the crowded airspace above our heads. In order to achieve this goal, the FAA segregates airspace users based primarily on the type of flight instrument flight rules (IFR) vs. visual flight rules (VFR), with the primary goal of restricting access to the most congested and busiest airspace around major hub airports as shown in the FAA graphic below.
Did you know a jet flying at 250 knots, travels 422 feet every second, and a mile every 12.5 seconds? Now consider that at any one time there are hundreds of planes in the skies over Northern California (NorCal) airspace and you get a sense of why traffic segregation is so critical to ensuring the safety of the airspace. Mixing high-speed IFR traffic with low-speed VFR traffic is something that controllers try to avoid whenever possible and the airspace classifications help to ensure this.
At cruise altitude where jets travel an average of 450 knots that translates to 760 feet per second and a mile every 6.9 seconds! That means that two jets flying on a converging course towards each other at 450 knots have a closure rate of 900 knots or 1,520 feet per second. That is a mile every 3.5 seconds.
Studies have shown that it takes a pilot at least 10 seconds to spot conflicting traffic, recognize it, and take action to avoid it. Given the closure rate mentioned above, that means in 10 seconds the two airplanes would have closed the distance between each other by almost 3 miles.
While modern traffic collision avoidance systems (TCAS) have greatly reduced the threat of a midair collision. This equipment is expensive and usually found only on commercial airliners and business jets and not small piston-engined general aviation planes. So segregation of high-speed IFR and low-speed VFR traffic remains critical, especially as traffic volumes at airports continue to increase.
Classes of Airspace
In the United States, the national airspace (NAS) is regulated and controlled by the FAA. The three-dimensional airspace is segregated into six classifications: A, B, C, D, E, and G. Each classification is governed by specific rules that restrict which type of traffic can operate in that particular airspace class. There are also special use airspaces that designate airspace that has restricted access for security reasons and or completely prohibits entry by any operator.
Generally speaking, Class B and C airspace when viewed from a 3-dimensional perspective appears like an upside-down wedding cake with at least three and sometimes four tiers. As shown in the image below the tiers increase in radius around the airport as the altitude increases. Individual tiers are usually several thousand feet in altitude and the upper limit of Class B and C airspace is normally 10,000 feet MSL. The shape and thickness of the tiers vary based on local geography, traffic patterns, and other features surrounding the individual airport.
CLASS A AIRSPACE
Class A airspace extends from 18,000 feet (5,500 m) MSL to 60,000 feet (FL600) throughout the contiguous United States and Alaska. Altitudes in Class A airspace are pressure altitudes referenced to a standardized altimeter setting of 29.92" Hg. and thus the true altitude depends on the local atmospheric pressure. VFR flights are not authorized in Class A airspace and all aircraft must be under positive control by ATC, flying on an instrument flight plan (IFR), and receive clearance prior to entering the airspace.
The only exception to IFR-only flight is the temporary authorization by the FAA of "Wave Windows". These blocks of airspace are occasionally opened up to allow gliders to take advantage of strong mountain waves which can carry them to altitudes in excess of 25,000 feet and in some rare occasions as high as 50,000 feet. The world altitude record for an unpowered glider is 76,100 feet which was set over the Andes Mountains in southern Argentina in 2018. One popular soaring area near the bay area frequented by gliders is located in Williams, California, northwest of Sacramento.
CLASS B AIRSPACE
Class B airspace surrounds the busiest airports in the United States. The criteria to establish Class B airspace is based on the number of IFR operations and total passenger traffic at the airport. There are currently 39 Class B airports in the U.S. There is only one Class B airport in the Bay Area, San Francisco International (KSFO).
All aircraft entering Class B airspace must obtain prior ATC clearance, must be equipped with a two-way radio for communication with ATC, and have an operating Mode C transponder and automatic altitude reporting equipment. This airspace class has the most stringent regulations of the six main classes and this is to limit the transit of VFR aircraft through this heavily trafficked IFR airspace around the busiest airports in the U.S.
These requirements are designed to exclude the majority of VFR flights to ensure adequate separation between low-speed piston-engined VFR traffic and high-speed IFR jet traffic. Most VFR flights will be instructed to remain clear of Class B airspace and will not receive clearance to enter or transit the airspace especially during high traffic volume periods at the airport. Class B airspace is indicated on aeronautical charts by a thick blue line as shown in the image below.
CLASS C AIRSPACE
Class C Airspace exists around airports with regular commercial passenger jet service of 100 passengers per flight or more. The FAA requirements for Class C airspace are an airport with an operational control tower, a radar-controlled approach system, and a minimum number of IFR approaches conducted per year. Airports that don't have a tower staffed 24-hours a day are only Class C airspace when approach control is operational. Outside of these times, the airspace reverts to Class D and when the tower is not operational it reverts to controlled Class E or uncontrolled Class G airspace.
Class C airspace varies from airport to airport based on local factors, but generally includes a 5 nautical mile cylinder that extends from the surface up to 4,000 feet AGL, and a 10 nautical mile radius cylinder that extends no lower than 1,200 feet up to 4,000 feet AGL. Clearance from ATC is not explicitly required to enter Class C airspace but controllers may instruct aircraft to remain outside the airspace upon initial contact. Class C airspace is indicated on aeronautical charts by a thick magenta line as shown in the image below.
Legal VFR flight inside Class C airspace requires the following minimum conditions: Three miles of visibility, at least 500 feet below, 1,000 feet above, and 2,000 feet lateral separation from clouds. Airplanes in Class C airspace must fly below 200 knots when at or below 2,500 feet above the ground and within four nautical miles of the airport. There are currently 120 Class C airports in the U.S. In the San Francisco Bay Area, there are two Class C airports, Oakland and San Jose.
CLASS D AIRSPACE
Class D Airspace exists around airports with a functioning control tower, that see very limited IFR approaches, usually a result of not having commercial airline service. This airspace is usually cylindrical in shape and extends from the surface up to 2,500 feet AGL. Class D airspace reverts to Class E or G during hours when the tower is not operational. Pilots must establish two-way communication with ATC before entering Class D airspace, but a transponder is not required to operate in the airspace. Minimum VFR flight rules are the same as those established for Class C airspace. Class D airspace is indicated on aeronautical charts by a dashed blue line as shown in the image below.
CLASS E AIRSPACE
Class E Airspace extends from the ground or 700 feet AGL in certain areas up to 18,000 feet, which is the boundary of Class A Airspace. All airspace above 60,000 feet is also considered Class E Airspace. Controlled airspace which is not covered by Class A, B, C, or D is considered Class E. The majority of controlled airspace in the United States is Class E. This airspace is used by aircraft to transition between the terminal and en-route environments (around non-towered airports).
No ATC clearance or radio communication is required for VFR flights in this airspace. VFR minimums are the same for Class E as Class C and D up to 10,000 feet MSL. Above 10,000 feet the minimum requirements increase to five miles visibility, 1,000 feet below, 1,000 feet above, and one-mile lateral separation from clouds. Class E Airspace is indicated on aeronautical charts in one of three ways.
- From the surface as a thin dashed magenta line
- From 700 feet AGL as a shaded magenta line
- From 1,200 feet AGL as a shaded cyan line
CLASS G AIRSPACE
Class G Airspace is classified as all uncontrolled airspace below 14,500 feet MSL. There are no requirements for aircraft to transit Class G airspace, even for IFR flight operations. Class G airspace except in mountainous areas usually doesn't extend above 1,200 feet AGL and is beneath Class E airspace and between Class B-D cylinders around towered airports. No radio communication is required in Class G airspace as it is completely uncontrolled by ATC.
Special Use Airspace
Special use airspace or special area of operation (SAO) is the designation the FAA uses for airspace which has restricted access or contains activities such as high-speed military training which must be confined to a specific area. Most special use airspaces are depicted on instrument charts. The depiction of the area on the map includes the area name or number, effective altitude, time, and weather conditions of operation as well as the controlling agency.
Special use airspaces may be any of the following:
- Prohibited Areas
- Restricted Areas
- Warning Areas
- Military Operation Areas (MOAs)
- Alert Areas
- Controlled Firing Areas (CFAs)
Prohibited Areas contain airspace of defined dimensions within which the flight of all aircraft is prohibited. These areas are usually established for national security reasons. Prohibited areas are designated on aeronautical charts as "P" followed by a number such as P-56 which includes the National Mall, U.S. Capitol Building, and the White House in Washington D.C. Washington Reagan National Airport as shown in the graphic below is located just south of this prohibited area, and as such departing and arriving aircraft are required to avoid this area which involves the use of specially developed procedures by the FAA. Violating prohibited areas either accidentally or purposefully can result in the loss of the aircraft operator's FAA license, jail time and monetary fines.
Restricted Areas are defined as areas where operations are hazardous to nonparticipating aircraft and contain airspace with which the flight of aircraft, while not wholly prohibited, is subject to restrictions. Restricted areas denote the existence of unusual, often invisible, hazards to aircraft such as artillery firing, aerial gunnery, or guided missiles. IFR flights may be granted access by the FAA to transit the airspace, however unauthorized penetration of restricted areas can be extremely hazardous to an aircraft and its occupants. Unlike Prohibited Areas in which no entry is permitted, Restricted Areas when not in use can be released to the FAA to allow civilian aircraft to use it. Under these conditions, aircraft don't have to have specific FAA clearance to utilize the airspace. Restricted Areas are shown on aeronautical charts with an "R" followed by a number as shown in the image below. Restricted Area R-4401 A, B & C is just southeast of Hattiesburg, Mississippi.
Warning Areas are similar to restricted areas; with one exception, the United States government does not have sole jurisdiction over the airspace. A Warning Area is an airspace of defined dimensions, extending from 3 nautical miles outward from the coast of the United States, containing activity that may be hazardous to nonparticipating aircraft. A Warning Area is designed to alert pilots to a potential hazard if they enter the area. A Warning Area may be located over domestic or international waters or both. The airspace is designated with a "W" followed by a number such as W-237A as shown in the image below, which is a location of the coast of Washington State.
MILITARY OPERATION AREAS (MOAs)
Military Operation Areas are airspace with defined vertical and lateral limits established for the purpose of military training exercises. Its purpose is to separate this special military traffic from other IFR traffic. When a MOA is in use, nonparticipating IFR traffic can still be cleared by the FAA through the airspace provided that IFR separation can be safely maintained by ATC. If not then ATC will reroute or restrict nonparticipating aircraft from entering the area. MOAs are depicted on sectional charts by name but are not numbered. The Fallon South 2 MOA shown in the image below, is located southeast of Fallon Naval Air Station in Nevada. Fallon NAS is home to the Navy's Fighter Weapons School better known as "Top Gun."
Alert Areas are areas that may contain a high volume of pilot training or another type of unusual aerial activity. Pilots should exercise caution in alert areas. All flight activity within the alert area is conducted in accordance with regulations, without a waiver, and pilots of both participating and transiting aircraft have shared responsibility for separation and collision avoidance. Alert Areas are indicated on Section Charts with an "A" followed by a number as shown in the chart below. Alert Area A-632C is an area used by the Navy for primary jet fighter training just west of Corpus Christi, TX.
CONTROLLED FIRING AREAS (CFAs)
Controlled Firing Areas contain activities involving the live use of ammunition that if not conducted in a controlled environment, could be hazardous to nonparticipating aircraft. The main difference between CFAs and other special use airspaces like MOAs or Restricted Areas is that access is not controlled or restricted and all activities must be suspended when a spotter aircraft, radar, or ground observer indicates a nonparticipating aircraft is approaching the area. Since access to these areas is not restricted there is not a need to chart them and pilots are free to transit the area at will without obtaining a special clearance.
Other Airspace Areas
Other Airspace Areas is a general term that incorporates the majority of the remaining airspace including:
- Local Airport Advisory (LAA)
- Military Training Route (MTR)
- Temporary Flight Restriction (TFR)
- Parachute Jump Aircraft Operations
- Published VFR routes
- Terminal Radar Service Area (TRSA)
- National Security Area (NSA)
- Air Defense Identification Zones (ADIZ)
LOCAL AIRPORT ADVISORY (LAA)
Local Airport Advisory is an advisory service provided by Flight Service Station (FSS) facilities, which are located on the landing airport, using a discrete ground-to-air frequency or the tower frequency when the tower is closed. The services provided by LAA include local airport advisories, automated weather reporting, and continuous Automated Surface Observing System (ASOS)/Automated Weather Observing Station (AWOS) data display, or manual observations.
MILITARY TRAINING ROUTES (MTRs)
MTRs are routes utilized by military aircraft to maintain proficiency in tactical flying maneuvers. These routes are usually below 10,000 feet MSL and are designated for operational speeds over 250 knots, which is the maximum legal airspeed for flight operations from the surface up to 10,000 feet. With rare exceptions, flight speeds on MTRs are usually limited to 420 knots. While operating on the MTR military aircraft squawk a Mode C Transponder code of '4000', which informs controllers that they are 'speeding' on a route. Only military aircraft are allowed to squawk 4000 and exceed 250 knots below 10,000 feet MSL. On aeronautical charts, these routes are identified as IFR (IR), and VFR (VR), followed by a number. MTRs with all segments below 1,500 above ground level (AGL) are identified by four numbers, while routes that include at least one segment above 1,500 AGL are identified by three number characters. Military Training Route VR1176 is a low altitude high-speed training route through the Sangre de Cristo Mountain Range north of Taos, New Mexico.
TEMPORARY FLIGHT RESTRICTIONS (TFR)
A Temporary Flight Restriction is a geographically-limited, short-term, airspace restriction put in place by the FAA for the purposes of excluding all traffic. TFRs are used to restrict aircraft for security reasons around major sporting events (NFL, MLB, NCAA football games), natural disaster areas, air shows, space launches, and Presidential movements. Prior to 9-11 most TFRs were small in area and placed limited restrictions on aircraft operations. Following 9-11 TFRs have routinely been imposed that restrict airspace for a 30 nautical mile radius around the President, with a 10 nautical mile radius "no-fly zone" for non-scheduled flights. The FAA's Office of System Operations Security is responsible for the screening of these requests and has the ultimate authority to authorize or deny the request. TFR's are published in real-time on the FAA website and can also be obtained from Flight Service Station Notice to Airman (NOTAM). The image below depicts a TFR in place around Dodger Stadium for the Padres-Dodger game which is active from the surface up to 3,000 feet beginning one hour prior to the game and extending until one hour after the game is over on 08/12/2020.
PARACHUTE JUMP AIRCRAFT OPERATIONS
Parachute Jump Aircraft Operations are areas that are used frequently for skydiving by civilian or military operators. Aircraft operators should use caution in these areas and avoid them if possible due to the danger of collision with the aircraft or skydivers. These areas are indicated on sectional charts with a parachute icon as shown in the image below which displays a popular area for skydiving just southeast of Portland, Oregon.
PUBLISHED VFR ROUTES
Published VFR Routes are used by aircraft operating under visual flight rules (VFR) to transition around, under, or through complex airspace and avoid heavily trafficked routes in Class B airspace. These routes are generally found on VFR terminal area planning charts.
TERMINAL RADAR SERVICE AREAS (TRSAs)
Terminal Radar Service Areas are areas where participating VFR pilots can receive additional radar services. While participation is voluntary for aircraft operating under VFR, pilots of these aircraft are encouraged to contact radar approach control and use the TRSA services as it helps ensure separation between all IFR and participating VFR aircraft. The primary airport(s) within the TRSA becomes Class D airspace. In the case of the example shown below this would be Green Bay-Austin Straubel International Airport outside Green Bay, Wisconsin.
NATIONAL SECURITY AREAS (NSAs)
National Security Areas are airspaces with defined lateral and vertical dimensions established at locations where there is a need for increased security and safety of ground infrastructure. While flight through these areas is not prohibited pilots are strongly advised to stay clear of NSAs or obtain prior authorization to transit them in order to reassure the controlling agency that the aircraft is not a threat to national security. NSAs are a compromise between normal and restricted or prohibited airspace and chan be temporarily converted into restricted airspace by NOTAMs. These areas are depicted on sectional charts with a heavy dashed magenta border and a special notation.
AIR DEFENSE IDENTIFICATION ZONES (ADIZ)
Air Defense Identification Zones are airspace over land or water in which the identification, location, and control of civil aircraft are performed in the interest of national security. The purpose of these areas is to provide advance warning and tracking of hostile aircraft that attempt to enter into U.S. airspace without authorization. The United States was the first country in the world to establish an ADIZ when on December 27, 1950, President Truman established one as a precaution during the Korean War. At present 20 countries have established ADIZ areas.
The United States and Canada jointly operate the North American ADIZ that covers the airspace surrounding the United States and Canada. The U.S. also maintains ADIZ areas in Alaska, Hawaii, and Guam. The ADIZ is jointly administered by the FAA and Transport Canada and the militaries of both countries all under the coordination of the North American Aerospace Defense Command (NORAD). The FAA handles requests from international aircraft and Transport Canada handles Canadian requests. Any aircraft that enters the North American ADIZ without prior authorization will be identified as a threat and treated as a hostile aircraft, and potentially be intercepted by fighter aircraft. To legally enter the ADIZ an aircraft is required to radio its intended course, destination, and trip details to ATC and must be equipped with a radar transponder to enable it to be tracked. All aircraft flying under IFR already meets the conditions as they are following an instrument flight plan under positive guidance by ATC.