Air Traffic Control System Structure PDF

Air Traffic Control System Structure / 163 In each class of airspace, both VFR and IFR pilots must comply with the regulations that have been previously mentioned as well as supplemental rules that may apply to flight operations in their specific airspace....

Air Traffic Control System Structure / 163 In each class of airspace, both VFR and IFR pilots must comply with the regulations that have been previously mentioned as well as supplemental rules that may apply to flight operations in their specific airspace. In general, Class A is most restrictive, whereas Class G is least. Class A Class A airspace is generally defined as the airspace extending from 18,000 Airspace feet MSL up to and including FL 600, including the airspace overlying the waters within 12 nautical miles off the coast of the forty-eight contiguous states and Alaska as well as the designated international airspace beyond 12 nautical miles off the coast of the forty-eight contiguous states and Alaska within areas of domestic radio navigational signal or ATC radar coverage and within which domestic procedures are applied. Class A airspace is not specifically charted. Class A airspace evolved from the jet advisory areas that were created in the 1960s to provide advisory services to civilian and military turbojet aircraft operating at high altitudes. When the jet advisory areas were first cre- ated, they extended from FL 240 to FL 410 and projected 14 nautical miles laterally on either side of every jet route. It was believed that pilots would be unable to “see and avoid” any other VFR or IFR aircraft operating at the same altitude at the high airspeeds at which these aircraft routinely operated. Within jet advisory areas, air traffic controllers were required to use radar to constantly monitor every IFR aircraft operating on a jet route and issue any heading changes (known as vectors) necessary to ensure that the IFR aircraft remained separated from any other aircraft observed on the controller’s radar display. The controllers were not usually in radio contact with the VFR aircraft, so it was impossible to determine their altitude, route of flight, or intentions. Because the actions of these aircraft could not be predicted, the controllers were forced to issue numerous unnecessary vectors to IFR aircraft to ensure that they would remain safely separated. Although this procedure might seem to decrease the probability of midair collisions, in many cases it actually made the situation more dangerous. Since the intentions of the VFR pilots were unknown, it was possible that heading changes could be issued to the IFR pilot at precisely the same moment that the VFR pilot began to maneuver to avoid the collision. This might create a situation even more dangerous than if no heading change had been issued at all. It was soon obvious that unless the controller could be in direct radio contact with every aircraft operating in the vicinity of the jet routes, it would be impossible to positively separate IFR from VFR aircraft. In an attempt to rectify this problem, the FAA has since classified all airspace between 18,000’ and 60,000” MSL as Class A airspace (see Figure 3–9). FAR 91.135 requires that every aircraft operating within Class A airspace operate under instrument flight rules and receive a clearance from ATC. This ATC separation of all aircraft is known as positive control. To operate within Class A airspace, pilots must comply with the following regulations. 164 / CHAPTER 3 Figure 3–9. Class A airspace.  Air Traffic Control System Structure / 165 FL 600 18,000 MSL CLASS A 14,500 AGL CLASS E CLASS B CLASS C CLASS D Nontowered 1,200 AGL Airport 700 AGL CLASS G CLASS G CLASS G Figure 3–10. U.S. airspace classifications. The pilot must be rated for instrument flight. The aircraft must be operated under instrument flight rules at a route and at an altitude assigned by ATC. All aircraft must be transponder equipped as specified in FAR 91.215. The creation of this airspace ensured that every aircraft operating at or above 18,000 feet MSL was provided separation services by air traffic control- lers. Since the creation of Class A airspace, high-altitude midair collisions have become extremely rare in this country. Figure 3–10 summarizes all the airspace classifications over the United States. Class B Even though the establishment of Class A airspace virtually eliminated high- Airspace altitude midair collisions, as traffic increased around airports, low-altitude col- lisions began to occur with increasing frequency. The FAA responded by creating a low-altitude version of Class A airspace called a terminal control area (TCA), which has since been reclassified as Class B airspace. Class B air- space is defined as the airspace that extends from the surface of the Earth up to 10,000 feet MSL surrounding the nation’s busiest airports in terms of IFR operations or passenger enplanements. The configuration of each Class B airspace area is individually tailored and consists of a surface area and two or more layers (some Class B airspace areas resemble upside-down wedding cakes) and is designed to contain all published instrument procedures once an aircraft enters the airspace (see Figure 3–11). An ATC clearance is required for all aircraft to operate in the area, and all aircraft that are cleared receive separation services within the airspace. Each successive layer of Class B airspace extends out from the central airport, with the floor of each layer raised to a slightly higher altitude. This 166 / CHAPTER 3 Figure 3–11. Graphic view of Class B airspace and the same airspace as depicted on a sectional chart.  Air Traffic Control System Structure / 167 design provides the controller with sufficient airspace to vector aircraft to an instrument approach at the primary airport. The separation procedures applied to aircraft operating within Class B airspace are similar to those applied to aircraft operating in Class A airspace. Prior to entering this airspace, both IFR and VFR pilots are required by FAR 91.131 to receive a clearance from the controlling ATC facility. While operating within the confines of Class B airspace, every pilot is required, if at all possible, to comply with the instructions issued by the controller. Air traffic controllers are responsible for the positive separation of every aircraft within Class B air- space, whether operating under instrument or visual flight rules. This generally means that aircraft operating at the same altitude must be kept at least 3 nauti- cal miles apart. This separation need not be applied if there is at least 1,000 feet of altitude between the aircraft. If either of the aircraft is VFR, the separation can usually be reduced to 1½ miles lateral or 500 feet vertical separation. If both aircraft are VFR or if one is VFR and the other is a small IFR, either 500 feet of vertical separation must be used, or the controller must ensure that the radar targets do not touch. This is known as target resolution. While operating within or, in some cases, near Class B airspace, every pilot must comply with the following FAR 91 regulations: Every aircraft must be equipped with appropriate communication and navigation radio equipment. This includes a two-way radio transceiver, VOR or TACAN navigation capability, and a transponder. (A transponder permits the controller to positively identify any particular aircraft when using radar for ATC separation. Transponders are discussed in detail in Chapter 8.) Aircraft may not operate within the airspace underlying Class B airspace at an indicated airspeed greater than 200 knots. Unless specifically authorized by the controller, every turbine-powered aircraft operating to or from the primary airport must operate above the floor while within the lateral confines of the Class B airspace. Every aircraft entering Class B airspace or operating within 30 nautical miles of the primary airport must be equipped with a mode C altitude encoder. This device permits the aircraft’s altitude to be shown directly on the controller’s radar display. Pilots operating on IFR flight plans do not need to specifically request per- mission to enter Class B airspace. VFR pilots, however, must request permission from the ATC facility prior to entering the airspace. Until permission is received from the controller, the VFR pilot is required to remain clear of Class B airspace. IFR aircraft operating within Class B airspace have priority over VFR aircraft. Air traffic controllers are permitted to deny VFR aircraft clearances if conditions are such that, in the opinion of the controller, the entry of the VFR aircraft might compromise safety. These conditions include, but are not limited to, weather, traffic conditions, controller workload, and equipment limitations. However, if the controller concludes that VFR operations can be safely approved, the pilot may be issued a VFR clearance to enter. Upon receiving the clearance, 168 / CHAPTER 3 and after entering, the VFR pilot is required to comply with any instruction issued by the controller but must also observe the basic VFR flight rules. At no time may the VFR pilot disregard VFR flight rules while attempting to comply with a controller’s request. If the pilot believes that the controller’s instructions might cause a violation of any VFR flight rule, the pilot is autho- rized by FARs 91.3 and 91.123 to disregard that instruction but must inform the controller as soon as feasible. The following terminal areas around the country are currently designated by FAR 71 as Class B airspace: Atlanta, GA Baltimore, MD-Washington, D.C. area Washington Dulles International Airport Washington National Ronald Reagan Airport Baltimore/Washington International Airport Boston, MA Charlotte, NC Chicago O’Hare, IL Cincinnati, OH-(Covington, KY) Cleveland, OH Dallas, TX Dallas/Fort Worth International Airport Dallas Love Field Airport Denver, CO Detroit, MI George Bush Intercontinental/Houston Airport Honolulu, HI Houston, TX John F Kennedy International Airport Kansas City, MO LaGuardia Airport Las Vegas, NV Los Angeles, CA Memphis, TN Miami, FL Minneapolis, MN New Orleans, LA New York, NY-Newark, NJ area Newark Liberty International Airport Orlando, FL Philadelphia, PA  Air Traffic Control System Structure / 169 Phoenix, AZ Pittsburgh, PA Saint Louis, MO Salt Lake City, UT San Diego, CA San Francisco, CA Seattle, WA Tampa, FL Washington Dulles International Airport Washington National Ronald Reagen Airport William P. Hobby Airport Class C Class C airspace was initially implemented in 1984 as airport radar service Airspace areas (ARSAs) to provide separation to aircraft flying within the vicinity of medium-sized airports that did not qualify for a TCA. After the airspace reclas- sification project, ARSAs became Class C airspace. Class C airspace in the United States surrounds medium-activity airports that have the capability to provide ATC services using radar. In general, the Class C airspace is a standard-shaped area that extends from the Earth’s sur- face, or from an intermediate altitude, up to a higher altitude approximately 4,000 feet above ground level. Within Class C airspace, every aircraft, both IFR and VFR, is subject to the operating rules and pilot and equipment require- ments specified in FAR 91. These requirements are similar to, but less restrictive than, the requirements to enter Class A airspace. Student pilot entry into Class A airspace is restricted, whereas student pilots are permitted to operate within Class C airspace under the same rules of operation as any VFR pilot. Class C airspace is defined as the airspace that extends from the surface to 4,000 feet above the airport elevation (charted using MSL) surrounding those airports that have an operational control tower, are serviced by a radar approach control, and have a certain number of IFR operations or passenger enplanements (see Figures 3–12 and 3–13). Although the configuration of each Class C airspace area is individually tailored, the airspace usually consists of a 5-nautical-mile radius core surface area that extends from the surface up to 4,000 feet above the airport elevation and a 10-nautical-mile radius shelf area that extends from 1,200 feet to 4,000 feet above the airport. An outer area extends 20 nautical miles outward from the center of the primary airport and extends from the lower limits of radar/radio coverage up to the ceiling of the approach control’s delegated airspace. Once the aircraft enters Class C airspace, the pilot is required to comply with any instruction issued by the controller but must still comply with the visibility and cloud avoidance requirements of FAR 91. At no time may a VFR pilot disregard the basic VFR rules when trying to comply with the controller’s clearance or subsequent instructions. If the pilot perceives that a controller’s request might force a violation of any of the visual flight rules, the pilot is 170 / CHAPTER 3 Core surface area 5 n mi Shelf area 10 n mi Outer area 20 n mi Height above airport 4,000 ft. Airport 1,200 ft. AGL Services upon establishing two-way radio communication and radar contact; Sequencing arrivals IFR/IFR standard separation IFR/VFR traffic advisories and conflict resolution VFR/VFR traffic advisories Figure 3–12. Depiction of Class C airspace. Figure 3–13. Class C airspace as depicted on a sectional chart.  Air Traffic Control System Structure / 171 authorized by FAR 91 to disregard that instruction but must inform the con- troller as soon as possible. Any VFR or IFR pilot who wishes to enter Class C airspace must comply with the following requirements: The pilot must establish communications with the appropriate air traffic control facility prior to entering. Unless the pilot is instructed to remain clear, the establishment of communication with the controller authorizes pilot entry into Class C airspace. While within Class C airspace, the pilot is required to comply with any of the instructions issued by the controller, unless these instructions will cause the pilot to violate a federal regulation, in which case the pilot is authorized to disregard the offending instruction. The aircraft must be equipped with an operable mode C transponder. The following airports have been established as Class C airspace primary airports: Alabama – Birmingham, Huntsville, Mobile Alaska – Anchorage Arizona – Tucson Arkansas – Little Rock, Fayetteville California – Beale Air Force Base, Burbank, Fresno, Monterey, Oakland International, Ontario, March Air Reserve Base, Sacramento, Santa Barbara, John Wayne Orange County, San José Colorado – Colorado Springs Connecticut – Hartford-Bradley International Florida – Daytona Beach, Fort Lauderdale-Hollywood, Jacksonville, Naval Air Station Whiting Field (South), Naval Air Station Pensacola, Naval Air Station Whiting Field (North), Palm Beach, Pensacola Regional, Southwest Florida- Fort Myers, Orlando-Sanford, Sarasota-Bradenton, Tallahassee Georgia – Columbus, Savannah Hawaii – Kahului-Maui Idaho – Boise Illinois – University of Illinois-Champaign-Urbana, Chicago Midway, Quad City-Moline, Greater Peoria, Capital-Springfield Indiana – Evansville, Fort Wayne, Indianapolis, South Bend Iowa – Cedar Rapids, Des Moines Kansas – Wichita Kentucky – Lexington, Louisville-Standiford Louisiana – Barksdale Air Force Base, Baton Rouge, Lafayette, Shreveport Maine – Bangor, Portland Michigan – Flint, Grand Rapids, Lansing Mississippi – Columbus, Air Force Base, Jackson 172 / CHAPTER 3 Missouri – Springfield Montana – Billings Nebraska – Lincoln, Offutt Air Force Base, Omaha Nevada – Reno New Hampshire – Manchester New Jersey – Atlantic City New Mexico – Albuquerque New York – Albany, Buffalo, Long Island MacArthur, Rochester, Syracuse North Carolina – Asheville, Fayetteville, Greensboro-Piedmont Triad, Pope Air Force Base, Raleigh-Durham Ohio – Akron-Canton, Columbus, Dayton, Toledo Oklahoma – Oklahoma City, Tinker Air Force Base, Tulsa Oregon – Portland Pennsylvania – Allentown-Bethlehem-Eastern Rhode Island – Providence South Carolina – Columbia, Charleston, Greenville-Spartanburg, Myrtle Beach, Shaw Air Force Base Tennessee – Nashville, Chattanooga, Knoxville Texas – Abilene, Amarillo, Austin, Biggs Army Airfield, Corpus Christi, Laughlin Air Force Base, Dyess Air Force Base, El Paso, Harlingen, Lubbock, Midland, San Antonio Vermont – Burlington Virginia – Norfolk, Roanoke, Richmond Washington – Spokane, Naval Air Station Whidbey Island, Fairchild Air Force Base West Virginia – Charleston Wisconsin – Green Bay, Milwaukee, Madison Puerto Rico – San Juan Virgin Islands – St. Thomas Class D Class D airspace is defined as the airspace extending from the surface to 2,500 Airspace feet above the airport elevation (charted using MSL) surrounding those air- ports that have an operational control tower (see Figure 3–14). The configura- tion of each Class D airspace area is individually tailored, and when instrument procedures are published, the airspace will generally be designed to contain the procedures. Pilots are required to establish two-way radio communication with the air traffic control tower providing ATC services prior to entry and thereaf- ter maintain those communications while in the Class D airspace. At airports where the control tower does not operate 24 hours a day, the operating hours  Air Traffic Control System Structure / 173 Figure 3–14. Class D airspace surrounding an airport with an operating control tower. of the tower are listed on the appropriate charts and publications. During the hours the tower is not in operation, Class E surface area rules or a combination of Class E rules to 700 feet above ground level and Class G rules to the surface are applicable as appropriate. Class D airspace areas are depicted on sectional and terminal charts with blue segmented lines and on IFR en route low-altitude charts with a boxed D. Arrival extensions for instrument approach procedures may be Class D or Class E airspace. As a general rule, if the extensions are all 2 miles or less in length, they remain part of the Class D surface area. However, if any one extension is greater than 2 miles, then all extensions become Class E. Due to the speed differential among aircraft operating near the airport, unless authorized or required by ATC, aircraft operating within Class D airspace at or below 2,500 feet above the surface are not permitted to operate at indicated airspeeds of more than 200 knots (230 mph). IFR aircraft are authorized to operate in Class D airspace if they are routed through by ATC clearance. VFR pilots are permitted to fly through Class D airspace as long as the basic VFR weather minima described in FAR 91 exist, the required cloud separation distances can be maintained, and permis- sion has been granted from the control tower. An additional requirement for VFR flight is that the cloud ceiling must be at least 1,000 feet above the ground if the pilot wishes to operate below the ceiling. This requirement ensures that VFR pilots will be able to maintain a distance of at least 500 feet below the clouds and 500 feet above the surface of the Earth, which is a FAR 91 require- ment for VFR flight. VFR pilots may operate above the 1,000 foot ceiling as long as they are able to climb above the ceiling while maintaining VFR conditions and can maintain the basic FAR 91 weather minima while flying above the ceiling. If the ceiling in the Class D airspace is less than 1,000 feet, or if the visibility is less than 3 miles, a VFR pilot is not permitted to operate within the air space. In these conditions, the pilot may request a special VFR (SVFR) clearance to operate. 174 / CHAPTER 3 Special VFR An SVFR clearance is a hybrid clearance in which VFR pilots navigate visually but are separated by the controller from other IFR or SVFR aircraft. Special VFR aircraft are required to remain clear of the clouds while operating within Class D airspace but can operate with visibility as low as 1 mile. Special VFR clearances may be issued only when requested by the pilot and when traffic conditions permit their use. In general, SVFR flights are allocated a fairly large block of airspace, since the pilot may need to navigate around clouds and obstructions. Special VFR operations generally reduce the number of IFR air- craft that can land at an airport. Because of this impact on IFR flights, FAR 91 Appendix D mandates that SVFR clearances cannot be obtained at some of the nation’s busiest airports. Class D airspace located at airports with control towers is indicated on VFR navigational charts using blue dashed lines. Class D airspace located at airports without control towers is indicated on VFR navigational charts using magenta dashes. Airspace where SVFR clearances cannot be issued is depicted on VFR navigation charts with the words NO SVFR in the airport data block (see Figure 3–15). Class E Generally, if the airspace is not Class A, B, C, or D, and it is controlled airspace, Airspace it is designated as Class E airspace. Class E airspace below 14,500 feet MSL is charted on sectional, terminal, world, and IFR en route low-altitude charts. Class E airspace generally has no defined vertical limit, but rather it extends upward to the overlying or adjacent controlled airspace. There are seven general forms of Class E airspace, all of which are defined to ensure that aircraft operating on IFR flight plans can remain in controlled airspace during their entire flight. Any normal IFR flight that leaves the con- fines of controlled airspace would no longer be offered ATC services, thereby negating the entire concept of air traffic control and separation. The seven forms of Class E airspace are as follows: Figure 3–15. Airport where no SVFR is allowed.  Air Traffic Control System Structure / 175 Figure 3–16. Surface area extensions. 1. Surface area designated for an airport. When designated as a surface area for an airport, sufficient Class E airspace will be designated around the airport to contain all instrument procedures. 2. Surface area extensions. Class E airspace areas can serve as extensions to Class B, C, and D surface areas designated for an airport (see Figure 3–16). Such airspace provides controlled airspace to contain standard instrument approach procedures without imposing a communications requirement on pilots operating under VFR. 3. Airspace used for transitions. Class E airspace areas beginning at either 700 or 1,200 feet AGL are used to transition to/from the terminal or en route environment. 4. En route domestic areas. Class E airspace areas that extend upward from a specified altitude and are en route domestic airspace areas provide controlled airspace in those areas where there is a requirement to provide IFR en route ATC services but the federal airway system is inadequate. Most of the United States airspace east of the Rocky Mountains and above 1,200 feet AGL has been designated as Class E airspace. 5. Offshore airspace areas. Class E airspace areas that extend upward from a specified altitude to, but not including, 18,000 feet MSL are designated as offshore airspace areas. These areas provide controlled airspace beyond 12 miles from the coast of the United States in those areas where there is a requirement to provide IFR en route ATC services and within which the United States is applying domestic procedures. 6. Continental airspace. Unless designated at a lower altitude, Class E airspace begins at 14,500 MSL to, but not including, 18,000 feet MSL overlying the forty-eight contiguous states including the waters within 12 miles from the coast of the forty-eight contiguous states; the District of Columbia; Alaska, including 176 / CHAPTER 3 the waters within 12 miles from the coast of Alaska, and that airspace above FL 600; excluding the Alaska peninsula west of long. 160°00-00-W; and the airspace below 1,500 feet above the surface of the Earth unless specifically so designated. 7. Federal airways. Federal airways are Class E airspace areas and, unless otherwise specified, extend upward from 1,200 feet to, but not including, 18,000 feet MSL. This includes the colored airway system that uses NDBs for navigation and the VOR airways. Federal FARs 71 and 93 define the structure of the federal airway system. The federal Airways airways are divided into two general types: colored airways and the VOR airway system. The colored airways use NDBs and four-course radio ranges for navigation. With the exception of coastal areas, colored airways no longer exist within the continental United States but are still used in Alaska and Canada. They are sometimes used to provide temporary airways when a VOR malfunctions or is being relocated. VOR-based airways have been the standard for aviation navigation in the continental United States since the late 1950s. Every federal airway is designated by the FARs as either a low-altitude airway or a jet route. Low-altitude airways are defined in FAR 71 and use both low- and high-altitude VORs for navigation. All low-altitude airways are assigned distinctive route numbers that are prefixed with the letter V and are known as victor airways (since victor is the phonetic pronunciation of the letter V). For example, V-251 is known as “victor two fifty-one.” Low-altitude airways extend from 1,200 feet above the surface of the Earth up to, but not including, 18,000 feet above MSL. Jet routes begin at 18,000 feet MSL and extend up to and including 45,000 feet MSL. High- altitude airways use high-altitude VORs exclusively, are assigned a route number, and are prefixed with the letter J. These airways are referred to as jet routes or simply jay routes. For example, J-155 would be pronounced as “Jay one fifty-five.” The FAA publishes both low- and high-altitude charts that depict federal airways. Figure 3–17 provides an example of a low-altitude chart; legends for reading the chart are provided in Appendix A. There are no airways or jet routes above 45,000 feet MSL. High-performance aircraft operating at these altitudes either use RNAV or fly directly from one VOR to the next. Flight Levels Since aircraft using high-altitude airways are usually traveling at high airspeeds, it is difficult to ensure that every aircraft operating within a given area is using the same altimeter setting. It is imperative that every altimeter measure altitude above the same reference plane (mean sea level). If two aircraft using different altimeter settings were flying in close proximity, they could conceivably be at the same altitude even though their altimeters indicated different altitudes. Improperly set altimeters increase the potential for near misses and actual mid- air collisions. This particular problem is solved for low-altitude aircraft by requiring that the pilot set the altimeter to the current station pressure at the controlling  Air Traffic Control System Structure / 177 ATC facility. This procedure ensures that every aircraft operating within the same area is using the same altimeter setting. This method is not so useful for aircraft operating at high altitudes, since they are usually flying at a fairly high airspeed, requiring pilots to constantly adjust their altimeter setting every few minutes as they pass from one area to another. The possibility that pilots could inadvertently use an incorrect altimeter setting increases every time they read- just the altimeter. The potential collision probability also increases any time a pilot fails to readjust the altimeter or when a controller fails to inform the pilot of the new altimeter setting. Since pilots operating high-altitude aircraft are not as concerned about their actual altitude above the ground as low-altitude pilots are, this potential collision problem can be solved by requiring pilots to reset their altimeters to 29.92 inches of mercury when operating their aircraft at or above 18,000 feet MSL. The setting of 29.92 inches is known as standard atmospheric pressure, and 18,000 feet MSL is known as the transition level. Setting the altimeter to standard pressure when operating at or above the transition level ensures that every aircraft is using the same altimeter setting and measuring altitude from a common datum. The only problem with this procedure is that the altimeter is no longer indicating the true altitude above MSL, which makes it difficult to determine the aircraft’s true altitude above an obstruction. For- tunately, few obstructions occur at these altitudes. Pilots flying near very high obstructions are routinely assigned altitudes high enough to guarantee obstacle clearance. To reduce the possibility of a pilot mistakenly using the local altimeter setting when flying on a jet route, any cruising altitude at or above 18,000 feet MSL is known as a flight level (FL). A flight level is defined as a level of con- stant atmospheric pressure related to a reference datum of 29.92 inches of mercury. Each flight level is stated using three digits that represent hundreds of feet. For example, FL 250 represents a barometric altimeter indication of 25,000 feet. Because every aircraft operating at or above 18,000 feet is using a com- mon altimeter setting, it can be safely assumed that an aircraft operating at FL 250 will always be 1,000 feet below an aircraft operating at FL 260. These two aircraft may not actually be at 25,000 feet and 26,000 feet MSL, respectively, but that is unimportant at these altitudes. The ATC system is primarily con- cerned that the aircraft are separated by at least 1,000 feet. As aircraft descend through the transition level (FL 180), pilots reset their altimeter to the local barometric pressure to again accurately indicate the aircraft’s altitude above mean sea level. This becomes increasingly important as the aircraft gets closer to the ground. The procedure of resetting the altimeter to 29.92 when passing through the transition level is used worldwide, but the transition level altitude varies among ICAO member nations. It is at 18,000 feet MSL in North America, but it may be as low as 3,000 feet MSL in some European countries. This may cause a problem when controllers are separating aircraft whose pilots are certified in another country and are accustomed to resetting their altimeter to standard Figure 3–17a Figure 3–17b Sample low-altitude en route chart. 180 / CHAPTER 3 pressure at a different transition altitude. Problems can also occur at airspace boundaries between countries with different transition levels. Flight levels are necessary to ensure that proper separation is being applied to aircraft operating at high altitudes, but whenever the local altimeter setting is less than 29.92 inches, FL 180 may actually be less than 1,000 feet above 17,000 feet MSL. Whenever the local altimeter setting is less than 29.92, FL 180 must be considered unusable. If the local barometric pressure drops below 28.92 inches, additional flight levels may become unusable. The following table from the Air Traffic Control Handbook demonstrates the lowest usable flight level that may be assigned to aircraft based on the local altimeter setting. Altimeter Setting Lowest Usable Flight Level 29.92 in. or higher FL 180 29.91 in. to 28.92 in. FL 190 28.91 in. to 27.92 in. FL 200 Airway The area reserved for aircraft operating along a federal airway includes the air- Dimensions space extending laterally 4 nautical miles on either side of the airway’s centerline. If the airway is more than 102 nautical miles from VOR to VOR, it is widened to take into consideration the spreading of the radials as they emanate from the VOR. At a point 51 nautical miles from the VOR, the boundaries of the airway begin to include the airspace between two lines that diverge from the VOR at an angle of 4.5° on either side of the airway centerline. If the airway changes direc- tion, it also includes that airspace enclosed by extending the boundary lines of each segment of the airway. The midway point of the airway is known as the changeover point (COP). This point is defined as the fix between the two naviga- tional aids that define that particular segment of the airway. The changeover point is where the pilot ceases to track from the first VOR and begins to track to the next VOR. Changeover points are not depicted on navigational charts unless they are located somewhere other than the exact midpoint of the airway. High-Altitude The high-altitude redesign (HAR) project is the first step in implementing some Redesign fundamental changes in structure to the en route portion of the national air- Project space system. The HAR project is an attempt to move away from the use of ground-based navaids and instead using RNP to provide navigation directly from the departure to destination airports. Pilots will have the flexibility to choose their routes taking into account personal and airline preferences, weather, and aircraft performance. This flexibility is known as nonrestrictive routing (NRR). HAR will be implemented in phases across the United States and will depend on both improved ATC and system user capabilities. Initial implemen- tation is planned for altitudes at or above FL 390 in the northwest portion of the United States. As program experience is gained, additional airspace and flight levels will be added until all high-altitude airspace overlying the domestic United States is included.  Air Traffic Control System Structure / 181 The concept of nonrestrictive routing is that pilots should be permitted to fly their aircraft on the shortest route from airport to airport. During the en route phase of flight, this is fairly easy to implement but cannot be used in busier, com- plex terminal areas or for the departure and arrival portions of a flight. Around busier airspace areas, transition points called “pitch” and “catch” points will be established for flights entering or exiting these busy areas. During the departure phase of flight, ATC will provide a route out to one of many defined “pitch” points, after which the pilot will be free to define his or her own route of flight. The pilot will have increased navigational flexibility en route but will be required to navigate to one of many “catch” points that will be established approximately 200 miles from the destination airport. These catch points will be established to improve ATC separation and entry into the terminal airspace around busy airports. New waypoints will be published on en route navigational charts to define pitch and catch points and also to facilitate navigation around areas of special use airspace. Other than these predefined airspace fixes, pilots will define their route of flight using the newly established navigation reference system (NRS) instead of using victor airways and jet routes. Navigation The navigation reference system is a grid of waypoints overlying the United Reference States that will be the basis for flight plan filing and operation in the redesigned System high-altitude environment. The NRS, as initially implemented, will establish waypoints every 30 minutes of latitude and every two degrees of longitude. Eventually, as experience is gained and airborne navigational receivers increase in capability and database storage capability, NRS waypoints will have a grid resolution of 10 minutes of latitude by 1 degree of longitude. Each NRS waypoint is assigned a five-character designator. The first char- acter for all waypoints within the contiguous forty-eight U.S. states will be a “K” (which is the ICAO identifier for the United States). The second character will designate within which ARTCC the waypoint resides. B Boston N New York W Washington J Jacksonville C Cleveland I Indianapolis T Atlanta R Miami H Houston F Fort Worth K Kansas City G Chicago P Minneapolis D Denver A Albuquerque U Salt Lake City L Los Angeles S Seattle The following two number/one letter combination will represent latitude and longitude but not in a typical lat-long format. The latitude increment num- bers start at the equator, which is designated “00”. Each 10-minute increment thereafter is then identified by a number between “01” and “90”. The latitude numbering sequence repeats each 15 degrees of latitude. The longitude letters start at the prime meridian and go from west to east around the globe repeating every 26 degrees. For example, the waypoint name KA03W can be identified as a U.S. waypoint, located in the Albuquerque ARTCC’s area at latitude N30-30-00 and longitude W104-00-00. 182 / CHAPTER 3 Although this referencing system may initially appear confusing, it will in fact be much easier to enter into a computer than a long string of latitude- longitude coordinates, and it is easier to build into navigation system internal error checking protocols. Example: KA03W Phraseology: “Kilo Alpha Zero Three Whiskey” K A 03 W United States Albuquerque Center 30-30-00. 104-00-00. north latitude west longitude Tango Routes In 2004, the Aircraft Owners and Pilots Association (AOPA) requested the FAA to establish RNAV routes around or through busy terminal areas. The fixed location of ground-based navaids precluded efficient routing of aircraft in and around busy terminal areas. The FAA used the flexibility provided by RNAV to develop a point-to-point route capability for busy terminal and other restrictive areas. Those routes are called Tango routes. Tango routes are not really airways in the classic sense but are designed to make it easier for aircraft to more efficiently avoid high traffic or restricted areas. Tango routes help reduce controller workload by providing a published route in lieu of controllers providing navigation services through use of radar vectoring along those flight paths. Class F Class F airspace is not used in the United States. It is used, however, internation- Airspace ally and will be described in Chapter 11. Class G Airspace defined as Class G airspace is uncontrolled airspace within which Airspace ATC separation services will not be provided to any aircraft, whether IFR or VFR. The regulations for flight in uncontrolled airspace are quite specific and place the burden of separation on the pilot. Most of the uncontrolled airspace in this country is located away from major airports below 1,200 feet AGL. The following procedures must be followed by any pilot flying in uncontrolled airspace. Uncontrolled Airspace IFR Flight IFR flight may be legally conducted in uncontrolled airspace, although no ATC separation services can be provided by the FAA. A pilot flying in IFR conditions in uncontrolled airspace assumes the entire responsibility for air traffic separation and terrain avoidance. Prop- erly qualified pilots may legally operate under instrument flight rules in uncon- trolled airspace as long as they adhere to the applicable FARs. Most of these regulations are found in FAR 91. Pilots operating in uncontrolled airspace under instrument flight rules are not required to file a flight plan nor will they receive clearance or sep- aration services from ATC. In fact, air traffic controllers are prohibited from issuing clearances or providing air traffic separation to IFR aircraft  Air Traffic Control System Structure / 183 operating in uncontrolled airspace. Since controllers are not informed of every aircraft operating in uncontrolled airspace, it is impossible for them to provide separation to these aircraft. In general, pilots wishing to con- duct IFR flight in uncontrolled airspace must comply with the following regulations. 1. The pilot of the aircraft must be properly rated, and the aircraft must be properly equipped for IFR flight as specified in the FARs. 2. The pilot is solely responsible for navigating and avoiding other IFR or VFR aircraft. 3. The pilot is responsible for operating the aircraft a safe distance above the ground. FAR 91 requires that pilots operating IFR in uncontrolled airspace maintain an altitude of at least 1,000 feet above any obstruction located within 5 statute miles of the course to be flown. This rule is not applicable to aircraft landing or taking off, during which it is the pilot’s responsibil- ity to operate the aircraft a safe distance above obstacles. In theory, during IFR flight in uncontrolled airspace, the pilot is required to fly at an altitude appropriate for the direction of flight. The altitudes are specified in FAR 91 but are seldom used since pilots rarely fly IFR in uncontrolled airspace for any length of time. IFR flight in uncontrolled airspace is usually limited to arrivals and departures from small airports with limited air traffic control services. Uncontrolled Airspace VFR Flight VFR pilots operating in uncontrolled airspace must adhere to the applicable regulations contained in FAR 91.155. This regulation specifies the weather conditions that must exist for the pilot to legally operate VFR. The required weather conditions vary depending on the aircraft’s cruising altitude and its actual altitude above the ground. To legally fly VFR in uncontrolled airspace, pilots must comply with the visibility and cloud distance minima contained in FAR 91. VFR pilots operating in uncontrolled airspace are not required to file any type of flight plan or to contact any air traffic control facility (unless they are entering a designated area where contact is mandatory). It is the responsibility of VFR pilots to see and avoid any other aircraft that might be within their immediate vicinity, regardless of whether that aircraft is operating under IFR or visual flight rules. ATC Services in Uncontrolled Airspace Air traffic control separation ser- vices are not offered to any aircraft operating in uncontrolled airspace unless an emergency exists. Additional ATC services can be provided, however, on a workload-permitting basis. If a controller finds it necessary to issue a clear- ance to an aircraft while it is still within uncontrolled airspace, the Air Traffic Control Handbook suggests that the following phraseology be used to ensure that the pilot is aware that ATC services will not begin until the aircraft enters controlled airspace: 184 / CHAPTER 3 [aircraft call sign], upon entering controlled airspace, [the clearance]. For example: “N512PU, upon entering controlled airspace, fly heading two seven zero and join victor two fifty-one.” Special Use Regulatory Special Use Airspace In numerous areas scattered around the Airspace United States, it is in national interest to either restrict or completely prohibit the flight of civilian aircraft. The U.S. government, through the FARs, has des- ignated these areas as special use airspace. Special use airspace is designed to either confine unique aircraft operations or to entirely prohibit flight within the specified area. Unless otherwise noted, all of the following examples of special use airspace are published on VFR and IFR navigation charts and are designated in appropriate aeronautical publications. Prohibited Areas A prohibited area is airspace where aircraft operations are absolutely prohibited by law. These areas are directly concerned with either national security or public safety. Among the prohibited areas are the White House, the Capitol Building, and Camp David. FAR 91.133 expressly prohib- its either IFR or VFR aircraft from entering such areas without specific (and very rarely granted) authorization. Air traffic controllers are not permitted to authorize civilian aircraft operations within these areas unless an emergency exists (see Figure 3–18). Every prohibited area is designated using a unique identifying number prefixed with the letter P. Prohibited areas are prominently marked on both IFR and VFR navigation charts to assist pilots in avoiding them. Federal airways are routed around prohibited areas, but VFR pilots must be familiar with their locations and plan their flight path accordingly. Restricted Areas Locations where aircraft operations are not absolutely prohibited but are subject to various restrictions, are labeled restricted areas. They are located where both airborne and ground-based activities are routinely conducted that may be hazardous to either the aircraft or its occupants. These activities include artillery firing, aerial gunnery, and high- energy laser and missile testing. Some restricted areas are in effect 24 hours a day, whereas others operate part-time. The part-time restricted areas, also known as joint use areas, are available for civilian flight whenever they are not active. The FAA facility that has been given responsibility for the airspace containing a joint use restricted area will be notified by the appropriate agency when the restricted area becomes active. Figure 3–18. Prohibited, At these times, it becomes the air traffic controller’s responsibil- restricted, or warning area as ity to issue clearances to keep IFR aircraft out of the restricted depicted on a sectional chart. area. VFR aircraft are expected to contact appropriate ATC facili- ties when approaching restricted areas to determine their status.  Air Traffic Control System Structure / 185 VFR pilots are required to provide their own separation from restricted areas, although they may request navigational assistance from ATC facilities. When the restricted area is not active, it may be released by the controlling agency to the appropriate ATC facility, and controllers may permit both IFR and VFR aircraft to use the restricted space. Restricted areas are prominently marked on both VFR and IFR charts and are identified by a unique number prefixed with the letter R (see Figure 3–18). Temporary Flight Restrictions The FAA may impose temporary flight restric- tions (TFRs) around any incident or accident that has the potential for attract- ing a sufficient number of aircraft to create a hazard to either other aircraft in the air or people on the ground. Temporary flight restrictions may be imposed around earthquake, flood, fire, or aircraft crash sites. TFRs essentially operate like temporary, ad-hoc restricted areas. When a temporary flight restriction is imposed, the FAA notifies pilots by issuing a notice to airmen (NOTAM). These notices are distributed nationwide to FAA air traffic control towers, air route traffic control centers, and flight ser- vice stations, who then relay the information to pilots. In addition, NOTAMs are transmitted to the airlines, military services, and many independent pilot- briefing companies who make the information available to their subscribers. When issued, a NOTAM defines the physical location, dimension, and duration of the restriction to flight. The NOTAM usually explains which aircraft are permitted to operate within the TFR. These aircraft include: Aircraft participating in disaster relief that have been approved by the FAA. IFR aircraft properly cleared through the restricted area by ATC. VFR pilots are required by FAR 91 to avoid these areas unless it is absolutely impossible to do so. IFR aircraft are rerouted by ATC around temporary flight restrictions. Domestic ADIZ As a result of the attacks on the Washington, D.C. area in 2001, the FAA has established the Washington, D.C. Metropolitan Area Air Defense Identification Zone (DC ADIZ) and the Washington, D.C. Metro- politan Flight Restricted Zone (DC FRZ). These zones are considered to be National Defense Airspace and there are very specific penalties for violating the rules pursuant to flying within this airspace. A pilot who violates the rules concerning operations in this airspace may be subject to both civil and criminal penalties under the law. Pilots who do not adhere to the proper procedures will likely be intercepted in flight, directed to a safe landing area, and detained and interviewed by law enforcement personnel. The DC FRZ extends outward roughly 30 nm in radius from Washington, D.C. and extends vertically from the ground up to, but not including, FL 180. Aircraft are to remain clear of this area unless they are properly equipped with radios and transponders, have filed a special flight plan and received clearance to enter or exit the ADIZ. Aircraft will be issued a special, discrete transponder 186 / CHAPTER 3 code and must maintain radio and radar contact with air traffic control at all times. Failure or inability to maintain contact requires that the pilot remains clear of the ADIZ. Nonregulatory Warning Areas A warning area is airspace located over international waters Special Use where operations that may be hazardous to nonparticipating aircraft are Airspace routinely conducted. The activities conducted in a warning area are usually similar to those performed in a restricted area (see Figure 3–18). Since the warning areas are located in international airspace, neither the United States nor any other government has the right to restrict the flight of aircraft through these areas. Both IFR and VFR aircraft may operate in warning areas, but they do so at their own risk. International civil aviation organization rules require that signatory nations advise each other when military activities are being conducted within warning areas. Fortunately, most of the developed nations of the world are members of ICAO and abide by this regulation. The military authority conducting the exercise will usually advise the responsible ATC facility of the type of activity and its expected duration. Military Operations Area A military operations area (MOA) is designated airspace where military flight training activities routinely take place that might prove hazardous to civilian aircraft. Some of the flight training being conducted by military aircraft requires acrobatic maneuvers to be practiced on or near a federal airway. Although acrobatic flight along a federal airway is forbidden by FAR 91, the Department of Defense has been exempted from this regulation if the maneuvers are conducted within an MOA. Although military training flights are usually conducted in VFR flight conditions, the rapid changes in aircraft attitude required during these training maneuvers make it extremely difficult for the military pilot to “see and avoid” civilian aircraft. It is for this reason that military operations areas were created. When the appropriate military authority advises the FAA that an MOA is active, air traffic controllers are required to reroute IFR aircraft around the MOA. VFR pilots are permitted to enter an MOA at any time but do so at their own risk. MOAs are depicted on both VFR and IFR navigation charts and are given identifying names followed by the letters MOA (see Figure 3–19). Alert Areas Alert areas are areas that may contain a large number of high- performance military training aircraft conducting routine training exercises (see Figure 3–20). Although there are no legal restrictions to civilian aircraft flying through an alert area, both IFR and VFR pilots transiting the area should be aware of the large numbers of VFR military aircraft that may be practicing nonacrobatic high-speed maneuvers there. Controlled Firing Areas Controlled firing areas contain activities that, if not conducted in a controlled environment, could be hazardous to aircraft. These areas are not identified on VFR and IFR charts since the controlling agency suspends its activities whenever nonparticipating aircraft approach the  Air Traffic Control System Structure / 187 Figure 3–19. Military operations area as depicted on a sectional chart. Figure 3–20. Alert area as depicted on a sectional chart. area. Such aircraft are usually detected by the use of spotter aircraft, radar, or ground-based observers. Whenever intrusion of a nonparticipating aircraft into a controlled firing area is detected, the test firings are halted until the aircraft in question has departed the area. Controlled firing areas predominantly affect low flying aircraft since most test firing is conducted at these altitudes. National Security Areas National security areas (NSAs) consist of airspace established at locations where increased security and safety of ground facili- ties are required. Pilots are requested to voluntarily avoid flying through NSAs whenever possible. When it is necessary to provide a greater level of security and safety, flight in NSAs may be temporarily prohibited by regulation under the provisions of FAR 99. Airport These nonregulatory areas exist at airports where a flight service station is Advisory located but where there is no operating air traffic control tower. An airport Areas advisory area is 10 statute miles in radius around the airport. Flight service station personnel will offer weather information and traffic reports to arriving and departing aircraft, but will not offer any separation services to aircraft. It is not mandatory that pilots use airport advisory services, but it is highly rec- ommended by the FAA that they do so. Military To remain sufficiently proficient to perform their duties, many military pilots Training are required to practice low-level, high-speed combat-training flights. The Routes maneuvers performed during these training flights make the “see and avoid” concept of traffic separation difficult without increased vigilance on the part of both military and civilian pilots. To assist civilian pilots to avoid these military aircraft, the FAA and the Department of Defense (DOD) have mutually agreed 188 / CHAPTER 3 Figure 3–21. Military training route as depicted on a sectional chart. to participate in the military training route (MTR) program. Through this pro- gram, designated MTR routes have been agreed to by both the FAA and the DOD and are depicted on VFR navigation charts (see Figure 3–21). Every military training route has been assigned a unique identifying des- ignator composed of two letters and either three or four numbers. The first two letters are either IR (instrument rules) or VR (visual rules) for the type of mili- tary operation that will be conducted. Military pilots flying on IR-designated routes are provided IFR separation and must remain in contact with FAA con- trollers during the entire flight. An IR MTR route is flown under instrument flight rules and requires the pilot to file a flight plan and receive an ATC clear- ance. Military aircraft operating on VR-designated routes use VFR “see-and- avoid” flight rules. These routes are used only when weather conditions permit the entire flight to be conducted in VFR conditions. An MTR designator containing three numbers signifies that the pilot will fly the MTR at an altitude that may be both above and below 1,500 feet AGL. Four numbers in the designator means that the entire MTR will be flown at an altitude at or below 1,500 feet AGL. For example, IR 101 is an MTR that would be flown in IFR conditions, with altitude segments that might be both above and below 1,500 feet AGL. VR 4002 is an MTR that would be flown in VFR conditions at or below 1,500 feet AGL. Civilian aircraft are not prohibited from flying in the vicinity of an MTR, but pilot contact with a nearby ATC facility is recommended. Any flight service station within 100 miles of the MTR route will be advised by the controlling authority when the MTR is active. It is the VFR pilot’s responsibility to determine whether the MTR is in use. Civilian IFR aircraft will always be separated from military aircraft operating on IR-designated MTRs but will not be separated from aircraft flying on a VR MTR. It is the civilian IFR pilot’s responsibility to remain vigilant and avoid any aircraft using a VR military training route.  Air Traffic Control System Structure / 189 KEY TERMS above ground level (AGL) high-altitude redesign (HAR) standard atmospheric pressure air traffic control jet advisory area standard terminal arrival route Air Traffic Control Handbook military operations area (MOA) (STAR) airport advisory areas military training route (MTR) target resolution airport radar service area (ARSA) minimum en route altitude temporary flight restriction alert areas (MEA) (TFR) changeover point (COP) mode C altitude encoder terminal control area (TCA) controlled airspace national airspace review (NAR) terminal radar service area controlled firing areas national security areas (NSAs) (TRSA) departure procedure (DP) notice to airmen (NOTAM) transition level Direct User Access Terminal positive controlled airspace (PCA) uncontrolled airspace (DUAT) positive separation vectors expect further clearance (EFC) prohibited area victor airways federal airways restricted area warning areas flight level (FL) special use airspace waypoint flight restricted zone (FRZ) special VFR (SVFR) workload permitting REVIEW QUESTIONS 1. What is the purpose of controlled airspace? 2. What is the difference between a jet route and an airway? 3. What must the pilot do when climbing through the transition level? 4. What is the primary difference between Class A and Class C airspace? 5. How are aircraft separated differently in Class B versus Class C airspace? 6. In what airspace areas are transponders mandatory?