Flying

How the U.S. Airspace System Works

By Fred Simonds, Special to Aviation.com

posted: 12 August 2008 11:31 am ET

: This profile shows how airspace is organized and divided in the U.S. Airliners and jet traffic dominate the Class A airspace between 18,000 and 60,000 feet. All aircraft in this space must be on instrument flight plans and under air traffic control (ATC). The green area is Class E or controlled airspace, in which ATC is capable of separating aircraft flying on instruments from one another. Aircraft flying visually can fly legally in Class E airspace subject to visibility and cloud clearance requirements. These rules minimize the prospect that an IFR airplane might emerge from a cloud and collide with a visual aircraft at the edge of the cloud. Class G is uncontrolled airspace in which ATC is not responsible for separating aircraft, due to limited radar visibility or other reasons. There is no Class F airspace in the U.S. Classes B or 'Bravo,' C or 'Charlie' and D or 'Delta' airspace surround busy, moderately busy and less busy tower-equipped airports respectively. Credit: FAA
: Class B, or 'Bravo,' airspace surrounds Chicago O’Hare and 11 other major U.S. airports. Often likened to an inverted wedding cake, Bravo airspace within the outermost 15-to-20-mile rings starts at 10,000 feet and descends to 3,600 or 4,000 feet. You can read this easily: The numbers that look like fractions indicate the top of the Class B, with 100 in the numerator meaning 10,000 feet. The denominator indicates the base of the Class B, either 40 for 4,000 or 36 for 3,600 as shown. Between 10 and 15 miles, the base descends to 3,000 feet. From 5 to 10 miles the base is 1,900 feet and it reaches the surface within 5 miles. Class B contains all the airspace needed for any desired instrument approach, making continuous ATC control available right down to the ground. Even the airspace outside the Bravo is protected. Within 30 miles of the airport and up to 10,000 feet, all aircraft must have a radar transponder with altitude reporting capability, called Mode C. This 30-mile radius has come to be known as the 'Mode C veil.' Credit: FAA
: Here’s a pilot’s-eye view of Class C airspace — the solid magenta ring surrounding Florida’s Palm Beach International Airport (KPBI). The outer ring extends between 5 and 10 miles from the airport and includes altitudes from 4,000 down to 1,200 feet. Within the 5-mile ring, the 'Charlie' airspace descends from 4,000 feet to the ground. The 'cutout' to the south of the airport permits small aircraft to access the non-tower Palm Beach County Park airport (LNA). Class C airspace exists at some 123 moderately busy airports in the United States. Permission to enter 'Charlie' airspace is automatic if the controller repeats the aircraft number back to the pilot after the pilot’s initial call. Credit: FAA
: Depicted is the Class D, or 'Delta,' airspace surrounding Boca Raton airport in southeastern Florida. Aircraft approaching an airport with an operating tower are expected to contact the tower before entering its airspace — a 5-mile circle around the airport extending to 2,500 feet above its surface. A simple radio call announcing aircraft type, number, location and request is all that’s needed. Of approximately 5,300 airports in the U.S., fewer than 500 have towers. Those 4,800 airports use long-established procedures, proper radio communications and 'eyeballs-outside' vigilance to operate safely. Credit: FAA

Since the genesis of powered flight with the Wright Brothers in 1903, the paramount objective of safety has driven evolutionary progress to organize and regulate airspace better. Other imperatives for the organization of airspace have been the need to make the best use of a finite medium and to make airspace usage convenient and logical for both pilots and controllers.

In recent years, the FAA has made U.S. airspace more consistent with international standards, so description of airspace organization in the United States is largely applicable anywhere in the world. Taken as a whole, today’s airspace organization is the best it has ever been.

Fundamentally, today’s airspace is designed to separate aircraft flying on instruments under what are called Instrument Flight Rules (IFR) from other IFR aircraft. Separating IFR aircraft from aircraft flying visually under Visual Flight Rules (VFR) comes second. Instrument aircraft can fly through clouds, whereas visual aircraft cannot. Almost without exception, airline traffic flies under the stricter instrument rules, whereas GA aircraft usually fly under visual rules — particularly those being flown recreationally.

Controlled airspace

Since IFR aircraft may be flying in clouds or in poor visibility where it is not possible for them to see or be seen by other aircraft, controlled or “Class E” airspace was created. In Class E airspace, air traffic control (ATC) becomes the eyes of instrument pilots.

Controlled airspace does not mean that all aircraft in that space are under ATC control. It means only that ATC is capable of separating instrument aircraft from other instrument and known visual aircraft within Class E airspace as depicted on aircraft charts.

Visual aircraft fly in Class E airspace legally all the time, subject to a defined set of visibility and cloud clearance requirements. These rules are designed to reduce the chance that an IFR airplane might emerge from a cloud and collide with an aircraft at the edge of the cloud.

The term 'controlled airspace' implies that there is also uncontrolled airspace. In uncontrolled or what pilots call “Class G” airspace, ATC has no responsibility to separate air traffic. Class G airspace is often close to the ground and hence below radar coverage, making separation via radar impossible. Flying under IFR as they mostly do, airliners rarely operate in Class G airspace.

Airliners, however, dominate the upper altitudes. Between 18,000 and 60,000 feet above mean sea level (MSL), all aircraft must be on IFR flight plans within what is called “Class A” airspace. Once at altitude and cruising along, you are almost certainly within Class A airspace.

Airspace near airports with control towers gets extra attention: the more traffic, the more restrictive the airspace. Accordingly, there are three categories of airspace designed to suit the activity levels of large-, medium- and smaller-volume airports.

Class B airspace

The largest airports are surrounded by highly regulated "Class B" airspace. In the United States, “Bravo” airspace is established around the 12 busiest airports, such as Atlanta’s Hartsfield and Chicago’s O’Hare.

Whether airborne or on the ground at the primary airport, explicit ATC permission must be obtained to enter or leave Class B airspace. Furthermore, a private pilot license or higher pilot qualification is generally required to operate in this sacrosanct airspace. For everyone’s safety, beginners should go elsewhere.

Woe betide the pilot who cuts a corner and enters Bravo airspace without permission. ATC can track an offending aircraft anywhere it might go, and the authorities will be waiting when he or she lands. The FAA takes airspace violations very seriously, and an airspace bust can earn a pilot a hefty 90-day suspension of his or her license. One story, possibly apocryphal, has it that New York controllers busted a pilot for being just 60 yards inside their Class B airspace.

Class C airspace and Mode C transponders

For some 123 airports not quite as busy, the FAA implemented "Class C" airspace. Similar in concept and form to Class Bravo, “Charlie” airspace extends to just 4,000 feet above the primary airport and reaches out just 10 miles.

A Mode C transponder — an avionics unit inside the aircraft that responds to ATC radar signals with a four-digit octal code (assigned by ATC) to identify the aircraft and another four-digit code to provide its pressure altitude — is required within the lateral limits of Class C airspace and upward to 10,000 feet MSL, but there is no "Mode C veil."

(A Mode C veil is a regulatory requirement associated with Class B airspace, requiring that all aircraft must be equipped with Mode C transponders while operating within airspace extending for a certain distance outside the Class B airspace itself. Another, more general, regulatory requirement is that all aircraft flying above 10,000 feet MSL must have Mode C transponders.)

Requirements to enter or leave Charlie airspace are also simpler than for Bravo. Permission to enter is automatic if the controller repeats your aircraft number back to you after your first call. Student pilots can operate legally within Charlie airspace.

The simplest class of airspace, "Class Delta," is effective at smaller airports with control towers, whenever their towers are operating. Extending five miles from the airport and up to 2,500 feet above it, permission is gained to enter the Delta upon pilot request. There are no minimum licensing or transponder requirements, and indeed many control towers don’t have radar at all.

Special use airspace

"Special Use Airspace" consists of prohibited areas which aircraft may not enter. Famous examples include P-40, the airspace above the Presidential Retreat at Camp David and P-51, the airspace above the White House.

“Hot” Restricted Areas contain operations that do not mix well with aircraft — little things like artillery, guided missiles or aerial gunnery. ATC clearance is required to enter, but these areas are sometimes “cold,” in which case there is no problem.

Temporary Flight Restrictions are put in place for events such as space shuttle operations, movements of the President or Vice President or disaster relief.

Since September 11, 2001, there has been a 30-mile circular Air Defense Identification Zone around Washington D.C., extending all the way up to Class A airspace at 18,000 feet.

While not required by law, pilots are asked not to fly near or “loiter” in the vicinity of nuclear power plants, power plants, dams, refineries, industrial complexes, military facilities and similar potentially sensitive facilities.

If you attend a major sporting event such as a major league baseball or football game or motor speedway event, you won’t see any low-flying aircraft overhead. Pilots are asked to stay 3 miles away and above 3,000 feet from such events.

Alert readers may have noticed something missing: "Class F." While the designation exists, there is no Class F airspace in the U.S.