Airspace

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Controlled airspace has become rather complicated and, ironically, it affects the VFR pilot to a much greater degree than the IFR pilot. The reason why airspace has become so complicated can be explained by its evolution. In the early days of aviation, all flying was visual; there was no such thing as controlled airspace or air traffic control. It wasn’t until the middle 1930s that blind flying using instruments became practical. Instrument flying came into its own after World War II with the development of navigational aids and communications.

The purpose of controlled airspace is to provide a safe environment for instrument operations. Controlled airspace established weather minimums. As one might expect, controlled airspace originally developed around airports where air traffic was congested. The next logical extension included the then-new electronic airway system.

As radios become more common, certain airspace required the pilot to establish radio communications with the controlling authority. As jet travel increased, all airspace in the contiguous United States above 14,500 feet became controlled, and flights above 24,000 feet required an IFR clearance. In the 1960s and 1970s, more and more airspace became controlled. Airspace that required a clearance for all aircraft was lowered to 18,000 feet, along with the airspace around major terminals. Specific communications and aircraft equipment requirements were established around smaller terminals.

The VFR pilot of the twenty-first century must contend with various weather minimums in an alphabet soup of controlled airspace, establish communications in certain areas, and make sure the aircraft has the required electronic equipment.

For our purposes, airspace on visual charts can be divided into two basic categories: controlled airspace and special-use airspace. Controlled airspace designated on visual charts is Class G, E, D, C, or B airspace. Their primary purpose is to protect IFR aircraft when weather conditions do not allow see-and-avoid separation. Class A airspace is not depicted. Special-use airspace is designated as prohibited, restricted, warning, alert, and military operations areas and military training routes. 

With the preceding as a background briefing of sorts, let’s see if we can make some sense out of the chaos. Areas with no air traffic control services are designated Class G airspace. Airspace not designated as Class E, D, C, or B on visual charts is Class G airspace. The Mammoth Lakes Airport is in Class G airspace.

If we were to fly west from Mammoth toward Merced, we would encounter a blue vignette encompassing airway Victor 230. (Note the number “115” in the box just below the airway designation. This is total mileage in nautical miles between NAVAIDs on the airway.) The blue vignette designates controlled Class E airspace at 1200 feet AGL. The dark edge of the vignette indicates the limit, and the vanishing edge is the direction of controlled airspace. Continuing west, we exit Class E airspace northwest of the airway.

When the base of Class E airspace is above 1200 AGL, the lower limit is printed on the chart. Continuing westbound, we come across another blue vignette (along the 119°30’ longitude line). The base of this Class E airspace is 12,000 feet MSL. 

The staggered thick-blue line (just east of Victor 165) indicates a change in the floor of Class E airspace. With no altitude specified, Class E airspace begins at 1,200 AGL.

From this point, we decide to fly direct to the El Nido VOR, near Merced. Class E airspace continues at 1200 AGL until we’re about two miles from the VOR, where we see a magenta vignette. The magenta vignette indicates that Class E airspace begins at 700 AGL.

If we were to proceed to the Merced Airport, we would encounter a magenta dashed line around the airport. The magenta dashed line means Class E airspace starts at the surface (surface-based Class E airspace). A magenta dashed line also designates surface-based Class E airspace associated with other types of airspace. For example, note the magenta dashed lines north of Castle AFB and south of Modesto.

At what altitude does Class E airspace begin over the Mammoth Airport? Unless designated at a lower altitude, Class E airspace begins at 14,500 MSL over the United States, except for airspace that is less than 1500 feet AGL (above mountains that are 13,000 or more feet MSL). (A sage pilot once observed that the first word in the Federal Aviation Regulations was “except.”) Class E airspace extends upward to but not including 18,000 feet.

The purpose of weather minimums is to allow enough ceiling, visibility, and cloud clearance for VFR and IFR aircraft to “see and avoid.” VFR weather minimums —that’s what they are, minimums— evolved in much the same way as controlled airspace. VFR weather minimums, especially below 10,000 feet, are much the same as they were in the beginning days of the Piper Cub and DC-3. With highperformance airplanes of all sizes and capabilities flying in the same airspace, a weather “minimum” does not necessarily equate to “safe”! 

As well as establishing weather minimums, Class D, C, B, and A airspace impose one or all of the following requirements:
• Communications
• AIRCRAFT EQUIPMENT
• ATC clearance
• Minimum pilot qualifications

Figure shows a vertical cross-section of the airspace with VFR minimums and certain equipment requirements.

All Class D airspace is surface-based. The upper limit is normally 2500 feet AGL. Modesto Class D airspace extends up to and includes 2600 feet MSL. This is indicated by the blue number “26” in the dashed blue box.

Class C airspace extends generally from the surface to 4000 feet AGL around airports with control towers and is served by a radar approach control. The boundaries of Class C airspace are individually tailored, based upon terrain and operational requirements. Class C airspace is charted using solid magenta lines. Various “shelves” exist beyond the surface-based airspace. Bases and tops of the “shelved airspace” are indicated in magenta (SFC/42 indicates “surface to 4200 feet MSL”; 14/42 indicates “1400 feet MSL to 4200 feet MSL”). Examine the Castle AFB Class C airspace.

Class B airspace surrounds the busiest airports. Class B generally consists of the airspace from the surface to 10,000 feet MSL with various shelves, sometimes referred to as an “upside-down wedding cake.” Class B airspace is charted using solid blue lines. Boundaries are defined by VOR radials, DME arcs, and prominent landmarks. Like Class C airspace, the bases and tops are charted. Like Class C airspace, bases and tops of Class B airspace are indicated, in this case in blue. 

Occasionally, Class D airspace extends to the base of overlying Class C or Class B airspace. This is one reason for nonstandard Class D airspace tops; in any case, the height is specified on the chart. When Class C airspace terminates at the base of Class B airspace, it is indicated by the magenta “T” (T/15 base of Class C 1500-feet upper limit base of overlying Class B airspace). The Oakland Class C airspace over the Hayward Airport. (The letter “T” is a leftover from the old airspace classification: the base of the terminal control area.) 

Class A airspace consists of that area from 18,000 to 60,000 feet MSL. Class E airspace is that area above 60,000 feet MSL. Class F is not an airspace designation in the United States; however, Class F is an International Civil Aviation Organization (ICAO) airspace classification. Where applicable, IFR and VFR flight are permitted. Air traffic advisory service and flight information service are provided on request. An ATC clearance is not required.

Fixed-wing special VFR is normally available in surface-based controlled airspace. In certain high-density surface-based airspace, special VFR is prohibited. This is indicated in the airport data block by “NO SVFR.”

Terminal radar service areas (TRSAs) designate airspace where traffic advisories, vectoring, sequencing, and separation of VFR aircraft are provided. TRSAs are designated Stage I, Stage II, or Stage III, which specify radar services that are available. The type of TRSA (Stage I, II, or III) can be found in the Airport/Facility Directory.

Visual charts depict special-use airspace (SUA) below 18,000 feet MSL. SUA consists of airspace where activities must be confined because they pose a hazard to aircraft operations. Prohibited, restricted, warning, alert, and military operations areas and military training routes are shown, with special military activity routes. Aircraft operations are prohibited within prohibited areas. These areas are established for security or other reasons associated with the national welfare. Aircraft operations are prohibited within restricted areas when the area is active. Restricted areas are established for unusual, often invisible, hazardous activities, such as artillery firing, aerial gunnery practice, or guided missile firing. Warning areas are established for the same hazards as restricted areas, but over international waters. Alert areas inform nonparticipating pilots of areas that might contain a high volume of training or unusual activity. Pilots should exercise extra caution within these areas.

Military operation areas (MOAs) alert pilots to military training activities. In addition to a possible high concentration of aircraft, military pilots might conduct aerobatic flight and operate at speeds in excess of 250 knots below 10,000 feet. High-speed low-level military operations are conducted along military training routes (MTRs). An MTR is designated IR when IFR operations are conducted within that route; VFR operations are designated VR. IR and VR routes operated at or below 1500 feet AGL will be identified by four-digit numbers (IR 1007, VR 1009). Operations that are conducted above 1500 feet AGL are identified by three-digit numbers (IR 205, VR 257). Special military activity routes alert pilots to areas where cruise missile tests are conducted.

Alert areas, like MOAs, advertise a high concentration of military activity. Figure shows alert area A-251 near Castle AFB, which warns pilots of military practice instrument approaches.

Figure contains symbols that alert pilots to parachute jumping, glider operations, and ultralight activity. An additional symbol has been added for hang-gliding activity. The symbol resembles a hang glider in flight. Where these symbols appear, pilots cannot expect to be alerted to the activity through NOTAMs. Details on the activity are normally found in the Airport/Facility Directory.

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