14.class-a-airspace. Class A Airspace
Class A airspace is the high-altitude controlled airspace where airline jets and high-performance turbine aircraft do most of their cruising. It is defined in 14 CFR 71.33 and described in Chapter 14 of the Pilot's Handbook of Aeronautical Knowledge.
Vertical and Lateral Limits
Class A airspace extends from 18,000 feet MSL up to and including FL600 (60,000 feet pressure altitude). Laterally, it covers the contiguous 48 states, the airspace within 12 nautical miles of the coast, and the airspace overlying Alaska — but it specifically excludes the Alaskan airspace north of 60° latitude and the airspace over Hawaii (high-altitude airspace over Hawaii is treated separately). Because the upper boundary is expressed as a flight level (FL180–FL600), all altitudes within Class A are flown using the standard altimeter setting of 29.92" Hg, which converts indicated altitude to pressure altitude referenced as flight levels.
Operating Rules
Class A is the most restrictive class of airspace in terms of pilot and equipment requirements. To operate in Class A, a flight must be:
- Conducted under Instrument Flight Rules (IFR) only — VFR operations are not permitted (14 CFR 91.135).
- Flown by a pilot holding at least an instrument rating appropriate to the aircraft (14 CFR 61.3).
- Operated on an ATC clearance received prior to entry, and in continuous two-way radio communication with ATC.
- Equipped with an operable Mode C (or Mode S) transponder with altitude reporting and, since January 1, 2020, ADS-B Out meeting the performance requirements of 14 CFR 91.225 and 91.227.
- Equipped with a DME when operating at or above FL240 if VOR navigation is used (14 CFR 91.205(e)).
Because Class A is IFR-only, there are no published VFR weather minimums, no VFR cloud clearances, and no see-and-avoid expectation by VFR traffic — separation is provided by ATC for all aircraft.
Charting
Class A airspace is not depicted with a special boundary on sectional or terminal area charts, because its lateral extent is essentially the entire country and its floor is a fixed altitude. Pilots are simply expected to know that 18,000 feet MSL is the floor. On en route high-altitude IFR charts, the entire chart represents operations within Class A.
Cruising Altitudes
When flying IFR in Class A, ATC assigns altitudes; the hemispherical (East/West) cruising altitude rules of 14 CFR 91.179 apply at and above FL180 in terms of available flight levels:
- Magnetic course 0°–179°: odd flight levels (FL190, FL210, FL230, …) up through FL410.
- Magnetic course 180°–359°: even flight levels (FL200, FL220, FL240, …) up through FL400.
Above FL410, non-RVSM separation applies and altitude assignments differ. From FL290 to FL410, Reduced Vertical Separation Minimum (RVSM) allows 1,000-foot vertical separation between properly equipped and authorized aircraft; without RVSM authorization, ATC must apply 2,000-foot separation in this band.
Transitioning Into and Out of Class A
A typical IFR flight climbs through Class E airspace and crosses 18,000 feet MSL with the altimeter already reset to 29.92" Hg. ATC will normally instruct, "Climb and maintain Flight Level one-niner-zero," or assign the appropriate flight level. On descent, the controller issues a local altimeter setting as the aircraft descends through FL180 back into Class E.
Example
A Citation departs Denver IFR and is cleared to FL410. Passing 18,000 feet, the pilot verifies 29.92" Hg set in both altimeters, confirms transponder and ADS-B Out are transmitting, and reports level at FL410 when assigned. The flight is in Class A airspace from FL180 through FL410 and remains under positive ATC control the entire time.
Why Class A Exists
The high-altitude environment is dominated by jets operating at high true airspeeds (often 400–500 knots), where closure rates make see-and-avoid impractical. By making the airspace IFR-only and procedurally controlled, the FAA ensures every aircraft is known to ATC, separated by controllers, and operated by an instrument-qualified crew. This standardization is what allows the National Airspace System to safely move thousands of high-altitude flights simultaneously across the country.
Key Numbers to Remember
- Floor: 18,000 ft MSL
- Ceiling: FL600
- Altimeter setting above 18,000 ft: 29.92" Hg
- RVSM band: FL290–FL410
- Required: IFR clearance, instrument rating, two-way radio, Mode C transponder, ADS-B Out