Regulation Text
Except as provided in paragraph (d) of this section, each person operating an aircraft in Class A airspace must conduct that operation under instrument flight rules (IFR) and in compliance with the following:
(a) Clearance. Operations may be conducted only under an ATC clearance received prior to entering the airspace.
(b) Communications. Unless otherwise authorized by ATC, each aircraft operating in Class A airspace must be equipped with a two-way radio capable of communicating with ATC on a frequency assigned by ATC. Each pilot must maintain two-way radio communications with ATC while operating in Class A airspace.
(c) Equipment requirements. Unless otherwise authorized by ATC, no person may operate an aircraft within Class A airspace unless that aircraft is equipped with the applicable equipment specified in § 91.215, and after January 1, 2020, § 91.225.
(d) ATC authorizations. An operator may deviate from any provision of this section under the provisions of an ATC authorization issued by the ATC facility having jurisdiction of the airspace concerned. In the case of an inoperative transponder, ATC may immediately approve an operation within a Class A airspace area allowing flight to continue, if desired, to the airport of ultimate destination, including any intermediate stops, or to proceed to a place where suitable repairs can be made, or both. Requests for deviation from any provision of this section must be submitted in writing, at least 4 days before the proposed operation. ATC may authorize a deviation on a continuing basis or for an individual flight.
[Docket 24458, 56 FR 65659, Dec. 17, 1991, as amended by Amdt. 91-314, 75 FR 30193, May 28, 2010]
Research Notes
Section 91.135 governs operations in Class A airspace — the IFR-only environment that extends from 18,000 feet MSL up to FL600 over the continental U.S.
Paragraph (a) — Operations under IFR only: Each person operating an aircraft in Class A must operate that aircraft under IFR (and in compliance with each requirement of § 91.181 (course to be flown), the IFR cruising altitudes, the radio communications rules, etc.). No VFR. No SVFR.
Paragraph (b) — Communications and transponder: Two-way radio communications with ATC must be maintained on the appropriate frequency. A Mode C transponder is required per § 91.215. ADS-B Out is required per § 91.225.
Paragraph (c) — Aircraft equipment: The aircraft must have equipment for radio communications (with ATC) and navigation (to determine position) appropriate to the air traffic control facilities being used. For routes in Class A, the navigation requirement typically means RNAV/GPS or VOR/DME, depending on the route structure.
Why Class A starts at 18,000 MSL: The transition altitude. Below 18,000, aircraft use local altimeter settings (per § 91.121). At and above 18,000, all aircraft use 29.92 inHg — which gives Flight Levels rather than indicated altitudes. The standardization is essential for separation in the busy upper-altitude environment.
RVSM — § 91.180: Between FL290 and FL410, RVSM (reduced vertical separation minimum) allows 1,000-foot separation rather than 2,000 feet. RVSM operations require special certification under § 91.180 and Appendix G to Part 91.
Reference: AIM 3-2-2 on Class A airspace; AIM 5-3 on enroute procedures.
Amendment History
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