14 CFR 91.135 — Operations in Class A Airspace
All operations in Class A airspace (18,000 MSL to FL600) must be conducted under IFR with an ATC clearance.
Regulation Text
Each person operating an aircraft in Class A airspace must conduct that operation under instrument flight rules (IFR) and must operate that aircraft under an IFR flight plan filed and activated with an ATC facility. The pilot must have received an ATC clearance. Altimeter must be set to 29.92 inches Hg (standard setting).
Note: This is an excerpt. Refer to the full regulation in eCFR for the complete text.
Plain-English Explanation
Class A airspace exists from 18,000 feet MSL (Flight Level 180) to FL600 (roughly 60,000 feet). There is no VFR in Class A — everything must be IFR with an ATC clearance. You must set your altimeter to 29.92 (standard pressure) because at these altitudes everyone uses flight levels, not MSL altitudes. This is where the airlines cruise, and IFR-only ensures positive separation for everyone.
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Related Regulations
14 CFR 91.131 — Operations in Class B Airspace
Requires an explicit ATC clearance to enter Class B airspace, operative transponder with Mode C, and specific pilot certificate requirements.
§91.16714 CFR 91.167 — Fuel Requirements for Flight in IFR Conditions
IFR fuel: enough to fly to destination, then to alternate (if required), then 45-minute reserve at normal cruising speed.
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