Title 14 of the Code of Federal Regulations, Part 91, is the backbone of general aviation regulations in the United States. It governs how virtually every civil aircraft may be operated in the National Airspace System. If you fly under a private or commercial certificate in the U.S., you are operating under Part 91. Every pilot must know its key provisions thoroughly.
Part 91 is organized into subparts spanning dozens of topics:
The general right-of-way rule is that aircraft in distress have right-of-way over all others. Among powered aircraft, the order of priority is: balloon, glider, airship, then aircraft towing or refueling other aircraft. When two aircraft of the same category converge head-on, each pilot shall alter course to the right. Aircraft being overtaken have right-of-way; the overtaking aircraft must pass to the right.
Except for takeoff and landing, no person may operate an aircraft below the following altitudes: over congested areas (cities, towns, settlements, or open-air assemblies) — at least 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet; over other than congested areas — at least 500 feet AGL; over open water or sparsely populated areas — no closer than 500 feet to any person, vessel, vehicle, or structure.
When flying VFR above 3,000 feet AGL in level cruising flight, pilots must maintain the prescribed cruising altitude: on a magnetic course of 0° through 179°, fly at an odd thousand-foot MSL altitude plus 500 feet (3,500; 5,500; 7,500; etc.); on a magnetic course of 180° through 359°, fly at an even thousand-foot MSL altitude plus 500 feet (4,500; 6,500; 8,500; etc.).
VFR weather minimums vary by airspace class:
| Airspace | Flight Visibility | Distance from Clouds |
|---|---|---|
| Class A | N/A (IFR only) | N/A |
| Class B | 3 SM | Clear of clouds |
| Class C | 3 SM | 500 below, 1,000 above, 2,000 horizontal |
| Class D | 3 SM | 500 below, 1,000 above, 2,000 horizontal |
| Class E (below 10,000 MSL) | 3 SM | 500 below, 1,000 above, 2,000 horizontal |
| Class E (at/above 10,000 MSL) | 5 SM | 1,000 below, 1,000 above, 1 SM horizontal |
| Class G (1,200 AGL, day) | 1 SM | Clear of clouds |
| Class G (1,200 AGL, night) | 3 SM | 500 below, 1,000 above, 2,000 horizontal |
| Class G (above 1,200 AGL, below 10,000 MSL, day) | 1 SM | 500 below, 1,000 above, 2,000 horizontal |
| Class G (above 1,200 AGL, at/above 10,000 MSL) | 5 SM | 1,000 below, 1,000 above, 1 SM horizontal |
Section 91.205 defines the minimum required instruments and equipment for VFR and IFR flight. The classic mnemonic for day VFR equipment is ATOMATOFLAMES: Airspeed indicator, Tachometer, Oil pressure gauge, Manifold pressure gauge (if altitude engine), Altimeter, Temperature gauge (liquid-cooled engines), Oil temperature gauge, Fuel gauge, Landing gear indicator (if retractable), Anticollision lights, Magnetic compass, ELT, Seat belts.
Night VFR requires all day VFR equipment plus FLAPS: Fuses (spare set), Landing light (if for hire), Anticollision lights, Position lights, and Source of power (adequate electrical power).
IFR flight requires all VFR equipment plus: two-way radio, navigation suitable for the route, gyroscopic rate-of-turn indicator, slip/skid indicator, sensitive altimeter adjustable for barometric pressure, clock with sweep seconds or digital, gyroscopic pitch and bank indicator, gyroscopic direction indicator, and a DME or RNAV system above FL240.
VFR (§91.151): No person may begin a flight under VFR conditions unless there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed, to fly after that for at least 30 minutes during the day or 45 minutes at night.
IFR (§91.167): No person may operate a civil aircraft in IFR conditions unless it carries enough fuel to complete the flight to the first airport of intended landing, fly from that airport to the alternate airport, and fly after that for 45 minutes at normal cruising speed. No alternate is required if the destination has a standard or special instrument approach and the weather is forecast to be at or above 2,000-foot ceiling and 3 SM visibility from 1 hour before to 1 hour after the estimated time of arrival (the "1-2-3" rule).
Part 91 governs general aviation — private, non-commercial flight operations. It applies to most flight training, personal flying, and certain corporate operations. Part 135 governs on-demand air carrier and commuter operations (charter flights, air taxi) and imposes additional requirements for crew training, currency, maintenance tracking, and flight and duty time. Part 121 governs scheduled air carrier operations (airlines) and is the most stringent, requiring extensive crew qualification, dispatch systems, and operational specifications. A pilot carrying passengers for hire crosses from Part 91 into Part 135 or Part 121 territory.
Part 91 contains the general operating and flight rules for civil aircraft in the U.S. It covers right-of-way rules, VFR weather minimums, altitudes, fuel requirements, equipment, maintenance, and much more.
Under §91.151, day VFR requires enough fuel to reach the destination plus 30 minutes at normal cruise speed. Night VFR requires enough fuel to reach the destination plus 45 minutes.
§91.205(b) requires the ATOMATOFLAMES equipment: Airspeed indicator, Tachometer, Oil pressure gauge, Manifold pressure gauge (altitude engines), Altimeter, Temperature gauge (liquid-cooled engines), Oil temperature gauge, Fuel gauge, Landing gear indicator (if retractable), Anticollision lights (if certified after 1996), Magnetic compass, ELT, and Seat belts.
Under §91.159, above 3,000 AGL in level VFR flight: magnetic courses 0–179° use odd thousands + 500 feet (3,500; 5,500; etc.); magnetic courses 180–359° use even thousands + 500 feet (4,500; 6,500; etc.).
Part 91 governs general aviation — private, non-commercial operations. Part 135 governs on-demand air carrier operations (charter, air taxi) and requires more stringent crew qualifications, dispatch requirements, and flight/duty time limits.