4.2 VFR Cruising Altitudes, Altimeter Settings, and Oxygen

PAR items on 91.159, 91.121, and 91.211 fail people who mix three different numbers that all happen to live in the teens: 3,000 feet AGL for cruising altitudes, 18,000 MSL for the altimeter transition, and 12,500 / 14,000 / 15,000 for oxygen. Keep the rules in separate drawers.

VFR cruising altitudes — 91.159

Except while holding in a holding pattern of 2 minutes or less, or while turning, each person operating an aircraft under VFR in level cruising flight more than 3,000 feet above the surface shall maintain the altitude or flight level below, unless ATC authorizes otherwise.

Below 18,000 feet MSL:

• On a magnetic course of 0° through 179°, any odd thousand-foot MSL altitude + 500 feet (the rule’s own examples: 3,500, 5,500, or 7,500).
• On a magnetic course of 180° through 359°, any even thousand-foot MSL altitude + 500 feet (4,500, 6,500, or 8,500).

Above 18,000 feet MSL, maintain the altitude or flight level assigned by ATC. A VFR private pilot in a typical training airplane never lives here; Class A is IFR airspace. Know the sentence so a “what if” item does not send you hunting for an odd-plus-500 Flight Level.

The 3,000-foot trigger is above the surface (AGL), not MSL. Over a 4,000-foot plateau, 7,000 MSL is only 3,000 AGL and 91.159 has not started. Climb another hundred feet in level cruise and it has. Climbing, descending, or turning is not “level cruising flight.” A two-minute-or-less hold is also carved out.

Magnetic course is not heading

Course is the intended track over the ground, referenced to magnetic north. Heading is where the nose points after you add wind-correction. 91.159 uses course. If the desired track is 175° magnetic and a west wind requires a 185° heading, you are still on a 0°–179° course. Fly 5,500 or 7,500, not 6,500. The exam loves a wind-correction stem that tempts you to read the heading bug.

East is odd-plus-500; west is even-plus-500. That jingle only works if “east/west” means magnetic course, not the wet compass in a slip.

IFR hemispheric altitudes (odd or even thousands, no +500) are 91.179, not this section. Do not put a VFR airplane at 6,000 because “west is even.” Westbound VFR is 6,500, not 6,000.

Altimeter settings — 91.121

Each person operating an aircraft shall maintain the cruising altitude or flight level by reference to an altimeter set as follows.

Below 18,000 feet MSL, set:

1. The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft; or
2. If there is no station in that area, the current reported setting of an appropriate available station; or
3. If the aircraft has no radio, the elevation of the departure airport or an appropriate altimeter setting available before departure.

At or above 18,000 feet MSL, set 29.92 inches of mercury.

The 100-nautical-mile station is along the route and near the airplane, not “whatever ATIS you copied at the departure airport two hours ago” and not automatically “the destination.” As you fly, update to a current reported setting from a station that still meets (1) or, if needed, (2). A no-radio Cub can legally launch on field elevation in the Kollsman window; a radio-equipped 172 that refuses to copy ASOS is not using paragraph (3) as a loophole.

When the altimeter is set to the current local setting, it reads approximately true altitude at the station. Climb into a lower-pressure area without updating and the altimeter overreads — you are lower than the needles say. That is why 91.121 exists below 18,000: so two airplanes using 5,500 on opposite courses are actually near 5,500. At or above 18,000 everyone uses 29.92 so Flight Levels line up with each other, not with the terrain. 91.121(b) then raises the lowest usable flight level when the current setting is below 29.92 (for example, 29.92 or higher allows FL180; 29.91 through 29.42 raises it to FL185). ATC assigns that level. A private VFR pilot’s operational takeaway is simpler: stay below 18,000 unless you are on an ATC-assigned Flight Level with 29.92 set.

Supplemental oxygen — 91.211

91.211 is written in cabin pressure altitude, which for an unpressurized training airplane is the same number as indicated altitude once the altimeter is set. A pressurized cabin uses the cabin controller’s altitude, not the Flight Level outside.

For a civil aircraft of U.S. registry, no person may operate:

1. At cabin pressure altitudes above 12,500 feet MSL up to and including 14,000 feet MSL unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration.
2. At cabin pressure altitudes above 14,000 feet MSL unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes.
3. At cabin pressure altitudes above 15,000 feet MSL unless each occupant of the aircraft is provided with supplemental oxygen.

Read the verbs. Between just above 12,500 and 14,000 inclusive, the crew must have oxygen and must use it after 30 minutes at those altitudes. A 20-minute hop at 13,500 with no bottle is legal for the crew under (a)(1); a 40-minute cruise at 13,500 is not, unless the crew is using oxygen for the time past 30 minutes. Above 14,000 the 30-minute grace period is gone: crew uses oxygen the whole time. Above 15,000 every occupant must be provided with oxygen. The regulation does not say passengers must use it. Provided versus required to use is a standard PAR discriminator. The crew rule above 14,000 is both provided and used.

Pressurized-airplane extras in 91.211(b) (10-minute dump supply above FL250; mask-on rules above FL350) are airline-shaped. Know they exist; PAR items almost always stay on the unpressurized 12,500 / 14,000 / 15,000 ladder.

Physiologically, hypoxia does not wait for 12,501 feet. 91.211 is the legal minimum, not a personal minimum. The test still wants the CFR numbers.

Special VFR — 91.157 at a high level

Special VFR is not a cruising-altitude rule, but it sits next to 91.159 in the book and shows up in the same ACS airspace cluster. 91.157 lets you operate under special VFR weather minimums, instead of 91.155, below 10,000 MSL within the airspace contained by the upward extension of the lateral boundaries of the controlled airspace designated to the surface for an airport — and not at the 12 airports listed in appendix D, section 3, where special VFR is restricted. You need an ATC clearance, must remain clear of clouds, and (except helicopters) need at least 1 statute mile flight visibility. Except for helicopters, night special VFR also requires an instrument-rated and current pilot and 91.205(d) instrument equipment. Takeoff and landing (other than helicopters) need 1 statute mile ground visibility, or 1 statute mile flight visibility if ground visibility is not reported. That is the whole private-pilot box. The airspace chapter works the Class B/C/D/E surface-area geometry; this chapter only needs you not to confuse special VFR with a 91.159 altitude.