Regulation Text
Unless otherwise authorized by ATC, the following rules apply—
(a) In controlled airspace. Each person operating an aircraft under IFR in level cruising flight in controlled airspace shall maintain the altitude or flight level assigned that aircraft by ATC. However, if the ATC clearance assigns “VFR conditions on-top,” that person shall maintain an altitude or flight level as prescribed by § 91.159.
(b) In uncontrolled airspace. Except while in a holding pattern of 2 minutes or less or while turning, each person operating an aircraft under IFR in level cruising flight in uncontrolled airspace shall maintain an appropriate altitude as follows:
(1) When operating below 18,000 feet MSL and—
(i) On a magnetic course of zero degrees through 179 degrees, any odd thousand foot MSL altitude (such as 3,000, 5,000, or 7,000); or
(ii) On a magnetic course of 180 degrees through 359 degrees, any even thousand foot MSL altitude (such as 2,000, 4,000, or 6,000).
(2) When operating at or above 18,000 feet MSL but below flight level 290, and—
(i) On a magnetic course of zero degrees through 179 degrees, any odd flight level (such as 190, 210, or 230); or
(ii) On a magnetic course of 180 degrees through 359 degrees, any even flight level (such as 180, 200, or 220).
(3) When operating at flight level 290 and above in non-RVSM airspace, and—
(i) On a magnetic course of zero degrees through 179 degrees, any flight level, at 4,000-foot intervals, beginning at and including flight level 290 (such as flight level 290, 330, or 370); or
(ii) On a magnetic course of 180 degrees through 359 degrees, any flight level, at 4,000-foot intervals, beginning at and including flight level 310 (such as flight level 310, 350, or 390).
(4) When operating at flight level 290 and above in airspace designated as Reduced Vertical Separation Minimum (RVSM) airspace and—
(i) On a magnetic course of zero degrees through 179 degrees, any odd flight level, at 2,000-foot intervals beginning at and including flight level 290 (such as flight level 290, 310, 330, 350, 370, 390, 410); or
(ii) On a magnetic course of 180 degrees through 359 degrees, any even flight level, at 2000-foot intervals beginning at and including flight level 300 (such as 300, 320, 340, 360, 380, 400).
[Docket 18334, 54 FR 34294, Aug. 18, 1989, as amended by Amdt. 91-276, 68 FR 61321, Oct. 27, 2003; 68 FR 70133, Dec. 17, 2003; Amdt. 91-296, 72 FR 31679, June 7, 2007]
Research Notes
Section 91.179 sets IFR cruising altitudes — the hemispheric rule for IFR aircraft.
Below FL290 (excluding Class A):
- Magnetic course 0° through 179° (basically east): ODD thousands (3,000, 5,000, 7,000, 9,000, etc.)
- Magnetic course 180° through 359° (basically west): EVEN thousands (2,000, 4,000, 6,000, 8,000, etc.)
Above FL290 (under RVSM): Standard 1,000-foot vertical separation per § 91.180. Eastbound: odd flight levels (FL310, FL330, etc.). Westbound: even (FL320, FL340, etc.).
Non-RVSM operations above FL290: 2,000-foot vertical separation. Eastbound: FL290, FL330, FL370, etc. Westbound: FL310, FL350, FL390, etc.
Below 18,000 MSL vs above: Below 18,000 MSL the altitude is expressed in feet MSL with local altimeter setting. At and above 18,000 MSL the altitude is expressed as Flight Level on 29.92 setting.
Why this matters: The 500-foot offset between IFR (full thousands) and VFR (thousands + 500) gives 500-foot vertical separation between VFR and IFR traffic on opposite tracks. Combined with the hemispheric rule, the structure dramatically reduces midair risk.
Reference: AIM 3-1-5.
Amendment History
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