FAR 91.179 — IFR Cruising Altitudes
FAR 91.179 sets IFR cruising altitudes and flight levels. Learn ATC-assigned altitudes, hemispheric rules, and RVSM levels for your IFR checkride.
In Plain English
FAR 91.179 tells you what altitude to fly when cruising under IFR. The big idea: in controlled airspace, you fly whatever altitude ATC assigns you. If ATC clears you for "VFR conditions on-top," you instead use the VFR cruising altitude rules from § 91.159.
In uncontrolled airspace (and you're not in a holding pattern of 2 minutes or less, or turning), you pick an altitude based on your magnetic course:
- Below 18,000 ft MSL:
- Course 0°–179° → odd thousands (3,000; 5,000; 7,000)
- Course 180°–359° → even thousands (2,000; 4,000; 6,000)
- 18,000 ft MSL up to FL290:
- 0°–179° → odd flight levels (FL190, FL210, FL230)
- 180°–359° → even flight levels (FL180, FL200, FL220)
- FL290 and above, non-RVSM: 4,000-ft intervals (eastbound FL290, 330, 370; westbound FL310, 350, 390)
- FL290 and above in RVSM airspace: 2,000-ft intervals (eastbound FL290, 310, 330…; westbound FL300, 320, 340…)
This matters because correct altitude selection in non-radar or lost-comm situations keeps you vertically separated from opposite-direction traffic.
Regulation Text
14 CFR § 91.179§ 91.179 IFR cruising altitude or flight level.
Unless otherwise authorized by ATC, the following rules apply—
(a)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)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]
Oral Exam Questions a DPE Might Ask
Q1When you're operating IFR in controlled airspace, what altitude are you required to maintain in level cruise?
Per FAR 91.179(a), I must maintain the altitude or flight level assigned by ATC. If they clear me for "VFR conditions on-top," I instead use the VFR cruising altitudes in § 91.159.
Q2You're IFR in uncontrolled airspace at 7,500 feet MSL on a magnetic course of 270°. Is that legal?
No. Under FAR 91.179(b)(1), on a westbound course (180°–359°) below 18,000 ft MSL I must fly an even thousand-foot altitude, such as 6,000 or 8,000 — not 7,500.
Q3How do IFR cruising altitudes change in RVSM airspace at and above FL290?
Per FAR 91.179(b)(4), in RVSM airspace flight levels are spaced at 2,000-ft intervals — odd FLs eastbound (FL290, 310, 330…) and even FLs westbound (FL300, 320, 340…), versus 4,000-ft intervals in non-RVSM airspace.
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Related Sections in Part 91