§ 91.179 IFR cruising altitude or flight level.
(a) In controlled airspace. Each person operating an aircraft under IFR in level cruising flight in controlled air-space shall maintain the altitude or flight level assigned that aircraft byATC. 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, 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).