##### § 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 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).