§91.179

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§ 91.179 IFR cruising altitude or flight level.

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).
Part §91 Subpart B — Flight Rules
§91.101 Applicability • §91.103 Preflight • §91.105 Crewmembers at stations • §91.107 Seatbelts • §91.109 Flight instruction • §91.111 Near other aircraft
§91.113 Right-of-way • §91.115 Right-of-way on water • §91.117 Speed Limits • §91.119 Minimum altitudes • §91.121 Altimeter • §91.123 ATC instructions • §91.125 Light signals
§91.126 Class G • §91.127 Class E • §91.129 Class D • §91.130 Class C • §91.131 Class B • §91.133 Restricted areas • §91.135 Class A
§91.137 TFRs • §91.138 Hawaiian TFRs • §91.139 Emergency traffic • §91.141 Presidential TFRs • §91.143 Space ops • §91.144 High barometric pressure
§91.145 Air shows & and sporting events • §91.146 Charity flights • §91.147 Paying-passenger flights
§91.151 VFR Fuel reserves • §91.153 VFR flight plans • §91.155 VFR weather minimums • §91.157 Special VFR • §91.159 VFR cruising altitude • §91.161 DC ADIZ
§91.167 IFR Fuel reserves • §91.169 IFR flight plans • §91.171 VOR test • §91.173 ATC clearances • §91.175 IFR Takeoffs and landing
§91.177 IFR Minimum altitudes • §91.179 IFR cruising altitudes • §91.180 RVSM • §91.181 Course flown • §91.183 IFR comm. • §91.185 IFR radio failure
§91.187 Malfunctions in IFR • §91.189 Cat II & III ops. • §91.191 Cat II & III manual • §91.193 Certain Cat II ops.