7.ifr-cruise-altitudes. IFR Cruising Altitudes
Under instrument flight rules in controlled airspace, the altitude or flight level at which an aircraft cruises is assigned by ATC. In uncontrolled airspace, or any time ATC issues a clearance to maintain "VFR-on-top," the pilot is responsible for selecting an appropriate IFR cruising altitude in accordance with 14 CFR 91.179. Understanding the hemispheric rule is essential because it provides the basic 1,000-foot vertical separation between opposing IFR traffic that the en route structure depends on.
The Hemispheric Rule (Below 18,000 ft MSL)
When operating IFR in level cruising flight below 18,000 feet MSL, the magnetic course (ground track) determines which altitudes are legal:
- Magnetic course 0° through 179° (generally eastbound): any odd thousand foot MSL altitude — 3,000; 5,000; 7,000; 9,000; 11,000; etc.
- Magnetic course 180° through 359° (generally westbound): any even thousand foot MSL altitude — 2,000; 4,000; 6,000; 8,000; 10,000; etc.
Note that the rule is based on magnetic course, not magnetic heading. Wind correction angle does not change the altitude selection. Note also that VFR cruising altitudes (the +500-foot rule of 91.159) do not apply to IFR flight; IFR aircraft cruise on the even thousands.
In Class A Airspace (18,000 ft MSL through FL600)
Class A is exclusively IFR. From 18,000 feet MSL up to and including FL410, in airspace designated as Reduced Vertical Separation Minimum (RVSM), aircraft and operators that are RVSM-authorized may be assigned 1,000-foot vertical separation using the same odd/even hemispheric convention expressed as flight levels:
- Eastbound (000°–179°): FL190, FL210, FL230 ... FL410 (odd)
- Westbound (180°–359°): FL180, FL200, FL220 ... FL400 (even)
For non-RVSM operations above FL290, separation reverts to 2,000 feet:
- Eastbound: FL290, FL330, FL370, FL410, FL450 ...
- Westbound: FL310, FL350, FL390, FL430 ...
Domestic U.S. RVSM is now applied between FL290 and FL410 inclusive, so most turbojet traffic in this band is separated by 1,000 feet. Above FL410 the 2,000-foot/4,000-foot non-RVSM scheme resumes.
When Pilots Actually Choose the Altitude
In practice, ATC assigns the cruise altitude in your IFR clearance, so the hemispheric rule is satisfied automatically. The pilot must apply 91.179 personally in three situations:
- IFR in uncontrolled airspace (Class G). Below the floor of controlled airspace, ATC is not separating traffic, but an IFR pilot must still cruise at an altitude consistent with the rule.
- VFR-on-top. A pilot on an IFR clearance may request "VFR-on-top" in lieu of a hard altitude. While VFR-on-top, the pilot flies at any VFR cruising altitude appropriate for the direction of flight (per 91.159: odd-thousand +500 eastbound, even-thousand +500 westbound) and complies with VFR cloud clearances, but otherwise remains on the IFR flight plan.
- Flight plan filing. The altitude listed in the flight plan must comply with 91.179, even though ATC may amend it.
Selecting an Appropriate Altitude
Several practical factors influence the request:
- Minimum IFR altitudes (91.177). The selected altitude must be at or above the MEA, MOCA (within 22 NM of the VOR for off-route navigation), or OROCA, as applicable, and high enough to clear obstacles by 1,000 feet (2,000 feet in designated mountainous terrain).
- Winds aloft and fuel burn. Higher altitudes generally yield better true airspeed and fuel specifics for turbine aircraft; piston aircraft trade climb fuel against tailwind component.
- Icing and weather. Pick an altitude that keeps you out of known icing or convective layers; freezing level forecasts and PIREPs drive this choice.
- Oxygen requirements (91.211). Required flight crew oxygen above 12,500 ft MSL after 30 minutes; continuous above 14,000 ft; supplemental for passengers above 15,000 ft.
- Terrain and lost-comm planning. Under 91.185, the altitude flown after lost comm is the highest of the MEA, the expected altitude advised by ATC, or the assigned altitude — pick a cruise altitude that keeps this manageable.
Examples
- Filed direct from KMEM (Memphis) to KATL (Atlanta), magnetic course about 105° — choose 5,000, 7,000, or 9,000 (odd).
- Filed KDEN to KSLC, magnetic course about 265° — choose 12,000, 14,000, or 16,000 (even). With terrain along the route, 16,000 is more realistic to satisfy MEAs.
- Cleared "climb and maintain VFR-on-top" eastbound — level off at 7,500, 9,500, or 11,500 (VFR cruising altitudes).
Knowing 91.179 cold prevents the embarrassing readback of an illegal altitude and helps you anticipate what ATC is likely to assign or amend on long cross-country IFR legs.