AIM Text
Altitude assignments for direction of flight follow a scheme of odd altitude assignment for magnetic courses 000-179 degrees and even altitudes for magnetic courses 180-359 degrees for flights up to and including FL 410, as indicated in FIG 4-6-1.
FIG 4-6-1
Flight Level Orientation Scheme
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-6-2.
The short answer
This AIM section is FAA advisory guidance describing the Flight Level Orientation Scheme for RVSM airspace. Altitude assignments follow odd flight levels for magnetic courses 000-179 degrees and even flight levels for magnetic courses 180-359 degrees, for flights up to and including FL 410, as shown in FIG 4-6-1. It supports the cruising-altitude rules of 14 CFR 91.159/91.179.
Research Notes
Common Questions
Which flight levels go with which magnetic course in RVSM?
The AIM specifies odd altitude (flight level) assignments for magnetic courses 000-179 degrees and even altitudes for magnetic courses 180-359 degrees, for flights up to and including FL 410, as indicated in FIG 4-6-1. This odd/even scheme keeps opposite-direction traffic vertically separated.
Up to what flight level does this odd/even orientation scheme apply?
The AIM states the Flight Level Orientation Scheme applies for flights up to and including FL 410. This matches RVSM airspace, which is implemented between FL 290 and FL 410 inclusive, where the one-thousand-foot vertical separation standard is applied.
A westbound flight is on a magnetic course of 270 degrees, so is it odd or even?
Even. Under the AIM scheme, magnetic courses 180-359 degrees are assigned even flight levels, so a 270-degree course uses even FLs. Magnetic courses 000-179 degrees are assigned odd flight levels. The note in FIG 4-6-1 restates this odd/even orientation by magnetic course.