Definition
14 CFR 91.159 is the Federal Aviation Regulation that prescribes the cruising altitudes pilots must use when operating under VFR in level cruising flight more than 3,000 feet above the surface. It assigns specific altitudes based on magnetic course: on a magnetic course of 0° through 179°, pilots must fly at an odd thousand-foot MSL altitude plus 500 feet (for example 3,500, 5,500, 7,500); on a magnetic course of 180° through 359°, pilots must fly at an even thousand-foot MSL altitude plus 500 feet (for example 4,500, 6,500, 8,500). The rule applies up to and including 18,000 feet MSL, with separate provisions above that.
Plain English
This is the rule that tells VFR pilots which altitude to cruise at once they are more than 3,000 feet above the ground. The altitude depends on which way they are heading. Eastbound (heading roughly north through south on the compass, 0–179°) you fly an odd thousand plus 500 feet. Westbound (180–359°) you fly an even thousand plus 500 feet. The whole point is to keep opposite-direction traffic at different altitudes so they don't meet head-on.
Context Anchor
Seen in discussions of IFR cruising altitude, VFR-on-top, and altitude selection rules in FAA handbooks and regulations.
Derivation
The number is a legal citation, not a word origin. “91” means the rule is in 14 CFR Part 91, which covers general operating and flight rules. “159” is the specific section number within that part.
Why Pilots Care
Knowing this regulation helps IFR pilots anticipate where VFR traffic is likely to be and maintain safe vertical separation.
Intuition Check
Do not read “91.159” as just a page or reference number. It points to a specific federal operating rule that can directly control what altitude you may use.
Example Sentence 1
Planning a VFR cross-country on a magnetic course of 090°, the pilot selected 7,500 feet MSL to comply with 91.159.
Example Sentence 2
While on an IFR clearance the controller noted nearby traffic complying with 91.159 at 5,500 feet.