Definition
14 CFR 91.181 is the Federal Aviation Regulation titled 'Course to be flown,' which requires that, unless otherwise authorized by ATC, no person may operate an aircraft within controlled airspace under IFR except along the centerline of a Federal airway, or, on any other route, along the direct course between the navigational aids or fixes defining that route. Reasonable deviations are permitted for safe operation, weather avoidance, or other necessary maneuvering.
Plain English
This rule says that when you're flying on instruments in controlled airspace, you have to stay on the centerline of the airway you've been cleared on, or fly directly between the navigation points that define your route. You can deviate when needed for safety, but otherwise you stick to the route.
Context Anchor
Seen in IFR en route navigation discussions, especially when learning how to follow airways, direct routes, and ATC clearances.
Derivation
The number is a regulation citation. “91” means the rule is in 14 CFR Part 91, which contains general operating and flight rules. “.181” identifies the specific section about the course to be flown under IFR.
Why Pilots Care
Maintaining the specified course provides the required obstacle clearance and traffic separation built into the airway or route structure.
Intuition Check
91.181 is not a route, altitude, or radio frequency. It is a rule citation that tells you how to fly an IFR route unless ATC authorizes a different course.
Example Sentence 1
During the checkride, the examiner asked the applicant to cite the regulation that requires IFR pilots to fly the centerline of a Federal airway, and the applicant correctly identified 91.181.
Example Sentence 2
When cleared direct between two fixes, 91.181 still requires the aircraft to track the precise course between them.