Definition
A section of Title 14 of the Code of Federal Regulations, Part 91, titled 'Course to be flown.' It requires that, unless otherwise authorized by ATC, an aircraft operated on an IFR flight plan must be flown 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. Off-course maneuvering is permitted only when necessary to pass well clear of other aircraft or for safe operation.
Plain English
When you're flying IFR, you have to stay on the centerline of the airway or on the direct line between the points that define your route. You can only deviate to avoid other traffic or for safety reasons.
Context Anchor
Seen in instrument flying discussions about en route navigation, assigned routes, and what course a pilot is expected to maintain after receiving an instrument clearance.
Derivation
14 CFR means Title 14 of the Code of Federal Regulations, the body of U.S. aviation rules. Part 91 covers general operating and flight rules. Section 91.181 is the specific paragraph addressing course to be flown.
Why Pilots Care
Compliance keeps the aircraft on its cleared route, maintains separation from other traffic, and prevents regulatory violations.
Grounding Statement
In practice, this rule is what keeps aircraft on predictable paths while they are being separated by air traffic control.
Intuition Check
Do not read this as just a legal citation with no flying meaning. It points to a specific operating rule: under instrument flight rules, fly the cleared course unless air traffic control clears you otherwise.
Example Sentence 1
Under 14 CFR 91.181, the pilot maintained the airway centerline rather than cutting the corner toward the next fix.
Example Sentence 2
Deviating from the route without ATC approval would violate 14 CFR 91.181 and risk loss of separation.