14 CFR 91.181 : Aeronautics And Space (§ 1.1 To Parts 1311-1399) — (§ 60.1 To Parts 108-109) — Federal Aviation Administration, Department Of Transportation (Continued) (§ 60.1 To Parts 108-109) — Air Traffic And General Operating Rules (§ 91.1 To Parts 108-109) — General Operating And Flight Rules (§ 91.1 To Appendix G To Part 91) — Flight Rules (§ 91.101 To §§ 91.195-91.199) — Instrument Flight Rules (§ 91.167 To §§ 91.195-91.199) — Course to be flown.

Unless otherwise authorized by ATC, no person may operate an aircraft within controlled airspace under IFR except as follows:

(a) On an ATS route, along the centerline of that airway.

(b) On any other route, along the direct course between the navigational aids or fixes defining that route. However, this section does not prohibit maneuvering the aircraft to pass well clear of other air traffic or the maneuvering of the aircraft in VFR conditions to clear the intended flight path both before and during climb or descent.