Unless otherwise authorized by ATC, no person may operate an aircraft
within controlled airspace under IFR except as follows:
(a) On a Federal airway, 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.