Rules of the Air Article 42

Except as provided for in Article 43 ,Article 44 and Article 46, an aircraft shall adhere to the current flight plan or the applicable portion of a current flight plan submitted for a controlled flight unless a request for a change has been made and clearance obtained from the appropriate air traffic control unit, or unless an emergency situation arises which necessitates immediate action by the aircraft, in which event as soon as circumstances permit, after such emergency authority is exercised, the appropriate ATS unit shall be notified of the action taken and that this action has been taken under emergency authority.Unless otherwise authorized or directed by the appropriate air traffic control unit, controlled flight shall, in so far as practicable, when on an established ATS route, operate along the defined centre line of that route; or when on any other route, operate directly between the navigation facilities and/or points defining that route.An aircraft operating along an ATS route segment defined by reference to very high frequency omnidirectional radio ranges shall change over for its primary navigation guidance from the facility behind the aircraft to that ahead of it at, or as close as operationally feasible to, the changeover point where established.