What was found
(CAN) THE ACFT WAS ON DEPARTURE, ENROUTE, APPROX 2 MINUTES INTO THE FLIGHT. THE FLIGHT CREW NOTICED A LOUD RUSHING AIR SOUND, WHICH WAS QUICKLY IDENTIFIED AS THE REAR DOOR COMING AJAR. AS THEY WERE FINISHING THE TURN, TO RETURN TO THE RUNWAY, REAR UPPER DOOR FLEW OPEN. THE ACFT RETURNED TO THE AIRPORT WITH NO FURTHER INCIDENT. THERE WAS SIGNIFICANT DAMAGE TO THE UPPER AND FWD HINGE (IT HAD SEPARATED). MX REPLACED THE DOOR AND DOOR HINGES PRIOR TO FURTHER FLIGHT.