What was found
(CAN) DURING TAKE OFF FOR A FERRY FLIGHT (NO PAX) A/C EXPERIENCED A PTU FAILURE. BOTH ISOLATION VALVES FUNCTIONED AS INTENDED. THE AIRCRAFT RETURNED TO DEPARTURE AIRPORT SAFELY. A/C CURRENTLY ON GROUND. IT WAS DETERMINED BY MAINTENANCE CREW THAT THE PTU FAILED DUE TO INTERNAL SEAL FAILURE AND RUPTURE OF THE PTU DRAIN LINE AT THE PTU CASING. PARTS HAVE BEEN PLACED ON ORDER FOR RECTIFICATION OF THE DEFECT.