What was found
AIRCRAFT RETURNED OF FLIGHT AS A CONSEQUENCE OF DURING CLIMB AT 33,000 FEET, AFT CARGO DOOR AND AFT STAIRWAY DOOR LIGHTS TURNED ON AND CABIN STARTED TO DEPRESSURIZE. CARGO COMPARTMENT DOORS WERE CHECKED, ALL CORRECT. AIRCRAFT WAS PRESSURIZED WITH A GROUND PNEUMATIC UNIT NO LEAK WAS FOUND. ENGINES PRESSURIZATION TESTS WAS PERFORMED, ALL CORRECT. AS A PREVENTIVE ACTION PRESSURIZATION CONTROLLER NR ONE WAS REPLACED. DURING OVERNIGHT SERVICE TESTS TO PRESSURIZATION SYSTEM WERE PERFORMED IN ACCORDANCE WITH MM 21-32-03, 52-31-00 AND 52-63-00, ALL CORRECT.