What was found
DURING CLIMB, THE CREW NOTICED THE CABIN ALTITUDE CLIMBING. FOLLOWING PROCEDURES IN THE AIRCRAFT FLIGHT MANUAL, THE CREW NOTIFIED ATC OF THE PROBLEM AND DESCENDED TO A LOWER ALTITUDE AND RETURNED TO YIP. MAINTENANCE PERSONNEL FOUND THE PRESSURIZATION DUCT TEMPERATURE LIMITER FLANGE BROKEN AND LEAKING. VALVE REPLACED WITH SERVICEABLE VALVE AND SYSTEM OPERATION TESTED. ALL WORK PERFORMED IN ACCORDANCE WITH THE LEARJET 35/36/35A/36A MAINTENANCE MANUAL.