What was found
WHILE CLIMBING THROUGH 20,000 FEET, THE PILOT NOTED ERRATIC PRESSURIZATION WITH NO RATE CONTROL. INVESTIGATION REVEALED THE OUTFLOW VALVE HAD FAILED INTERNALLY WHEN THE 146067-2, DIAPHRAGM-RETAINING RING CRACKED. THE SAFETY VALVE WAS INSPECTED AND FOUND TO HAVE FAILED IN THE SAME MANNER AS THE OUTFLOW VALVE. THE DIAPHRAGM-RETAINING RING IN THE SAFETY VALVE IS DATE STAMPED MAY 1980. THE DATE STAMP ON THE DIAPHRAGM-RETAINING RING IN THE OUTFLOW VALVE IS UNREADABLE. BOTH OUTFLOW AND SAFETY VALVES ARE DATE STAMPED JANUARY 1985. BOTH VALVES ARE APPARENTLY THE ORIGINAL VALVES IN THE AIRCRAFT. (X)