What was found
CABIN BEGAN TO CLIMB AT 2000 FPM DURING CRUISE AT 12,500 FT. DESCENT TO 10,500 FT. CABIN WOULD ONLY MAINTAIN 2.5 PSID. CHECKED OUTFLOW AND SAFETY VALVES, MOISTURE DRAINS, LANDING GEAR BOOTS, FAILEADS ALL FOUND OK. PERFORM GROUND CHECK OF SYSTEM FOUND NO AUDIBLE LEAKS ELSEWHERE IN CABIN AND DIFFERENTIAL CHECKED OK. PERFORMED SECOND GROUND CHECK AND PULLED CABIN AIR VALVES ONE AT A TIME ISOLATED PROBLEM TO RIGHT SOURCE. FOUND DUCT PARTIALLY BLOWN OFF OF SONIC VENTURI. REMOVED DUCT AND FOUND .7500 INCH TEAR WHERE IT PASSES THROUGH WING RIB. MFG NO LONGER STOCKS A 33.5 DUCT, ONLY 38 INCH. -38 CUT DOWN IN THE FIELD AND INSTALLED BOTH WINGS. LT DUCTING REPLACED MARCH 2006. RT DUCTING WAS FOUND AIRWORTHY AT T