What was found
(CAN) CLIMBING THROUGH 28,000FT THE CREW HEARD A LOUD POP FROM THE BACK OF THE AIRCRAFT FOLLOWED BY A LOSS OF PRESSURIZATION AND A CABIN CLIMB RATE OF 2400 FT/MIN. OXYGEN MASKS WERE MANUALLY DROPPED AND AN IMMEDIATE DESCENT MADE TO 10,000FT. UPON INVESTIGATION MAINTENANCE FOUND THE AIR CYCLE MACHINE HOT AIR BYPASS DUCT LOWER FLEXI JOINT HAD BLOWN OFF THE DUCT CAUSING THE LOUD POP AND LOSS OF PRESSURIZATION. FROM THE PICTURE IT CAN BE SEEN THAT THE FLEXI JOINT LOWER CLAMP IS STILL ATTACHED TO THE JOINT AND STAYED ATTACHED TO THE JOINT DURING SEPARATION. UPON FURTHER CHECKS IT WAS FOUND THAT THE CLAMP WAS TIGHT ENOUGH TO STAY ON THE JOINT BUT LOOSE ENOUGH TO ALLOW THE JOINT TO BLOW PAST THE BEAD ON THE DUCT. THE UPPER CLAMP AND THE 2 CLAMPS ON THE SECOND FLEXI JOINT ON THE BYPASS DUCT WERE CHECKED AND FOUND TO REQUIRE 1 TURN TO BE TIGHT. AS A PRECAUTION A SECOND C560 IN OUR FLEET WAS CHECKED, THIS WAS A 2007 MODEL WITH 1780HRS SINCE NEW, THE SAME CLAMPS IN THAT AIRCRAFT REQUIRED 1.5 TURNS TO BE TIGHT.