A CANADAIR CL65 AT FL280 INITIATED A RAPID DSCNT TO 10000 FT DUE TO LOSS OF CABIN PRESSURE CAUSED BY A FAILED DUCT ON THE L PACK AIR CYCLE MACHINE.

Narrative

WHILE IN CRUISE AT FL280 CREW RECEIVED A CABIN ALT CAUTION FROM CREW ALERTING SYS INDICATING CABIN ALT RISEN ABOVE 8500 FT. FO WAS FLYING; CAPT ATTEMPTED TO CONTACT ATC FOR LOWER ALT; BUT DUE TO FREQ CONGESTION WAS UNABLE FOR ABOUT A MIN. CABIN ALT CONTINUED TO RISE. CAPT AND FO DONNED OXYGEN MASKS. CAPT WENT OFF FREQ TO ADVISE FA TO DON MASK. FO WAS ABLE TO COM WITH ATC FOR LOWER ALT AND WAS ASSIGNED FL240. AT APPROX FL260; CREW ALERT SYS ANNUNCIATED A WARNING INDICATING CABIN ALT HAD RISEN ABOVE 10000 FT. CAPT RAN CHKLIST TO ASSUME MANUAL CTL OF PRESSURIZATION. FO RECEIVED ATC TO FL180 AND PROGRESSIVELY TO 10000 FT. SLIGHT BUFFET AROUSED CONCERN FOR STRUCTURAL FAILURE; BUT THIS WAS UNFOUNDED CAPT ACHIEVED MANUAL CTL AND FLT CONTINUED TO DESTINATION WITH ADVICE OF COMPANY MAINT UNEVENTFULLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF CABIN PRESSURE WAS A BROKEN PRESSURIZED DUCT ON THE AIR CYCLE MACHINE. THE RPTR SAID SOME DIFFICULTY WAS ENCOUNTERED CONTACTING ATC CTL DUE TO FREQ CONGESTION. THE RPTR SAID MANUAL CTL WAS ALSO INEFFECTIVE AND THE FLT WAS CONTINUED AT 10000 FT.