CABIN DOOR OR DOOR SEAL FAILURE ON ASTRA SPX RESULTS IN EMER DSCNT AND DIVERSION.

Narrative

THE ACFT WAS ON AUTOPLT AT FL430. ABOUT 45 MINS INTO THE FLT; THE 'CABIN DOOR' CAUTION LIGHT ILLUMINATED ON THE ANNUNCIATOR PANEL. THE PNF READ THE CHKLIST FOR THAT ITEM. THE CHKLIST SAID TO REMAIN BELOW FL390 IF NO PRESSURIZATION PROBS WERE PRESENT. (NONE WERE AT THE TIME AND THE ACFT WAS HOLDING PRESSURIZATION WITH A CABIN ALT OF 7500 FT.) WE REQUESTED LOWER ALT WITH ATC AND WERE GIVEN FL380. I BEGAN A DSCNT ON AUTOPLT WITH A VERT SPD OF APPROX 2000 FPM. AT ABOUT FL420 AND DSNDING; A LOUD AUDIBLE SQUEAL WAS HEARD FROM THE MAIN CABIN DOOR. THE SQUEAL BECAME VERY LOUD FOLLOWED BY A WHOOSHING SOUND AND PHYSICAL RUSH OF AIR. I LOOKED AT THE CABIN PRESSURIZATION INDICATOR. CABIN ALT VERT SPD/VSI WAS PEGGED AT +6000 FPM; THE AIRPLANE WAS DEPRESSURIZING VERY RAPIDLY. THE RED 'CABIN ALT' LIGHT ILLUMINATED ON THE ANNUNCIATOR PANEL; MEANING CABIN ALT WAS ABOVE 10000 FT MSL. WE INITIATED A RAPID DSCNT TO A SAFE ALT AND DECLARED AN EMER. ONCE STABILIZED AND LEVEL; WE ASSESSED THE SIT AND DETERMINED THE ACFT HAD NO STRUCTURAL PROBS. WE DIVERTED TO ZZZ. THE CABIN OXYGEN MASKS DROPPED. THE CABIN ALT REACHED A MAX OF APPROX 25000 FT.