ACR LGT EXPERIENCES AN ACFT EQUIP PROB MALFUNCTION THAT MANDATES A RETURN LAND.

Narrative

ON CLBOUT THE COCKPIT HAD NO AIRFLOW AND WAS VERY WARM -- REST OF AIRPLANE (ALL OF CABIN) WAS NORMAL. AT ABOUT 30000 FT THE CABIN ALT REACHED 10000 FT WITH A WARNING LIGHT. I MADE A DSCNT AND STARTED BACK FOR MIA. CABIN ALT STOPPED AT 10000 FT AND STARTED DOWN SO A FULL EMER DSCNT WAS NOT CALLED FOR. AT AROUND 15000 FT AT EXPLOSION OF SNOW AND ICE CAME OUT OF THE COCKPIT AIR VENTS. THIS WAS FOLLOWED BY A SURGE OF AIR AND THE PRESSURIZATION INDICATIONS RETURNED TO NORMAL. WITH THE AIRPLANE LEVELED AT 11000 FT WE CONSULTED WITH COMPANY MAINT AND DISPATCH. ON THE BASIS OF THE INFO I HAD AND THE SNOW/ICE CLRING OUT; WITH DISPATCH'S CONCURRENCE; I RESUMED THE FLT TO MEX. AT FL280 PRESSURIZATION BEGAN ACTING UP AGAIN WITH A SLOW RISE IN CABIN ALT WHICH WE COULD NOT STOP. MADE A DSCNT TO 11000 FT AND RETURNED FOR AN UNEVENTFUL LNDG. THE SNOW AND ICE CLRING WITH SUCH FORCE; FOLLOWED BY NORMAL SYS INDICATIONS MADE US FEEL THE PROB WAS FIXED AFTER THE FIRST DSCNT. MAINT FOUND A BROKEN AIR CONDITIONING PACK LINE ON THE GND THAT WAS OUR PROB.