A LEARJET 25D AT FL410 STARTING CLIMB TO FL430 HAD THE CABIN ALTITUDE WARNING SOUND AND MADE AN IMMEDIATE DSCNT CAUSED BY A FAILED ENTRANCE DOOR SEAL.

Narrative

ACFT WAS LEVEL AT FL410. COPLT WAS ASKED TO CALL CENTER FOR A CLIMB TO FL430. AS THE CLIMB WAS STARTED THE CABIN ALT WARNING AND ANNUNCIATOR WENT OFF. CAPTAIN TOLD COPLT TO PUT ON HIS MASK. CAPTAIN DONNED HIS. COPLT WAS INSTRUCTED TO TELL CENTER THAT WE WERE MAKING AN IMMEDIATE DSCNT AND THAT WE HAD A CABIN PRESSURE PROBLEM. CENTER COMPLIED. WE DSNDED AND COMPLETED THE APPROPRIATE CHKLISTS; REGAINED CONTROL OF THE CABIN AND LANDED AT DEN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS WAS NOT A RAPID LOSS OF PRESSURIZATION AS THE CABIN ALTITUDE WAS HELD AT 9500 FT. THE RPTR SAID THE CAUSE OF CABIN PRESSURE LOSS WAS THE FORWARD CABIN ENTRANCE DOOR SEAL. THE RPTR STATED THE DOOR SEAL ON ITS HORIZONTAL SURFACE SHOULD HAVE THREE SMALL HOLES TO ADMIT CABIN PRESSURE WHICH INFLATES THE SEAL BETWEEN THE DOOR AND THE DOOR FRAME. THE RPTR SAID THIS SEAL WAS A FACTORY INSTALLATION WHEN THE AIRPLANE WAS NEW AND THE THREE INFLATION HOLES WERE MISSING. THE RPTR STATED THE SEAL PERFORMED SATISFACTORY FOR 12 YEARS UNTIL THIS INCIDENT.