FLT CREW OF CL60 MISHEAR CLRED ALT AND CLB THROUGH CLRED ALT.

Narrative

ON THE LAST OF 4 LEGS WE DEPARTED WEST PALM BEACH (PBI) ENRTE WASHINGTON (IAD). THE ACFT WAS PASSED FROM PBI TWR TO DEP CTL. WHEN COMS WAS ESTABLISHED WITH PBI DEP ALL XMISSIONS FROM DEP CTL WERE VERY WEAK AND NUMEROUS READBACKS WERE REQUIRED. I WAS ASSIGNED WHAT I THOUGHT WAS 13000 FT AND SET THE ALT ALERT FOR THAT ALT. AS THE ACFT WAS PASSING THROUGH 12800 FT WE WERE INFORMED BY DEP THAT OUR ASSIGNED ALT WAS 12000 FT. THE SIC; WHO WAS FLYING THE ACFT; IMMEDIATELY DSNDED TO 12000 FT. AS THE DSCNT TO 12000 FT WAS INITIATED I TOLD THE CTLR THAT I THOUGHT WE WERE ASSIGNED 13000 FT AND SHE SAID NO IT WAS 12000 FT. I DO NOT REMEMBER THE ALT THAT I READ BACK TO HER BUT I ASSUME SINCE I SET 13000 FT IN THE ALT ALERT THAT IS WHAT I READ BACK. IN MY OPINION SOME CONTRIBUTING THINGS HERE ARE: 1) THE SIGNAL FROM DEP CTL WAS VERY WEAK. I TOLD THEM WE WERE RECEIVING THEM WEAK AT OUR INITIAL CONTACT. 2) NORMAL PROCS ARE TO USE HEADSET AND BOOM MIKE BELOW 10000 FT BUT WE WERE USING THE ACFT SPEAKER SINCE ONE HEADSET WAS INOP. 3) THE FULL DEP WAS NOT ENTERED INTO THE FMS. 4) THE OLD COMPANY POLICY WAS THAT HEADSETS WERE SUPPLIED BY THE COMPANY AND REMAINED IN THE ACFT. THE NEW POLICY IS EACH CREW MEMBER PROVIDES HIS/HER OWN HEADSET AND THEY CARRY IT WITH THEM WHEN THEY LEAVE THE ACFT. NOT ALL CREW MEMBERS HAVE PURCHASED A HEADSET AND THERE IS NO STANDARD TYPE SPECIFIED TO PURCHASE SO NOT ALL HEADSET MIKE LEVELS ARE THE SAME FOR THE INTERCOM SYS.