B752 CREW MISINTERPED CLRNC AND OVERSHOT ASSIGNED ALT.

Narrative

I WAS THE PF; DSNDING ON THE MILTON 3 ARR FOR LGA. WE HAD PASSED OVER VIBES INTXN AT 13000 FT. THE CAPT WAS JUST COMING BACK UP ON THE PRIMARY RADIO; AFTER COMMUNICATING WITH COMPANY ON THE SECONDARY RADIO. ATC GAVE US A CLRNC TO DIRECT BEUTY INTXN. I DID NOT CLRLY HEAR THE ATC INSTRUCTION; BUT I DID HEAR THE CAPT SAY BEUTY AT 10000 FT. THE CAPT DID NOT HAVE HIS RADIO PANEL SET UP CORRECTLY; AND I WAS DISTR AS I WATCHED AND LISTENED TO HIM MAKING THE AUDIO PANEL CORRECT FOR BOOM MIKE XMISSIONS. AT THE TIME I DID NOT KNOW THAT THE CAPT WAS STILL COMMUNICATING WITH COMPANY ON THE SECONDARY RADIO; AND THAT HE COULD NOT CLRLY HEAR BOTH COMS FROM THE COMPANY AND ATC AT THE SAME TIME. AS WE DSNDED TOWARDS 10000 FT; ATC TOLD US THAT THE CLRNC WAS 'MAINTAIN 13000 FT CLRED DIRECT BEUTY INTXN.' WE WERE AT 11100 FT AND WE WERE INSTRUCTED TO CLB TO 12000 FT. THERE WAS NO TCASII OR OTHER INDICATION OF ANY TFC CONFLICT. AS A CREW WE FAILED TO FOLLOW PROPER COM PROCS. WHEN THE CAPT STATED THAT HE WAS GOING TO COMPANY ON THE SECONDARY RADIO; I SAID THAT I HAD THE PRIMARY RADIO. IN REALITY; THE CAPT ATTEMPTED TO MONITOR AND COMMUNICATE ON BOTH RADIOS AT THE SAME TIME. IN THE CONFUSION OF HIS SWITCHING BACK AND FORTH BTWN RADIOS; I FAILED TO CLRLY UNDERSTAND THE ATC INSTRUCTION; AND WE FAILED TO CHALLENGE AND RESPOND TO ALT AND NAV FIX CHANGES. AS SOON AS THE CAPT RESPONDED TO ATC; HE SWITCHED IMMEDIATELY BACK TO THE OTHER RADIO TO FINISH COMMUNICATING WITH COMPANY. BECAUSE OF THE CONFUSION; I AM NOT ACTUALLY SURE IF PERHAPS ATC MAY HAVE MISTAKENLY GIVEN US BEUTY AT 10000 FT. THE SOLUTION TO THIS SIT IS WHEN THE RESPONSIBILITY TO COVER A RADIO IS GIVEN TO A CREW MEMBER; YOU SHOULD NOT CONTINUE TO MAKE THE COMS YOURSELF; WHILE SIMULTANEOUSLY COMMUNICATING ON ANOTHER RADIO. ADDITIONALLY THE ESTABLISHED PROC FOR CHALLENGING AND RESPONDING TO ALL NAV AND ALT CHANGES MUST BE ACCOMPLISHED BY BOTH THE PF AND PNF.