AS MLG HAS AN ALT EXCURSION IN ZDC AIRSPACE BECAUSE OF THE PF MISTAKES A HDG FOR AN ASSIGNED ALT.

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I; AS THE CAPT; WAS THE PNF. CTR ASKED US TO TURN L TO HDG 330 DEGS. I READ BACK THE CLRNC AND VERIFIED IT WITH THE FO WHO WAS THE PF. THE VERY NEXT MOMENT; MY ATTN WAS DISTR BY THE INTERPHONE TO THE FLT ATTENDANT IN THE BACK. AS I WAS TALKING TO HER; CTR CAME UP ON THE RADIO AND ASKED US TO MAINTAIN FL350. I LOOKED UP AT THE ALTIMETER AND LOW AND BEHOLD I SAW US DSNDING THROUGH FL346. I GRABBED THE YOKE AND DISCONNECTED THE AUTOPLT AND CLBED US BACK TO FL350. I ASKED THE FO WHY HE BEGAN A DSCNT. HE TOLD ME THAT HE HAD NOT HEARD THE CTR'S CLRNC; AND WHEN HE LOOKED AT ME HE HEARD ME SAY '340;' SO HE THOUGHT THAT MEANT DSND TO FL340. I TOLD HIM WE NEVER USED '340' AS AN ALT. HE SUDDENLY REALIZED THIS AND SAID HE JUST HAD NOT HEARD THE CLRNC. I TOLD HIM THAT THE NEXT TIME HE DOESN'T HEAR A CLRNC; PLEASE LET ME KNOW SO WE CAN VERIFY IT. I FAULT MYSELF FOR NOT WATCHING HIM AND ALLOWING MYSELF TO BECOME DISTR. IT HAPPENED SO QUICKLY. SUPPLEMENTAL INFO FROM ACN 413649: ...AND HEARD HIM SAY '340.' I INCORRECTLY UNDERSTOOD THAT TO MEAN FL340 AND ENTERED THE ALT FL340 INTO THE MCP AND ON THE CRUISE PAGE OF THE FMC. WE STARTED DSNDING 1000 FT AND AT APPROX FL346 ZDC INSTRUCTED US TO MAINTAIN FL350 AND TURN L TO A HDG OF 340 DEGS. THE CAPT LOOKED UP FROM THE PAPERWORK AND WE DISCONNECTED THE AUTOPLT; CLBED BACK TO FL350 AND COMMENCED A TURN L TO A HDG OF 340 DEGS. THE REMAINDER OF FLT WAS UNEVENTFUL. IN DISCUSSION WITH THE CAPT AND REFLECTION ON WHAT OCCURRED; I BELIEVE THE PRIMARY CAUSE WAS TEMPORARY BREAKDOWN IN CREW COORD IN CARRYING OUT STANDARD PROCS IN SETTING AN ALT/FLT LEVEL INTO THE MCP. OUR COMPANY'S SOP FOR SETTING ALTS/FLT LEVELS IS FOR THE PF TO SET THE ALT AND FOR PLT MONITORING TO AUDIBLY VERIFY THAT THE CORRECT ALT/FLT LEVEL WAS ENTERED. ADDITIONALLY; COMPLACENCY ON MY PART CONTRIBUTED BECAUSE I ALLOWED MYSELF TO GET IMMERSED IN NON FLYING ACTIVITY; PARTICULARLY WHEN HAVING JUST RETURNED TO THE FLT DECK AND NOT READJUSTING HEADSET VOLUME AS WELL AS SITUATIONAL AWARENESS; IE; POS OF THE ACFT. ALSO; FL340 IS NOT A STANDARD CRUISING FLT LEVEL; WHICH SHOULD HAVE SET OFF A WARNING FLAG IN MY OWN MIND. FINALLY; I SHOULD HAVE; AT A MINIMUM; QUERIED THE CAPT ON WHAT THE CLRNC WAS GIVEN THAT I DID NOT HEAR THE RADIO XMISSION OR RESPONSE; AND I SHOULD HAVE REPEATED THE FLT LEVEL/ALT I SET IN THE MCP UNTIL IT ELICITED A RESPONSE FROM THE CAPT; WHICH WOULD HAVE BROKEN THE CHAIN AND PREVENTED THE OCCURRENCE.