AN ACR'S HEADING IS QUESTIONED WHEN THE ACFT APPEARS TO BE FLYING THE PREVIOUSLY ASSIGNED DEP PROC FROM NARITA NEW TOKYO INTL ARPT EVEN THOUGH ATC HAD ALREADY ISSUED 2 SUBSEQUENT HEADINGS.

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POSSIBLE HEADING DEV ON DEP. FO FLYING. CLBING OUT OF NARITA ARPT ON THE NARITA REVERSE SEVEN DEP HANEDA TRANSITION RWY 16. LNAV; VNAV; AUTOPLT ENGAGED AT 1000 FT. INITIAL CLRNC ALT 8000 FT. AUTOPLT STARTED TURN AT 14 DME NRE VOR. NARITA DEP ON FREQ 124.2 ISSUED NEW CLRNC TO TURN TO HEADING OF 040 DEGS AND ACFT CLBED TO 8000 FT. AUTOPLT LEVELED ACFT AT 8000 FT AND CONTINUED TURN TO A HEADING OF 040 DEGS. NARITA DEP GAVE NEW CLRNC TO CLB TO FL350 AND HEADING 360 DEGS. FO SET 000 DEGS IN THE HEADING WINDOW AND FL350 IN ALT WINDOW. FO CALLED FOR CLB CHK PASSING 10000 FT. I DID THE CLB CHK. WHILE DOING CHK NARITA DEP ASKED WHAT IS YOUR HEADING WHILE CLBING THROUGH 12000 FT. I LOOKED AT THE HEADING 000 DEGS SET IN COMMAND WINDOW AND REPLIED HEADING 360 DEGS. ACFT HAD PASSED HEADING 360 DEGS AND WAS STARTING TO INTERCEPT 314 DEG COURSE INBOUND TOWARDS NRE VOR AS DEPICTED ON THE SID. FO ENGAGED HEADING MODE AND ACFT TURNED BACK TO 360 DEGS. NO FURTHER COMMENTS FROM NRT DEP. WE BOTH MISSED THAT HEADING SELECT WAS NOT SELECTED WHEN HEADING WAS GIVEN. THE TURN ON THE SID OCCURRED AT THE SAME TIME DEP GAVE US A HEADING. THE ACFT STARTED A TURN SO WE BOTH THOUGHT HEADING WAS SELECTED HOWEVER LNAV WAS STILL ENGAGED. THE ZONE DIFFERENCE WAS A FACTOR; NEW TO AIRPLANE; BOTH PLTS. ALWAYS VERIFY EVEN IF YOU ARE PNF.