WDB FLC SETS ALTIMETER 29 PT 92 PRIOR TO DEPARTING THE TRANSITION ALT.

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WE WERE GIVEN A SID ON DEP FROM HEATHROW ARPT. ALT ON SID WAS 6000 FT FOR LEVEL OFF (FL'S BEGIN AT 6000 FT). CLBING THROUGH 5000-5500 FT WE DISCUSSED FL600 AND RESET OUR ALTIMETERS TO 29.92 AS WE BEGAN LEVELING OFF. WE FLEW FOR SEVERAL MINS BEFORE BEING INSTRUCTED TO CLB TO 10000 FT. THE CTLR SAID NOTHING TO US AND WE PROCEEDED ON OUR DEP. WE LATER DISCOVERED THAT WE SHOULD NOT HAVE CHANGED OUR ALTIMETERS UNTIL AFTER LEAVING 6000 FT. ACCORDING TO THE DEFINITION IN OUR PUBS; FL'S BEGIN AFTER FLYING THROUGH THE INITIAL ALT; BUT NOT IF YOU LEVEL OFF AT THE INITIAL ALT. DUE TO THE LCL ALTIMETER SETTING THAT DAY; WE WERE FLYING 500 FT LOWER (5500 FT) VERSUS 6000 FT AFTER WE RESET TO 29.92. THIS WAS THE FIRST TIME I EVER HAD TO LEVEL OFF AT THE TRANSITION LEVEL. OUR DEPS HAVE ALWAYS BEEN CLRED TO CLB TO A HIGHER ALT BEFORE REACHING THE TRANSITION ALT. NEXT TIME I WILL NOT FORGET! A FOOTNOTE ON THE SID PUT NEAR THE ALT BLOCK WOULD HELP REMIND PLTS. EXAMPLE: 6000 FT LEVEL OFF IS LCL ALTIMETER; NOT TRANSITION LEVEL OF 29.92.