FAILURE OF CAPTAIN TO RESET HIS ALTIMETER AT THE TRANSITION LEVEL RESULTS IN ALTITUDE DEVIATION INBOUND TO EGLL.

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DSNDING TO 5000 FT DURING VECTORS FOR APCH WHILE ON AUTOPLT. MOMENTARY ALTDEV OCCURRED AND DSNDED TO APPROX 4640 FT WHEN AUTOPLT DID NOT CAPTURE 5000 FT. THE REASON FOR THE FAILURE TO CAPTURE APPEARS TO BE MY LATE ACTION TO RESET ALTIMETER TO THE LCL SETTING OF 993 MILIBARS. THE PNF CALLED OUT THE ALTIMETER SETTING AND I MISSED THE CALL DUE TO RADIO TFC. HE CALLED IT OUT AGAIN AND I RESET MY ALTIMETER AT APPROX SAME TIME AS AUTOPLT SHOULD HAVE CAPTURED 5000 FT. DISTRS CONSULTING THE LHR CONSTANT DSCNT APCH CHART AND USING VERT SPD 1200 FT DOWN RATHER THAN FLCH CONTRIBUTED TO THE PROB. SHOULD HAVE TAKEN ADVANTAGE OF THE RELIEF PLT AND HAD HER CONSULT THE CDA GRAPH SO I COULD PAY MORE ATTN TO FLYING THE ACFT. SUPPLEMENTAL INFO FROM ACN 810178: CONTRIBUTING FACTORS WERE THE AMOUNT OF TIME LAPSED BTWN THE START OF THE APCH DSCNT CHKLIST AND FINALLY REACHING THE TRANSITION LEVEL (ALMOST 30 MINS); AND THE LOW ALTIMETER SETTING. SUPPLEMENTAL INFO FROM ACN 809961: ALL 3 OF US LOOKED AT THE CONSTANT DSCNT CHART TO ARRIVE AT AN APPROPRIATE DSCNT RATE. I BELIEVE THE PREOCCUPATION WITH THIS NEW CONSTANT DSCNT APCH; WAS THE PRIMARY FACTOR IN NOT GETTING THE ALTIMETER SET AT THE APPROPRIATE TIME. I HAVE ONLY FLOWN INTO HEATHROW A FEW TIMES SINCE THE EMPHASIS ON THE CONSTANT DSCNT APCH STARTED; BUT I HAVE YET TO SEE ANYONE NOT SACRIFICE SOME PORTION OF ACFT CTL IN AN ATTEMPT TO KEEP A CONSTANT DSCNT. THE CONSTANT DSCNT APCH FORCES PLTS TO USE THE AUTOPLTS IN MODES THAT ARE NOT THE NORM. THE CAPT HAD APPROPRIATELY DISCUSSED THAT FL8000 WAS THE LOWEST USABLE FLT LEVEL BECAUSE OF THE LOW ALTIMETER; BUT WE DIDN'T DISCUSS ENSURING ALTIMETER CHANGE TO 993 MB EARLY ENOUGH TO PREVENT THIS ALTDEV.