FLC OF A WDB DSNDED BELOW ASSIGNED ALT DUE TO THE FAILURE TO RESET THE STANDBY ALTIMETER DURING DSCNT FROM HIGH ALT.

Narrative

ON DSCNT INTO LONDON GATWICK; RECEIVED CLRNC TO DSND FROM FL110 TO FL90. FL90 WAS ENTERED IN ALT SELECT AS WELL AIN ALT WARNING INDICATOR (AWI). ON FLT GUIDANCE PANEL; ALT SELECT WAS SELECTED. AS ACFT WAS PASSING THROUGH 8800 FT INSTRUCTOR CAPT (OCCUPYING R SEAT) NOTED ACFT GONE TOO LOW; CALLED IT OUT AND CAPT UNDER INSTRUCTION (IN L SEAT) COMMENCED PULL OUT -- BOTTOMING OUT AT AN INDICATED 8700 FT ON BOTH ALTIMETERS. SIMULTANEOUS WITH THIS 'BOTTOM OUT;' ATC CALLED AND INSTRUCTED 'WE SHOW YOU 8700 FT; CLB IMMEDIATELY TO 9000 FT.' AFTER RETURNING TO 9000 FT WE SET ABOUT TO DETERMINE WHAT HAD CAUSED THIS PROB. THIS 747 USES THE CAPT'S STANDBY ALTIMETER TO MAINTAIN SELECTED CRUISE FLT LEVEL. AS A RESULT; IF AN ACFT LEVELS OFF HIGH OR LOW; THE CAPT WILL CHANGE THE ALTIMETER SETTING FROM 29.92 IN THE KOLSMAN WINDOW TO DRIVE THE ALTIMETER INDICATOR HIGHER OR LOWER TO GET THE RPTING MAIN ALTIMETER TO READ AT THE CORRECT FLT LEVEL. SURE ENOUGH; ON THE FLT; THIS PROC HAD BEEN USED AT FL350 TO KEEP THINGS RPTING OK. AT THE TIME OF THE INCIDENT; THE STANDBY ALTIMETER HAD 29.99 RATHER THAN 29.92. WE CORRECTED THIS ALTIMETER SETTING TO 29.92 AND THE ACFT ACQUIRED 9000 FT JUST FINE. HOW TO FIX THIS PROB? PERHAPS A PROCEDURAL REQUIREMENT TO RETURN STANDBY ALTIMETER TO 29.92 'AT TOP OF DSCNT' BY THE WAY: SO (ME) IS AN INSTRUCTOR/LINE CHK AIRMAN. CAPT IN R SEAT IS INSTRUCTOR/CHK AIRMAN FOR OUR B757 PROGRAM!