Narrative:

On descent into london gatwick, received clearance to descend from FL110 to FL90. FL90 was entered in altitude select as well ain altitude warning indicator (awi). On flight guidance panel, altitude select was selected. As aircraft was passing through 8800 ft instructor captain (occupying right seat) noted aircraft gone too low, called it out and captain under instruction (in left 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, climb immediately to 9000 ft.' after returning to 9000 ft we set about to determine what had caused this problem. This 747 uses the captain's standby altimeter to maintain selected cruise flight level. As a result, if an aircraft levels off high or low, the captain will change the altimeter setting from 29.92 in the kolsman window to drive the altimeter indicator higher or lower to get the reporting main altimeter to read at the correct flight level. Sure enough, on the flight, this procedure had been used at FL350 to keep things reporting 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 aircraft acquired 9000 ft just fine. How to fix this problem? Perhaps a procedural requirement to return standby altimeter to 29.92 'at top of descent' by the way: so (me) is an instructor/line check airman. Captain in right seat is instructor/check airman for our B757 program!

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Original NASA ASRS Text

Title: 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!

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.