Narrative:

During arrival into gatwick airport, ATC cleared us from FL80 to 4000 ft and stated qnh 997 millibars. No mention of transition level was made by ATC or in ATIS. During descent, pilots incorrectly set 29.97 inches in altimeter instead of 997 millibars, which resulted in leveling off approximately 300 ft lower than assigned. ATC queried our altitude and crew realized the error and immediately climbed to 4000 ft. Time spent off altitude approximately 15-20 seconds. Remainder of flight uneventful. No conflicts with other aircraft and no evasive action needed. VMC at all times. Error occurred during a very busy phase of flight. Transition level was nonstandard for 'domestic oriented' crew. ATC controller gave us current altimeter setting but did not advise of us the transition level to revert to qnh nor was it covered in arrival ATIS information. Similarity in numbers, eg, 997 millibars and 29.97 inches, made it easy to set wrong measure of units. All 3 crew members had less than 125 hours in DC10 and less than 90 days since completing DC10 training. Last flying assignments were purely domestic in united states of america. Crew was very busy briefing autoland procedures for approach in busy terminal environment. Supplemental information from acn 469890: in descent, ATC cleared us to 4000 ft. When passing approximately 7000 ft (transition level unknown and not reported on ATIS), ATC said 'altimeter 997 qnh.' all cockpit altimeters were set to 29.97 in headquarters. At approximately 3700 ft, ATC had us check our altimeter for the correct setting. We were approximately 300 ft low and climbed back to 4000 ft using the correct setting. Time off altitude was approximately 15-20 seconds. Transition level unknown and not reported on ATIS.

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Original NASA ASRS Text

Title: DC10 CREW DSNDED BELOW CLRED ALT WHEN THEY SET QNH IN INCHES INSTEAD OF HPA.

Narrative: DURING ARR INTO GATWICK ARPT, ATC CLRED US FROM FL80 TO 4000 FT AND STATED QNH 997 MILLIBARS. NO MENTION OF TRANSITION LEVEL WAS MADE BY ATC OR IN ATIS. DURING DSCNT, PLTS INCORRECTLY SET 29.97 INCHES IN ALTIMETER INSTEAD OF 997 MILLIBARS, WHICH RESULTED IN LEVELING OFF APPROX 300 FT LOWER THAN ASSIGNED. ATC QUERIED OUR ALT AND CREW REALIZED THE ERROR AND IMMEDIATELY CLBED TO 4000 FT. TIME SPENT OFF ALT APPROX 15-20 SECONDS. REMAINDER OF FLT UNEVENTFUL. NO CONFLICTS WITH OTHER ACFT AND NO EVASIVE ACTION NEEDED. VMC AT ALL TIMES. ERROR OCCURRED DURING A VERY BUSY PHASE OF FLT. TRANSITION LEVEL WAS NONSTANDARD FOR 'DOMESTIC ORIENTED' CREW. ATC CTLR GAVE US CURRENT ALTIMETER SETTING BUT DID NOT ADVISE OF US THE TRANSITION LEVEL TO REVERT TO QNH NOR WAS IT COVERED IN ARR ATIS INFO. SIMILARITY IN NUMBERS, EG, 997 MILLIBARS AND 29.97 INCHES, MADE IT EASY TO SET WRONG MEASURE OF UNITS. ALL 3 CREW MEMBERS HAD LESS THAN 125 HRS IN DC10 AND LESS THAN 90 DAYS SINCE COMPLETING DC10 TRAINING. LAST FLYING ASSIGNMENTS WERE PURELY DOMESTIC IN UNITED STATES OF AMERICA. CREW WAS VERY BUSY BRIEFING AUTOLAND PROCS FOR APCH IN BUSY TERMINAL ENVIRONMENT. SUPPLEMENTAL INFO FROM ACN 469890: IN DSCNT, ATC CLRED US TO 4000 FT. WHEN PASSING APPROX 7000 FT (TRANSITION LEVEL UNKNOWN AND NOT RPTED ON ATIS), ATC SAID 'ALTIMETER 997 QNH.' ALL COCKPIT ALTIMETERS WERE SET TO 29.97 IN HQ. AT APPROX 3700 FT, ATC HAD US CHK OUR ALTIMETER FOR THE CORRECT SETTING. WE WERE APPROX 300 FT LOW AND CLBED BACK TO 4000 FT USING THE CORRECT SETTING. TIME OFF ALT WAS APPROX 15-20 SECONDS. TRANSITION LEVEL UNKNOWN AND NOT RPTED ON ATIS.

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.