Narrative:

On autoplt/FMC altimeter was misset in descent to 9000' maximum allowable. Aircraft leveled off at approximately 9700' and entered holding at pivot. Outbnd, landing checklist was began and 'altimeter' is first item. Correct setting put us at 9700' vice 9000' we resumed descent to 9000'. Some confusion as another TCAS aircraft was querying ATC about our altitude and we were trying to find out what our altitude was. Our company uses 3 altimeters with the middle one set at MSL and the ca/first officer set at afl. The captain set the MSL in his altimeter and the afl in the center so the autoplt leveled off at 9000' plus 667' (ord field elevation). 9000' indicated. Factors. All crew were busy. Copilot 'flying' programming FMC to enter hold, captain monitoring ord 'preapch' with approximately 10 aircraft. International relief pilot (me) on ATIS trying to find out why ord approach had refused to accept approachs (strong winds forced an unplanned change of runways). Ground of 9 hour flight from europe. Fatigue! Captain was 'behind' the aircraft after jumping up to relieve himself descending through FL240. He was buckling his seatbelt at FL180 but didn't reset his altimeter until later and then didn't set both to MSL but consulted the landing data sheet (which I prepared) and set both the MSL and afl as we were close to 10000'. Weak implementation of third altimeter in aircraft the 'center' altimeter is on the far left of a very wide cockpit. It is as inaccurate as the normal altimeters are razor accurate. Below 10000', however it is the only altimeter indicating MSL. Summary: unusual blunder by the captain not noticed by the first officer or io due to interruption of routine, fatigue, and task saturation. Note: although unique to the afl/3 altimeter system this error is very unusual. I've never seen anyone else make this mistake or even try to because the afl setting is always (except in death valley) a smaller number than MSL. Note: the first think I looked at was the TCAS to see if anyone was above us in holding. No one was, but it was nice to have that information.

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Original NASA ASRS Text

Title: ACR WDB ALT DEVIATION UNDERSHOT DURING DESCENT. FLT CREW USING ANTIQUATED COMPANY ALT SETTINGPROC DESIGNED FOR AG OPERATION AND NOT FOR INTERNATIONAL ACR OPERATION. AN ACFT IN THE AREA WITH TCAS II QUESTIONED THE ALT BRINGING THE ERROR TO THE FLT CREW'S ATTENTION. ALTIMETERS WERE SET ON QNE THEN QFE WHEN THEY SHOULD HAVE BEEN ON QNH.

Narrative: ON AUTOPLT/FMC ALTIMETER WAS MISSET IN DSNT TO 9000' MAXIMUM ALLOWABLE. ACFT LEVELED OFF AT APPROX 9700' AND ENTERED HOLDING AT PIVOT. OUTBND, LNDG CHKLIST WAS BEGAN AND 'ALTIMETER' IS FIRST ITEM. CORRECT SETTING PUT US AT 9700' VICE 9000' WE RESUMED DSNT TO 9000'. SOME CONFUSION AS ANOTHER TCAS ACFT WAS QUERYING ATC ABOUT OUR ALT AND WE WERE TRYING TO FIND OUT WHAT OUR ALT WAS. OUR COMPANY USES 3 ALTIMETERS WITH THE MIDDLE ONE SET AT MSL AND THE CA/FO SET AT AFL. THE CAPT SET THE MSL IN HIS ALTIMETER AND THE AFL IN THE CTR SO THE AUTOPLT LEVELED OFF AT 9000' PLUS 667' (ORD FIELD ELEVATION). 9000' INDICATED. FACTORS. ALL CREW WERE BUSY. COPLT 'FLYING' PROGRAMMING FMC TO ENTER HOLD, CAPT MONITORING ORD 'PREAPCH' WITH APPROX 10 ACFT. INTL RELIEF PLT (ME) ON ATIS TRYING TO FIND OUT WHY ORD APCH HAD REFUSED TO ACCEPT APCHS (STRONG WINDS FORCED AN UNPLANNED CHANGE OF RWYS). GND OF 9 HR FLT FROM EUROPE. FATIGUE! CAPT WAS 'BEHIND' THE ACFT AFTER JUMPING UP TO RELIEVE HIMSELF DSNDING THROUGH FL240. HE WAS BUCKLING HIS SEATBELT AT FL180 BUT DIDN'T RESET HIS ALTIMETER UNTIL LATER AND THEN DIDN'T SET BOTH TO MSL BUT CONSULTED THE LNDG DATA SHEET (WHICH I PREPARED) AND SET BOTH THE MSL AND AFL AS WE WERE CLOSE TO 10000'. WEAK IMPLEMENTATION OF THIRD ALTIMETER IN ACFT THE 'CTR' ALTIMETER IS ON THE FAR L OF A VERY WIDE COCKPIT. IT IS AS INACCURATE AS THE NORMAL ALTIMETERS ARE RAZOR ACCURATE. BELOW 10000', HOWEVER IT IS THE ONLY ALTIMETER INDICATING MSL. SUMMARY: UNUSUAL BLUNDER BY THE CAPT NOT NOTICED BY THE F/O OR IO DUE TO INTERRUPTION OF ROUTINE, FATIGUE, AND TASK SATURATION. NOTE: ALTHOUGH UNIQUE TO THE AFL/3 ALTIMETER SYSTEM THIS ERROR IS VERY UNUSUAL. I'VE NEVER SEEN ANYONE ELSE MAKE THIS MISTAKE OR EVEN TRY TO BECAUSE THE AFL SETTING IS ALWAYS (EXCEPT IN DEATH VALLEY) A SMALLER NUMBER THAN MSL. NOTE: THE FIRST THINK I LOOKED AT WAS THE TCAS TO SEE IF ANYONE WAS ABOVE US IN HOLDING. NO ONE WAS, BUT IT WAS NICE TO HAVE THAT INFO.

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.