SETTING 29 POINT 98 INCHES INSTEAD OF 998 MILLIBARS RESULTS IN DSCNT 350 FT BELOW CLRED ALT AND BELOW THE MSA DEPICTED ON THE ARPT APCH CHARTS AT BRUSSELS.

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OUR FLT WAS CLRED TO 2000 FT WITH AN ALTIMETER OF 998. THE CAPT AND FO BOTH MISTAKENLY SET 29.98 INCHES INSTEAD OF 998 MILLIBARS. I WAS THE RELIEF PLT AND DIDN'T CATCH THE ERROR. AS WE DSNDED TO 2000 FT; WE ACTUALLY WENT A LITTLE LOWER DUE TO THE INCORRECT ALTIMETER SETTING. ATC ADVISED US TO MAINTAIN 2000 FT. THIS CLUED US IN TO THE MISTAKEN ALTIMETER SETTING. THE CAPT AND FO SET 998 MILLIBARS AND SAW THAT WE WERE NOW BELOW 2000 FT. WE MADE AN IMMEDIATE CORRECTION BACK TO 2000 FT. THIS IS THE CLASSIC ALTIMETER ERROR THAT WE ARE TAUGHT TO BE AWARE OF. SUPPLEMENTAL INFO FROM ACN 565880: WE WERE OVER 350 FT LOW WHEN APCH QUERIED US AS TO OUR ALT. THIS EVENT OCCURRED ON A WX DIVERT AFTER FLYING ALL NIGHT. I; THE CAPT; HAD NOT FLOWN TO EUROPE SINCE APR/XX/02 AND IT HAD BEEN 4 YRS SINCE MY LAST EUROPEAN TRIP PRIOR TO THIS TRIP. IT WAS THE CLASSIC 998 VERSUS 29.98 ERROR. I FELT THAT THIS EVENT WAS CRITICAL AND SHOULD BE ADDED TO THE LONG LIST OF ALTIMETER SETTING ERRORS. THIS COULD HAVE BEEN A MUCH MORE SERIOUS/FATAL ERROR IN A DIFFERENT SCENARIO. CONTRIBUTING TO THIS WAS THE FACT THAT WE HAD DIVERTED TO BRU AND WE WERE ALL FATIGUED AFTER THE LONG FLT COUPLED WITH HOLDING AT LONDON -- FOLLOWED BY A DIVERT TO BRU.