AN ACR IN THE EUROPEAN OP SET AN ALTIMETER TO INCHES OF MERCURY INSTEAD OF HECTOPASCALS WHEN BELOW THE TRANSITION LEVEL. THE GPWAS WENT OFF BECAUSE OF LOW ACTUAL ALT.

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ON APCH TO RWY 14 ZURICH; SWITZERLAND; CLRED TO MAINTAIN 4000 FT AND CLRED FOR THE ILS RWY 14 APCH. CTLR EXPRESSED CONCERN WITH OUR ALT AT ABOUT THE SAME TIME AS A TERRAIN ALERT OCCURRED ON GPWS. WE WERE ON TOP OF AN OVCST LAYER; CLRLY NOT IN ANY DANGER OF A COLLISION WITH TERRAIN AS NO TERRAIN WAS VISIBLE ABOVE THE OVCST LAYER AND WE WERE IN LEVEL FLT. I ASKED THE CTLR TO CONFIRM THE ALTIMETER SETTING OF 29.91 IN HG. SHE STATED THE ALTIMETER WAS 991 MB. THE RELIEF PLT HAD COPIED THE ATIS ALTIMETER AS 29.92 SETTING US UP TO HEAR 299X. THERE CLRLY WAS NO REASON TO GO AROUND AS THE CORRECT ALTIMETER SETTING WAS IN; WE WERE RECLRED FOR THE APCH AND 900 FT ABOVE THE CLOSEST TERRAIN AT OUR ACTUAL ALT OF 3400 FT. IN THE FUTURE; WE NEED TO ENSURE WE LISTEN FOR THE ALTIMETER SETTING IN MB IN EUROPE AND NOT LET OUR EARS LEAD US TO THE WRONG SETTING OF 299X IN HG BECAUSE 99X MB SOUNDS LIKE WHAT WE'RE USED TO HEARING. AND THE CTLR SHOULD LISTEN CAREFULLY TO THE READBACK BY CREWS BASED IN COUNTRIES THAT USE 'INCHES HG' AS THEIR PRIMARY ALTIMETER SETTING REF; ESPECIALLY WHEN THE ALTIMETER SETTING IS 99X MB.