A DSNDING MD11 FAILS TO CAPTURE ITS LEVELOFF ALT OF 10000 FT WHILE APCHING FRA; FO.

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WHILE DSNDING INTO FRA THE ATIS ALTIMETER SETTING WAS VERY HARD TO UNDERSTAND. THE 2 PLTS FLYING FOUND A DISCREPANCY BTWN THE MILLIBAR READING AND INCHES OF MERCURY READING WHICH WOULD HAVE CAUSED AN ALT ERROR. I WAS TOLD TO LISTEN TO THE ATIS AND TRY TO GET A CLR READING. AS I WAS DOING THIS THE ACFT DSNDED THROUGH THE LEVELOFF ALT OF 10000 FT BY ABOUT 300 FT. FRA RADAR TOLD THE PLTS TO CONTINUE DSCNT TO FL90. NO OTHER ACFT WAS NEAR; AND NO TFC WAS DIVERTED. I THINK THE AUTOPLT WAS CONFUSED BY GOING BACK AND FORTH BTWN MILLIBARS AND INCHES OF HP I AM NOT SURE. I COULD HAVE CONFUSED 29.92 AND ACTUAL INCHES OF HP OR MILLIBARS. THE FLT CONTINUED WITHOUT INCIDENT. SUPPLEMENTAL INFO FROM ACN 392208: I REMEMBER SEEING THE AMBER 1000 FT TO GO LIGHT AND CALLED 'OUT OF 11 FOR 10.' I THEN GOT INVOLVED IN AN ALTIMETER SETTING DISCREPANCY WITH THE FO WHO WAS FLYING AND WHEN I LOOKED UP AGAIN THE ALT WAS 9700 FT. JUST AS I NOTICED THIS; THE CTLR SAID TO CHK OUR ALT AND I SAID 'YAH; THIS THING DIDN'T CAPTURE' OR SOMETHING TO THAT EFFECT. THE CTLR SAID 'THAT'S OK; JUST MAINTAIN FL90.' THIS AIRPLANE; IE; MD11 HAS FAILED A COUPLE OF TIMES TO CAPTURE AND HOLD AN ALT FOR ME; ESPECIALLY AFTER BEING GIVEN 'HIGH SPD' (BELOW 10000 FT) BY THE CTLR WHICH WAS THE CASE HERE. THERE WAS A HECTARE PASCALS (MILLIBARS)/INCHES CONFUSION BY ME AND THE FO. I THOUGHT ATIS SAID '1021 HP = 3024 INCHES. HE THOUGHT IT SAID 1031 HP = 3044 INCHES. THIS MEANT THE RELIEF PLT; WHO WAS TALKING TO A FLT ATTENDANT ABOUT WHEELCHAIRS; HAD TO LISTEN AGAIN TO ATIS. ALL THIS HAPPENED BELOW 15000 FT. THE FO WAS TAUGHT TO 'PRESET' THE BAROMETRIC SETTING TO THE LCL VALUE SO AT TRANSITION LEVEL ONLY 1 'CLICK' HAD TO BE MADE TO RESET IT. I THINK HE MAY HAVE BEEN DOING THIS AT FL110 WHEN AUTOPLT #2 (WHICH WAS FLYING THE ACFT) WAS CAPTURING THE ALT. THIS COULD HAVE 'CONFUSED' THE AUTOPLT AND IT MAY HAVE GONE INTO CWS (CTL WHEEL STEERING). IT AMAZES ME HOW WE DON'T HEAR FROM FLT ATTENDANTS FOR HRS; AND 15 MINS BEFORE LNDG; ALL 10 OF THEM CALL OR COME TO THE COCKPIT TO ASK WHAT GATE; WHAT TIME; ETC; ETC. THIS INCIDENT COULD HAVE BEEN AVOIDED BY MY DEMANDING A LITTLE MORE COCKPIT DISCIPLINE AT A BUSY TIME. SUPPLEMENTAL INFO FROM ACN 392207: THERE WERE SEVERAL CONTRIBUTING FACTORS THAT LEAD TO THIS INCIDENT. THE FLC HAD BEEN AWAKE ALL NIGHT ON FLT TO FRA. THERE WAS CONFUSION IN THE COCKPIT OVER THE FIELD ALTIMETER. THE FLT ATTENDANTS CALLED THE COCKPIT AND ASKED HOW FAR OUT WE WERE DURING THE DSCNT CHKLIST. THE FO PRESET THE FIELD ELEVATION A SECOND TIME WHEN THE RELIEF PLT CONFIRMED THE 1031 SETTING. THE FO INADVERTENTLY MUST HAVE PULLED THE MINIMUMS KNOB INSTEAD OF THE ALTIMETER KNOB (THESE 2 KNOBS ARE LOCATED TOGETHER ON THE GLARE SHIELD CTL PANEL). THIS SECOND RESET ATTEMPT OCCURRED AROUND FL110. THE ACFT SHOULD HAVE LEVELED OFF HIGH. ALSO; THE CAPT HAD NOTED PRIOR TO THIS INCIDENT; HE HAD OBSERVED THE MD11 LEVEL OFF LOW DURING HIGH SPD DSCNTS SIMILAR TO THE ONE ON JAN/MON/98.