FO OF A BEECH 1900D AIRLINER; BE02; DSNDED BELOW INITIAL APCH ALT WHILE RETURNING TO LAND DUE TO A PROP ICING SYS PROB. APCH CTLR INTERVENED AND ALERTED HIM TO HIS LOW ALT TO WHICH HE IMMEDIATELY CORRECTED BACK.

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WE DEPARTED PWM. DURING CLBOUT; I (FO PF) NOTICED THAT THE R PROP HEAT WAS NOT WORKING PROPERLY. I ALERTED THE CAPT AND THE DECISION WAS MADE TO RETURN TO THE FIELD (PWM). WE WERE IN ICE CONDITIONS. AS WE WERE BEING VECTORED FOR THE ILS RWY 11; THE CAPT WAS BUSY WITH TALKING ON THE OTHER RADIO WITH OPS; THE PAX; AND DISPATCH. THE VECTOR WAS SOMEWHAT TIGHT RESULTING IN HAVING TO REINTERCEPT THE LOC. TO ASSIST MY WORKLOAD; I SELECTED THE FLT DIRECTOR; WHICH APPEARED TO GIVE FALSE INFO. I THEN SELECTED THE FLT DIRECTOR TO OFF. WHILE ALL THIS WAS HAPPENING; APCH CALLED AND SAID 1800 FT. I WAS APPROX 1600-1500 FT. I IMMEDIATELY CORRECTED BACK TO 1800 FT. MY OMISSION OF THE ALTIMETER IN MY SCAN WAS DUE TO: HIGH WORKLOAD (INTERCEPT; REINTERCEPT; WHILE DSNDING TO 1800 FT AND REDUCING SPD). DISTR (FLT DIRECTOR). TIGHT VECTOR (ATC). PNF NOT IN THE LOOP AT CRITICAL TIME. (IN HIND SIGHT WHEN CAPT ANNOUNCED HE WOULD BE ON #2 RADIO; I SHOULD HAVE REQUESTED 'STAY WITH ME UNTIL I AM ESTABLISHED/STABILIZED ON THE APCH.)