A BUSY FLC IN A TWIN ENG ACR STARTS TO PERFORM THEIR DUTIES OUT OF SEQUENCE. THE CAPT TURNED OFF THE 'A' HYD SYS INSTEAD OF THE ANTI-ICE SYS AND THEY ASSUME THAT THE 'A' SYS HAD FAILED AND THEY ACT ACCORDINGLY.

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PRIOR CREW BRIEFED 'A' HYD QUANTITY GAUGE FLICKERING; THEN BACK TO NORMAL. FLYING INTO SLC BROKEN CLOUD LAYER INTO ARPT; RPTED LIGHT TO MODERATE RIME ICING (CONFIRMED; PICKED UP MODERATE RIME). AT 10500 FT MSL; RPTED FIELD IN SIGHT (AT APPROX 4000-6000 FT E (ABEAM) FIELD. CLRED VISUAL APCH; BUT TO REMAIN WITHIN 12 NM OF FIELD. WE NEEDED TO GET DOWN (LOSE ALT AND CONFIGURE ASAP -- CONTRIBUTING FACTOR). I; AS PNF; WAS ACCOMPLISHING APCH CHKLIST; WHEN CAPT ASKED TO TURN OFF ANTI-ICE. CAPT; SEEING I WAS OCCUPIED WITH CHKLIST DUTIES; REACHED UP TO TURN OFF THE ANTI-ICE. UNBEKNOWNST TO ME; IN DOING SO; ACTUALLY TURNED OFF 'A' SYS HYDS. CAPT DECLARED EMER. I; THE PNF; PROCEEDED TO RUN LOSS OF SYS 'A' HYDS CHKLIST. UPON REACHING POINT IN CHKLIST TO TURN PUMPS OFF; I LOOKED UP AND SAW THE PUMPS IN THE OFF POS. KNOWING THAT THE CAPT KNEW I WAS VERY BUSY; I ASSUMED HE TURNED THE SWITCHES OFF (CONTRIBUTING FACTOR; NOT QUESTIONING MY ASSUMPTION). I ALSO; AFTER COMPLETING THE EMER CHKLIST AND LNDG CHKLIST; FINALLY REACHED UP TO TURN ANTI-ICE OFF PRIOR TO LAND. CHKLISTS AND LNDG WERE UNEVENTFUL. DISCUSSING THE SIT ON THE GND; WE DISCOVERED OUR MISTAKE. CONTRIBUTING FACTORS: PREBRIEFED HYD PROB. LACK OF COM -- BOTH FROM MYSELF (QUESTIONING ASSUMPTION AND CAPT FOR TURNING ANTI-ICE OFF). PREVENTION OF PROB OCCURRING: BETTER COM. ANY TIME A SWITCH IS MOVED; INFORM OTHER PERSON. SUPPLEMENTAL INFO FROM ACN 428458: I KNEW THE FO WAS EXTREMELY BUSY AND WE WERE SOMEWHAT COMPRESSED FOR TIME. SO; MY FIRST MISTAKE WAS; I REACHED UP TO TURN OFF THE ANTI-ICE TO 'HELP' EASE THE FO'S WORKLOAD. MY SECOND MISTAKE; I DIDN'T FINISH LOOKING WHERE MY HAND WENT AND DIDN'T TELL THE FO. AS A RESULT; UNKNOWN TO ME; I TURNED OFF OUR 'A' SYS HYD PUMPS. BEING IN A HIGH WORKLOAD TURN TO FINAL; I HAD JUST HELPED CREATE OUR EMER; WITH RESULTING MASTER CAUTION AND HYD LIGHTS COMING ON. WE LANDED UNEVENTFULLY. AFTER LNDG; WE DISCUSSED WHAT HAD HAPPENED (WE WERE STOPPED ON TXWY). IT WAS THEN I REALIZED MY ERROR. WE TURNED THE 'A' PUMPS BACK ON AND ALL SYS WERE NORMAL. ANOTHER PIECE OF THIS PICTURE THAT CAUSED US TO AFFIRM HYD FAILURE WAS A PREVIOUS WRITE-UP OF A HYD QUANTITY GAUGE MALFUNCTION. IT WAS ACTUALLY WORKING FINE; BUT BECAUSE OF MY ERROR; WE THOUGHT IT MAY HAVE BEEN READING WRONG DUE TO ZERO 'A' PRESSURE (THE WRITE-UP HAD BEEN CLRED). MY LOGBOOK WRITE-UP REFLECTED MY ERROR IN TURNING OFF 2 GOOD 'A' PUMPS TO PREVENT ANY UNNECESSARY MAINT. I ALSO SPOKE WITH OUR OPS DIRECTOR WITHIN 2 HRS OF INCIDENT AND GAVE HIM A FULL VERBAL ACCOUNT OF WHAT HAPPENED.