AN SF340A HYD SYS FAILURE WHILE TAXIING LED TO LOSS OF ACFT CTL WHICH IN TURN RESULTED IN TXWY EXCURSION.

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HYD CAUTION LIGHT ILLUMINATED WHILE TAXIING TO RWY 3C AT TXWY K AND TXWY K15. I BEGAN REQUIRED PROCS AND COMPLETED THE QRH CHKLIST. THE CAPT CONTACTED MAINT AND REQUESTED A TOW BACK TO THE GATE. WE BOTH WERE APPLYING PRESSURE TO THE BRAKES. I BEGAN TO MAKE AN ANNOUNCEMENT TO THE PAX WHEN THE ACFT BEGAN TO MOVE. THE CAPT BEGAN TO SHUT THE ENG DOWN. THE ACFT BEGAN TO YAW TO THE L. THE CAPT APPLIED DIFFERENTIAL PWR AND REVERSE. THE ACFT WOULD NOT STOP. WHEN IT WAS CLR WE HAD NO BRAKES TO STOP THE PLANE; THE CAPT IMMEDIATELY SHUT BOTH ENGS DOWN. WE ROLLED TO A STOP IN THE GRASS. ALL PAX WERE SAFE; CREW WAS SAFE. ATC AND MAINT WERE NOTIFIED. A VERY POORLY WRITTEN QRH EMER CHKLIST; I BELIEVE; SHOULD BE MODIFIED AND IMPROVED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT; AFTER HYD CAUTION LIGHT CAME ON; CAPT NOTICED BRAKING ACTION WAS DEGRADING; SO RPTR BEGAN TO ASSIST IN APPLICATION OF BRAKES. RPTR WENT THROUGH THE QRH CHKLIST. IN COMPLETING CHKLIST PROC; THE ELECTRIC HYD PUMP WAS TURNED OFF. ACFT BEGAN TO MOVE. RPTR STATED TURNING HYD PUMP BACK ON DID NOT HELP; SO CAPT BEGAN SHUTTING DOWN ENGS; L ENG FIRST. ACFT CONTINUED MOVING; YAWING TO L. DIFFERENTIAL REVERSE THRUST DID NOT STOP ACFT; SO CAPT SHUT DOWN SECOND ENG. ACFT STOPPED. RPTR ADMITS CREW DID NOT EMPLOY THE MANUAL HYD PUMP. RPTR'S CONTENTION THAT CHKLIST IS AMBIGUOUS IS BASED ON HER OBSERVATION; APPARENTLY CONCURRED WITH BY RPTR'S CHIEF PLT; THAT THE CHKLIST IS FOR USE INFLT; NOT ON THE GND. SHE HAD NO SUGGESTIONS AS TO HOW THE CHKLIST COULD BE IMPROVED. RPTR ALSO FEELS THAT SAAB 340A'S SINGLE HYD SYS; WITH NO BACKUP; IS REFLECTION OF POOR DESIGN. RPTR STATED THAT; ALTHOUGH HER CHIEF PLT AGREED WITH HER CONTENTION THAT THE CHKLIST WAS NOT APPLICABLE TO A HYD FAILURE/PROB WHILE TAXIING; NO CHANGES TO THE CHKLIST HAVE BEEN MADE IN THE 2 MONTHS SINCE THE INCIDENT OCCURRED.