A B737-300 ON TAXI OUT FOR MAINT ENG RUN EXITED THE TXWY DUE TO HIGH PWR APPLIED TO THE #1 ENG WITH THE #2 ENG AT IDLE.

Narrative

I WAS ASSIGNED R SEAT DUTY FOR AN ENG HIGH PWR RUN FOLLOWING A VARIABLE STATOR VANE RIG ADJUSTMENT. THE HIGH PWR PROC INVOLVES A TURBINE CASE CLRNC DEACTIVATION PROC WHICH INVOLVES A SHORT 3 SECOND ACCELERATION TO 92% N2 FOLLOWED BY RETURN TO IDLE PWR SETTING. ON TAXI-OUT THE #1 ENG WAS THROTTLED UP; #2 ENG AT IDLE. AS THE L ENG APCHED 80% N1; THE AIRPLANE NOSE GEAR SKIDDED TO THE R OF TXWY CTRLINE. BRAKES WERE APPLIED BUT CTL WAS NOT RECOVERED. THE NOSE GEAR LEFT THE TXWY AND ONTO THE GRASS. AFTER ACFT RECOVERY; NOSE TIRES WERE CHANGED; NOSE GEAR WAS WASHED; LUBED AND GEAR SWING ACCOMPLISHED. INSPECTION FOUND NO DAMAGE. CONTRIBUTING FACTORS WERE THE ACFT'S LIGHT FUEL LOAD AND THE FORWARD MOMENTUM ALREADY ESTABLISHED DURING TAXI-OUT. IN RETROSPECT; THE ACFT SHOULD HAVE BEEN STOPPED WITH PARKING BRAKE SET. A HEAVIER FUEL LOAD WOULD ALSO HELP IN HOLDING THE ACFT UNDER FULL BRAKING. SUPPLEMENTAL INFO FROM ACN 410303: AT ABOUT 80% N1; THE AIRPLANE SWERVED TO THE R. BRAKE PEDALS WERE FULLY DEPRESSED; NOSE LNDG GEAR STEERING WAS FULL L. PWR WAS BROUGHT BACK TO IDLE. WITHIN 3 SECONDS THE AIRPLANE HAD LEFT THE TXWY CTRLINE AND THE NOSE LNDG GEAR HAD STOPPED 15 FT INTO THE GRASS. I SHOULD HAVE FULLY STOPPED AIRPLANE BEFORE STARTING THE TKOF PWR RUN. I'VE BEEN TAXIING B737'S SINCE 1989 AND THOUGHT THE BRAKES WERE STRONGER THAN THE ENGS. MORE WT WOULD HAVE KEPT THE WHEELS FROM SKIDDING WITH THE BRAKES LOCKED. A 2 ENG PWR RUN; BOTH TO TKOF PWR REQUIRES 20000 LBS OF FUEL. WE HAD LESS THAN 7000 LBS; SO I KNEW BETTER THAN TO HAVE 2 ENGS AT HIGH PWR. I TAXI EVERYTHING ACR HAS AT ZZZ OVERNIGHT; SURE LEARNED A LOT FROM MY 3 SECOND RIDE. MAYBE DOING THE RUN A320 STYLE; WITH 75% PWR ON THE 'NON-TESTED' ENG AND 95% ON THE 'TESTING' ENG WOULD HAVE KEPT ME ON A STRAIGHT SLIDE DOWN THE TXWY. I DIDN'T HAVE ENOUGH WT TO FEEL GOOD ABOUT DOING THAT.