AN ACR LTT TAXIED OFF THE TXWY AFTER A MECHANICAL FAILURE.

Narrative

WE TAXIED CLR OF ACTIVE RWY AND WHILE TAXIING TO THE GATE AREA THE L ENG WAS SHUT DOWN AND THE TAXIING WAS DONE ON THE R ENG. THIS IS A NORMAL PROC. WHEN THE FO WENT TO START THE APU FOR GND COOLING WHILE WE WERE AT THE GATE; ONE OF THE OVER VOLTAGE PROTECTION RELAYS TRIPPED; CAUSING US TO LOSE ALL ESSENTIAL PWR TO THE ACFT. THIS CAUSED THE FAILURE OF THE HYD PUMP ON THE L SIDE. THIS PROVIDES HYD PRESSURE FOR THE NOSEWHEEL STEERING. WITH THE LOSS OF HYD PRESSURE; THE NOSEWHEEL WENT HARD L CAUSING THE NOSEWHEEL AND L MAIN TO EXIT INTO THE GRASS. R MAIN STAYED ON TXWY. NO DAMAGE WAS DONE AND NO ONE WAS INJURED. THE LTT ACFT SEEM TO HAVE THIS PROB. THIS IS THE 4TH TIME IN 2 MONTHS I KNOW THIS HAS HAPPENED. I WAS THE FIRST PLT THAT WAS ON A NARROW TXWY WHEN IT HAPPENED. AS WE STARTED TOWARDS THE EDGE OF THE TXWY; I IMMEDIATELY SHUT DOWN THE R ENG JUST IN CASE WE WERE TO HIT A TAXI LIGHT CAUSING FOREIGN OBJECTS TO BE THROWN INTO THE SIDE OF THE ACFT. THIS COULD HAPPEN INFLT CAUSING A VERY SERIOUS PROB IF ALL ESSENTIAL PWR WAS LOST. THIS NEEDS TO BE LOOKED INTO VERY CLOSELY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING PLT IS A FORMER MGMNT PERSON WHO WENT BACK TO THE LINE WHEN SEVERAL SMALL ACR'S WERE CONSOLIDATED. HE WAS AGAINST THE POLICY OF TAXIING ON 1 ENG AS THIS PUT MANY OF THE SYS ON THEIR BACKUP SYS INCLUDING THE NOSEWHEEL STEERING USING A BACKUP ELECTRICAL PUMP. HE WAS OVERRULED AND FOLLOWED THE PROC. AS THE PAX DOOR IS ON THE L; SINGLE ENG TAXI IS DONE WITH THE R ENG. THE FO FOLLOWED THE PROC AS LISTED ON THE CHKLIST AND THE ACFT WENT OFF THE TXWY WHEN THE ELECTRICAL PUMP COULD NOT COME ON THE LINE. NORMAL BRAKING WAS LOST WITH THE ANTISKID SWITCH ON AND IT WAS FURTHER DOWN THE CHKLIST. THERE WAS NO DAMAGE TO THE ACFT NOR INJURY TO PERSONNEL. WITHIN 2 DAYS THE COMPANY HAD CHANGED THE PROC SO THAT THE APU WILL NOT BE STARTED UNLESS BOTH ENGS ARE OPERATING OR THE ACFT IS CHOCKED. THIS DOES NOT REALLY FIX THE PROB OF THE OVER VOLTAGE RELAY; BUT DOES PREVENT THE ACFT LEAVING THE PAVEMENT.