Narrative:

We taxied clear of active runway and while taxiing to the gate area the left engine was shut down and the taxiing was done on the right engine. This is a normal procedure. When the first officer went to start the APU for ground cooling while we were at the gate, one of the over voltage protection relays tripped, causing us to lose all essential power to the aircraft. This caused the failure of the hydraulic pump on the left side. This provides hydraulic pressure for the nosewheel steering. With the loss of hydraulic pressure, the nosewheel went hard left causing the nosewheel and left main to exit into the grass. Right main stayed on taxiway. No damage was done and no one was injured. The light transport aircraft seem to have this problem. This is the 4TH time in 2 months I know this has happened. I was the first pilot that was on a narrow taxiway when it happened. As we started towards the edge of the taxiway, I immediately shut down the right engine just in case we were to hit a taxi light causing foreign objects to be thrown into the side of the aircraft. This could happen in-flight causing a very serious problem if all essential power was lost. This needs to be looked into very closely. Callback conversation with reporter revealed the following information: the reporting pilot is a former management person who went back to the line when several small air carrier's were consolidated. He was against the policy of taxiing on 1 engine as this put many of the system on their backup system including the nosewheel steering using a backup electrical pump. He was overruled and followed the procedure. As the passenger door is on the left, single engine taxi is done with the right engine. The first officer followed the procedure as listed on the checklist and the aircraft went off the taxiway 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 checklist. There was no damage to the aircraft nor injury to personnel. Within 2 days the company had changed the procedure so that the APU will not be started unless both engines are operating or the aircraft is chocked. This does not really fix the problem of the over voltage relay, but does prevent the aircraft leaving the pavement.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.