B777 CREW HAS UNCOMMANDED ENG ACCELERATION AFTER AUTOSTART WAS COMPLETE.

Narrative

I WAS THE CAPT ON AN B777 FLT. THE INCIDENT INVOLVED AN UNCOMMANDED DOUBLE ENG ACCELERATION DURING ENG START AT THE CONCLUSION OF THE PUSHBACK PROC AFTER THE PARKING BRAKES WERE SET. THE MECH REQUESTED PERMISSION TO DISCONNECT FROM THE ACFT AND PERMISSION WAS GRANTED. THE ENTIRE START PROC USING THE AUTOSTART SYS WAS NEARING COMPLETION. THE FO BELIEVES AND I BELIEVE I HEARD HIM SAY 'ROLLBACK;' WHICH WOULD SIGNAL THE END OF THE START PHASE. THE MECH AND TUG WERE NOT YET VISIBLE TO ME; SO I GLANCED DOWN AT MY TAXI CHART WHEN I HEARD WHAT I ORIGINALLY THOUGHT WAS THE AIR CONDITIONING PACKS AUTOMATICALLY TURNING ON AFTER ENG START. WITHIN A HALF OF A SECOND; I REALIZED WHAT I WAS HEARING WAS THE ENGS ACCELERATING. I IMMEDIATELY RETARDED THE THROTTLES AND MANUALLY APPLIED FULL BRAKE PRESSURE TO THE BRAKE PEDALS. THE ACFT HAD NOT MOVED; BUT APPLYING THE BRAKES AND RETARDING THE THROTTLES WERE JUST GUT REACTIONS TO SOMETHING BEING WRONG. WE WERE CONVINCED THAT THE AUTOTHROTTLE SYS HAD SOMEHOW MOVED THE THROTTLES FORWARD. I DISCUSSED AT LENGTH ON THE GND WITH OUR MAINT PEOPLE WHAT HAD OCCURRED. I WENT THROUGH ALL THE MAINT PAGES IN THE FMC; BUT COULD FIND NOTHING. I ASKED IF THE ACFT HAD DOWNLOADED ANY DISCREPANCIES ON ITS OWN; BUT MAINT CTL COULD FIND NONE. WE AGREED TO FLY THE ACFT. ONCE LINED UP ON THE RWY; I PUSHED THE TO/GA SWITCH TO AUTOMATICALLY ADVANCE THE THROTTLES; BUT NOTHING HAPPENED. I PUSHED THE SWITCH A SECOND TIME AND THE THROTTLES ENGAGED AND THE SYS OPERATED NORMALLY THEREAFTER. I AGAIN CALLED OUR MAINT PEOPLE TO FURTHER PURSUE THE PROB. THE MAINT CTLR SUGGESTED WE SET UP A CONFERENCE CALL WITH THE AUTOTHROTTLE ENGINEER; WHICH WE DID. AFTER A LONG DISCUSSION ON ALL THE POSSIBILITIES WITH THE AUTOTHROTTLE; I BEGAN TO WONDER IF THE THROTTLES MIGHT NOT HAVE BEEN IN THE IDLE POS BEFORE WE EVER LEFT THE GATE. I ASKED IF THE FLT RECORDER WOULD SHOW WHAT THE THROTTLE POS WAS AND THEY THOUGHT IT MIGHT. I THEN ASKED THAT THE RECORDER BE REMOVED SO THAT WE MIGHT FIND THE ANSWER. THIS WAS DONE ON OUR ARR. I AM SURE AS I CAN REASONABLY BE THAT I CHKED THE THROTTLE POS DURING MY COCKPIT SETUP. THERE WAS MUCH ACTIVITY IN THE COCKPIT ABOUT 25-30 MINS BEFORE PUSHBACK WITH A COUPLE OF PLT OBSERVERS; A MAINT MAN; A FUELER; FLT ATTENDANT AND THE 2 OF US. I LEFT FOR A WHILE TO GO TO THE LAVATORY AND I SUPPOSE IT WOULD BE POSSIBLE FOR ONE OF US; PERHAPS GETTING IN OR OUT OF OUR SEATS; TO HAVE MOVED THE THROTTLES INADVERTENTLY. IF THAT TURNS OUT TO BE THE CASE; THEN WE NEED TO THINK ABOUT HAVING SOME KIND OF WARNING SYS THAT WOULD ALERT US BEFORE OR DURING ENG START. I AM SOMEWHAT SURPRISED THAT THE TKOF CONFIGN WARNING DID NOT SOUND; EVEN THOUGH THE FLAPS WERE UP. PERHAPS THE THROTTLES WERE NOT AS ADVANCED AS I THOUGHT THEY WERE. I AM RPTING THIS BECAUSE THERE WAS THE POTENTIAL FOR AN INCIDENT OR ACCIDENT NO MATTER HOW THE THROTTLES GOT WHERE THEY WERE.