A SAAB 340B ON LNDG ROLLOUT THE ACFT BEGAN YAWING UNCTLABLY DUE TO THE R ENG SURGING CAUSED BY A FAILED AUTO COURSING COMPUTER.

Narrative

FLT FROM GTR-TUP. AFTER BEING CLRED TO LAND; WE FLEW A STRAIGHT-IN VISUAL APCH AND MADE A NORMAL LNDG IN THE TOUCHDOWN ZONE. AS SOON AS WE TOUCHED DOWN AND THE FO; WHO WAS FLYING; PULLED BACK THE PWR LEVERS; WE BEGAN TO PULL HARD R. I CORRECTED TO THE L USING THE TILLER WHEN I REALIZED HE HAD FULL L RUDDER. THE ACFT BEGAN TO SURGE L AND THEN BACK TO THE R AGAIN. I WAS ABLE TO KEEP THE AIRPLANE OVER THE CTRLINE; BUT WE WERE YAWING UNCTLABLY L AND R. I FELT PWR SURGES AND HEARD ABNORMAL ENG NOISES. THE ACFT BECAME MORE UNCTLABLE AS WE SLOWED AND WE SEEMED TO ACTUALLY SPD UP FOR A SECOND DURING A FINAL PWR SURGE. WHEN I REALIZED THAT THE ENGS WERE CAUSING THIS; I SHUT DOWN BOTH TO MAINTAIN DIRECTIONAL CTL. AS SOON AS THE ENGS SHUT DOWN AND THE PROPS FEATHERED; WE WERE ABLE TO BRING THE ACFT TO A CTLED STOP ON THE CTRLINE; ABOUT 1/2 WAY DOWN THE RWY. I DID NOT DECLARE AN EMER BECAUSE I DID NOT BELIEVE WE WERE IN ANY MORE DANGER AFTER THE ACFT CAME TO A STOP. WE DID NOT GET ON THE BRAKES IN WHAT I WOULD CONSIDER TO BE AN ABNORMAL MANNER AND THE ENG INDICATIONS APPEARED VERY NORMAL AFTER SHUTDOWN. I MADE THE DECISION NOT TO EVAC (OR DEPLANE) ALSO; BECAUSE OF THE ABOVE FACTORS AND BECAUSE I THOUGHT IT WOULD CREATE MORE PROBS THAN IT WOULD SOLVE. TWR IMMEDIATELY ASKED IF WE NEEDED ASSISTANCE WHEN THEY SAW OUR ROLLOUT AND CFR RESPONDED SWIFTLY EVEN THOUGH WE DID NOT REQUEST IT. I INITIALLY NOTIFIED TWR THAT I DID NOT WANT TO RESTART AND WOULD ASK TO BE TOWED OFF THE RWY. AFTER TALKING TO MAINT; HOWEVER; I MADE THE DECISION TO RESTART THE R ENG. THEY WERE ABLE TO CONVINCE ME THAT ANY ABNORMALITIES AS I DESCRIBED WOULD NOT ENDANGER THE PAX AND THAT I COULD SHUT DOWN AND DISCONTINUE THE START IF NECESSARY. WE PERFORMED THE RESTART AND TAXIED BACK TO THE GATE WITHOUT FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE YAWING PROB WAS CAUSED BY THE R ENG CHANGING PWR OUTPUT ABOVE AND BELOW IDLE OR BETA. THE RPTR SAID THE AUTO COURSING COMPUTER ON THE R ENG WAS MALFUNCTIONING.