MD83 FLC HAS RWY EXCURSION DURING LNDG ROLL.

Narrative

FLT WAS NORMAL IN ALL RESPECTS TILL TOUCHDOWN. IT WAS NIGHT AND THE RWY WAS WET; BUT IT WAS NOT RAINING. TOUCH DOWNED ON TOUCHDOWN ZONE ON CTRLINE; ON SPD; WITH NORMAL RATE OF DSCNT. WHEN THRUST REVERSERS DEPLOYED; THE NOSE OF THE ACFT RAPIDLY VEERED ABOUT 20 DEGS L. I IMMEDIATELY PUSHED THE REVERSE THRUST LEVERS TO REVERSE IDLE; ADDED R RUDDER AND EXPECTED THE NOSE TO MOVE BACK TO THE R. IT DID NOT. I FELT AS IF I WAS SKIDDING AND THUS WAS VERY CAUTIOUS ABOUT OVERCORRECTING AND MAKING A BAD SIT WORSE. I INCREASED R RUDDER SMOOTHLY. ALL I COULD SEE WAS THE RWY EDGE LIGHTS AND I KNEW I WAS GOING TO STAY ON THE RWY SIDE OF THEM. FOR A BRIEF PERIOD; I FELT A VIBRATION AND WONDERED IF IT WAS THE SHOULDER OF THE RWY OR NOT. ONCE BACK ON CTRLINE; I ROLLED TO THE END AND CLRED THE RWY. EVERYTHING LOOKED AND FELT NORMAL; SO WE TAXIED TO THE GATE. WANTING TO KNOW FOR SURE IF I WENT OFF THE RWY; I DID A POSTFLT INSPECTION AND SAW THE GRASS AND MUD. I HAD EXITED THE RWY WHEN THE VISUAL CUES AVAILABLE HAD INDICATED THAT I HAD NOT. I BELIEVE RWY EDGE LIGHTS SHOULD BE AT THE RWY EDGE OF CONCRETE OR NOT SO NAMED. I CAN'T SAY WHY THE NOSE VEERED IN THE FIRST PLACE. IT MAY BE DUE TO THE ACFT; IE; UNEVEN MAIN STRUTS; UNUSUALLY EASY TO USE REVERSE LEVERS THAT WORKED WAY TOO WELL AND BROUGHT ON TOO MUCH PWR TOO SOON; OR ME BEING IMPATIENT AND NOT WAITING FOR THE NOSEWHEEL STRUT TO COMPRESS AND DEFINITELY HAVE WT ON IT; ETC. WHATEVER THE REASON INITIALLY; IT SHOULD NOT HAVE BEEN ENOUGH TO EXIT THE RWY. HOWEVER; THE WET RWY ADDED TO THE PROB. I BELIEVE THE RAPID YAW CAUSED A HYDROPLANE EFFECT WHICH HELPED DELAY THE ONSET OF RECOVERY. I FELT LIKE I WAS SKIDDING AND THAT I NEEDED TO BE CAREFUL NOT TO OVER-CONTROL THE RECOVERY AND MAKE A BAD SIT WORSE. AS I EASED THE NOSE AROUND SMOOTHLY; I USED UP MORE OF THE WIDTH OF THE RWY; BUT I COULD SEE THE 'RWY EDGE' LIGHTS AND WAS NOT WORRIED. I MAY HAVE USED IT ALL (THE WIDTH) UP; BUT I WAS GOING TO BE ABLE TO SAFELY RETURN THE ACFT TO THE CTRLINE WITHOUT GETTING SIDEWAYS IN THE OTHER DIRECTION; AND I WAS GOING TO KEEP IT ON THE RWY. LITTLE DID I KNOW THAT THE RWY EDGE LIGHTS WERE NOT ON THE RWY EDGE BUT SOME 6 FT OUTSIDE. I COULD NOT SEE THIS ON A DARK NIGHT WITH A WET RWY WITHOUT THE OPPORTUNITY TO STOP AND TAKE MY TIME TO SEE IT. THE LIGHTS WERE NOT WHAT I THOUGHT THEY WERE. I WAS VISUALLY DECEIVED.