A C172 LANDED WITH AN UNSPECIFIED CTL ANOMALY CAUSING THE ACFT TO EXIT THE RWY BUT RESULTING IN NO HARM OR INJURY.

Narrative

I WAS ON A VISUAL APCH TO BRACKETT ARPT (POC); LNDG ON RWY 26L. MY MAIN LNDG GEAR TOUCHED DOWN ABOUT 200 FT PAST THE 1000 FT MARKER AND MY NOSEWHEEL SHORTLY THEREAFTER. I BEGAN TO APPLY EVEN BRAKE PRESSURE WHEN THE ACFT YAWED SHARPLY TO THE R. I APPLIED PRESSURE ON THE L RUDDER PEDAL IN ORDER TO CORRECT THE YAW AND MAINTAIN DIRECTIONAL CTL. THE L RUDDER INPUT DID NOT CHANGE THE DIRECTION OF THE ACFT OR STOP THE YAW. I FELT INCREASING THE AMOUNT OF L RUDDER INPUT WOULD ONLY CAUSE A SHARP YAW TO THE L; INCREASING THE CENTRIFUGAL FORCE TO THE R AND POSSIBLY ROLLING THE ACFT. THEREFORE; I HELD THE L RUDDER PRESSURE CONSTANT; WHICH EVENTUALLY STOPPED THE YAW TO THE R AND REGAINED DIRECTIONAL CTL. ONCE DIRECTIONAL CTL WAS REGAINED; THE ACFT WAS AT A 45 DEG ANGLE TO THE RWY AND HEADING FOR THE R EDGE. I FELT MY SAFEST OPTION WAS TO KEEP THE ACFT HEADED STRAIGHT; MAINTAIN DIRECTIONAL CTL AND REAPPLY BRAKE PRESSURE. DECIDING TO NOT CHANGE THE DIRECTION OF THE ACFT CAUSED ME TO EXIT THE RWY SURFACE AND ENTER THE GRASS AREA N OF RWY 26L. I CONTINUED TO APPLY THE BRAKES AS THE ACFT CAME TO A STOP ON TXWY E. I ADVISED THE TWR THAT WE WERE OK AND WERE CLRED TO TAXI TO PARKING. I INSPECTED THE ACFT AND DID NOT FIND ANY VISIBLE DAMAGE. BEFORE TAKING OFF; I TESTED THE BRAKES AND FOUND THEM TO WORK PROPERLY. I DO NOT KNOW WHAT CAUSED THE SUDDEN YAW DURING LNDG; BUT I AM CONFIDENT MY CORRECTIVE ACTION PREVENTED SUBSTANTIAL ACFT DAMAGE AND POSSIBLY AN ACCIDENT.