A SHORTS 330 L BRAKE WAS DRAGGING AFTER LNDG CAUSING THE ACFT TO VEER L OFF OF THE RWY.

Narrative

I WAS CONDUCTING A CHK RIDE IN VMC. THE PLT EXECUTED ILS (LOC ONLY) RWY 6 AT ZZZ WITH A CIRCLE TO RWY 32. WINDS WERE RPTED OUT OF 300 DEGS WITH GUSTS UP TO 20 KTS. THE PLT FLEW THE AIRPLANE TO A STABLE 1 MI FINAL WITH A RED-OVER-WHITE INDICATION ON THE VASI. THE PLT MAINTAINED 94 KTS (VREF +10) THE ENTIRE FINAL APCH UNTIL WE NEARED THE RWY THRESHOLD. WE TOUCHED DOWN JUST BEYOND THE 1000 FT MARKERS AT ABOUT 80 KTS (VREF -4). THE PLT APPLIED LIGHT BRAKING INITIALLY BECAUSE OF UNCERTAINTY ABOUT WHAT WAS CAUSING THE PLANE TO VEER L. AS WE SLOWED; THE PLT ASKED; 'HEY; WHAT IS THAT?' IN RESPONSE TO AN UNUSUAL RUMBLING NOISE. THE AIRPLANE CONTINUED TOWARD THE SIDE OF THE RWY; AND I VERBALLY INSTRUCTED THE PLT TO TAKE PRESSURE OFF THE L BRAKE. THE PLT INDICATED PRESSURE WAS NOT ON THE BRAKES AND SHE WAS UNABLE TO TURN THE AIRPLANE. AT THIS POINT; I ASSUMED THE CTLS AND APPLIED FULL R RUDDER. BY THE TIME I WENT TO REACH FOR R BRAKE AND SPLIT THE PWR; THE ACFT HAD LEFT THE RWY. THE ACFT CAME TO REST IN A DITCH. WHEN THE AIRPLANE CAME TO A STOP; I SECURED THE ACFT AND DID A WALKAROUND. UPON FURTHER REFLECTION; ANOTHER PLT HAD MENTIONED THAT HE'D EXPERIENCED A STRONG PULL TO THE L AFTER TOUCHDOWN. THE PULL TO THE L WAS SO SEVERE THAT ATC ASKED IF THEY NEEDED ANY HELP. THE ONLY WRITE-UP WAS FOR A FLAT-SPOTTED L MAIN TIRE. I SAW THIS DISCREPANCY WAS 'CLOSED' BEFORE THE FLT; BUT IT DID NOT DESCRIBE THE PULLING SENSATION AND; THEREFORE; DID NOT JOG MY MEMORY.