A LUSCOMBE STUDENT PLT-OWNER; WHILE FLYING WITH HIS CFI; FLIPPED HIS ACFT OVER DURING LNDG AT 47K.

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I DEPARTED FROM A GRASS STRIP AND FLEW TO 47K. THE FIRST LNDG WAS A WHEEL LNDG. SLIGHT PIO (PLT INDUCED OSCILLATION) WAS ARRESTED BY FORWARD STICK PRESSURE. AS THE ACFT SLOWED DOWN THE TAILWHEEL SLOWLY DSNDED DOWN TO THE RWY. IMMEDIATELY; THE ACFT TURNED L. FULL R RUDDER WAS APPLIED. THE ACFT DEPARTED THE RWY; FULL BRAKES WERE APPLIED; AND THE ACFT WAS ALMOST STOPPED WHEN IT HIT A RUT AND FLIPPED. THE OCCUPANTS WERE NOT INJURED. THE ACFT WAS RIGHTED AND DID NOT RECEIVE SUBSTANTIAL DAMAGE. THE TAILWHEEL WAS INSPECTED AND FOUND TO BE IN THE CASTERING MODE -- NOT ALIGNED WITH THE AIRPLANE. THE ACFT IS EQUIPPED WITH HEEL BRAKES. IT IS DIFFICULT TO APPLY FULL BRAKE AND FULL R RUDDER UNDER PRESSURE. IF FULL R BRAKE WERE APPLIED THE TURN MAY HAVE BEEN ARRESTED. SUPPLEMENTAL INFO FROM ACN 595794: PRIOR TO THE FLT; THE FLT INSTRUCTOR DISCUSSED WITH THE STUDENT PLT/OWNER THE PERFORMANCE OF THE TAILWHEEL DURING GND OPS. IT WAS NOTED THAT THE ACFT'S TAILWHEEL HAD A TENDENCY TO NOT UNLOCK OR 'SWIVEL' WHEN TURNING TO THE L. IT WAS ALSO NOTED THAT THERE WAS DIFFICULTY LOCKING THE TAILWHEEL ON HARD SURFACES AFTER IT HAD UNLOCKED. WE DISCUSSED WHETHER MAINT OR REPLACEMENT WOULD BE AN OPTION. WE ASSUMED THAT THIS WAS A 'QUIRK' OF THE AIRPLANE; RATHER THAN A DEFICIENCY. AS THE ACFT SLOWED; THE TAIL CAME DOWN. WHEN THE TAIL TOUCHED; THE ACFT MADE AN IMMEDIATE TURN TO THE L. THE TAILWHEEL WAS OBSERVED TO BE UNLOCKED. WHILE THE ACFT WAS PUSHED TO THE RAMP; THE TAILWHEEL FAILED TO LOCK IN THE TRAIL POS AFTER A L TURN; IE; THE ACFT CONTINUED TO TURN L. THE ACFT WAS NOT EQUIPPED WITH BRAKES ON THE R SIDE. AS AN INSTRUCTOR; I WAS SITTING IN THE R SEAT. WE SHOULD NOT ALLOW OURSELVES TO ACCEPT A MECHANICAL DEFICIENCY AS A 'QUIRK' OF THE AIRPLANE. BOTH OF THE PLTS WERE UNDER PRESSURE TO FINISH THE RATING. WE SHOULD HAVE MADE SURE THE ACFT WAS NOT DEFICIENT IN ANY WAY.