AN INSTRUCTOR PLT LET HIS STUDENT LOSE CTL AND RUN OFF OF THE RWY ON LNDG.

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UPON APCH AND LNDG AT MCCARRAN INTL ON RWY 19R; WE LOST DIRECTIONAL CTL ON ROLLOUT AND WENT OFF THE RWY INTO A GRAVEL DIRT AREA BTWN THE TXWY AND 19R. DAMAGE TO THE AIRPLANE WAS A BENT STRUT ON THE R MAIN GEAR AND BROKEN SCISSORS ARM. I WAS ACTING AS AN INSTRUCTOR BEING RECENTLY CHKED OUT AFTER GND SCHOOL AND FLT TRAINING TO MANUFACTURER'S SYLLABUS. THE PF WENT TO THE SAME GND SCHOOL; IS ALSO AN ATP WITH MEL; CURRENT IFR. WE HAD DONE A PREVIOUS FLT INCLUDING AIRWORK AND A LNDG. I WAS ACTING AS PNF AS WELL AS AN INSTRUCTOR. CROSSED THE THRESHOLD AT BTWN 25 FT AND 50 FT; 105 KIAS; AND THE PF TOOK HAND OFF THE PWR LEVER AND PUT IT ON THE YOKE AND ASKED ME TO DO THE PWR. I DID NOT EXPECT THIS; BUT PUT HANDS ON THE PWR LEVERS AND CONTINUED TO REDUCE PWR UNTIL TOUCHDOWN. AT TOUCHDOWN WE WERE LONG ON THE 5001 FT BY 75 FT RWY BECAUSE OF EXTENDED FLARE AND SLIGHT TAILWIND. THE AIRPLANE YAWED AND I TRIED TO ADJUST OUR IDLE TO PREVENT THE YAW. THE PF LOST STEERING CTL. THE AIRPLANE WENT R; THEN L. THE PF WENT HARD ON BRAKES. I ASSISTED IN HOLDING THE RUDDERS STABLE AND TOLD THE PF TO HOLD STRAIGHT AS WE WERE GOING OFF THE RWY TO THE R. I DID HAVE SOME EFFECT WITH THE PWR LEVERS. THE PF SAID 'LET'S GAR.' I SAID 'NO; WE WILL HOLD STRAIGHT.' WE WENT OFF THE RWY AND THE R GEAR CAUGHT A DRAINAGE DITCH AND BROKE THE SCISSORS. I SHOULD HAVE HAD MORE AGGRESSIVE ATTN TO POTENTIAL PROBS. I DID NOT EXPECT HIS ACTIONS TO LEAVE THE PWR LEVERS AT THIS POINT WITHOUT A PREVIOUS BRIEFING. I SHOULD HAVE ORDERED A GAR OR TAKEN THE AIRPLANE MYSELF.