DURING A COMMERCIAL PLT EXAMINATION; THE PLT EXAMINER'S SEAT FAILED TO HOLD ITS POS AT VARIOUS PHASES OF FLT; INCLUDING USING THE BRAKES DURING TAXIING. THE ACFT STALLED WHEN DURING SLOW FLT; THE EXAMINER'S SEAT SLID BACK CAUSING A SHIFT IN THE ACFT CTR OF GRAVITY.

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I HAVE OVER 200 HRS IN A CESSNA 172. THESE ACFT ARE KNOWN FOR HAVING THEIR SEATS SLIDE BACK IF THEY ARE NOT LOCKED INTO PLACE. THEY ALSO HAVE LOCKS; AIRWORTHY DIRECTIVES; AND OTHER MEASURES TO TRY AND PREVENT THE PLTS FROM SLIDING BACK DURING A CRITICAL PHASE OF FLT AND TAKING THE CTLS WITH THEM. ON MY FIRST FLT IN THE 1998 MOONEY M20J ALLEGRO TOP-OF-THE-LINE ACFT I WAS SADDENED TO FIND THE SAME TYPE OF SEAT RAILS. AFTER A COMPLETE CHK OUT IN THIS NEW ACFT BY AN EXCELLENT CFI; I TOOK MY COMMERCIAL CHK RIDE. ON THE PREFLT PAX BRIEFING I MADE THE EXAMINER AWARE OF HAVING TO ENSURE HIS SEAT WAS LOCKED INTO PLACE. WHEN HE TESTED IT THE FIRST TIME; HE SLID BACK. HE REPOSITIONED AND TRIED AGAIN AND IT HELD; UNTIL HE TESTED THE BRAKES ON HIS SIDE DURING THE TAXI. HE REPOSITIONED AGAIN; I OFFERED THE 'STAND ON THE LOCK-DOWN BAR' METHOD AND WE TRIED IT. AGAIN IT SEEMED TO WORK. ON ROTATION; HE ENDED UP IN THE BACK SEAT AS HIS SEAT SLID BACK. SAME THING WHEN WE WERE CRUISING TO THE XCOUNTRY DEST AND HIT TURB WITH ABOUT 200 FPM CLB. ONCE AGAIN DURING SLOW FLT WHERE IT CAUSED ENOUGH OF A CTR OF GRAVITY CHANGE TO STALL THE ACFT (2-3 KTS DIFFERENCE). THE LAST TIME WAS DURING A GAR AT THE INITIAL APPLICATION OF PWR PLUS THE PITCH UP. AFTER THE CHK RIDE CAME TO ITS SUCCESSFUL CONCLUSION I ASKED IF THESE SEAT EXCURSIONS WERE THE 'REALISTIC DISTRACTIONS.' HE ASSURED ME WITH ALL THE WT GIVEN A RETIRED FAA INSPECTOR THAT NONE OF THOSE SLIDES WERE INTENTIONAL.