MD-83 ACFT EQUIP PROB. AUTOTHROTTLES WENT TO IDLE DURING A GAR. AIRSPD DECREASED TO STICK SHAKER.

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FLYING A VISUAL APCH; AUTOPLT OFF; AUTOTHROTTLES ON. SMALL TURBOPROP FAILED TO TURN OFF RWY AND WE WERE GIVEN A GAR AT ABOUT 75 FT AGL. THROTTLES HAD GONE INTO RETARD MODE. TOGA BUTTONS WERE PUSHED; FMA SHOWED GAR AND THROTTLES ADVANCED. AT APPROX 600 FT AGL WE WERE GIVEN A L TURN TO 210 DEGS THEN A CLRNC TO 3000 FT. THE COPLT WAS SETTING THE HEADING AND I REACHED TO SET 3000 FT. JUST AS I SET 3000 FT THE COPLT SAID 'WATCH YOUR AIRSPD (ATTITUDE/ANU DEGS WAS NORMAL) LOWER THE NOSE.' I SAW THE AIRSPD DECELERATING THROUGH 120 KTS (VREF FLAPS 28 DEGS AND 134 KTS). I SMOOTHLY LOWERED THE NOSE AND REACHED FOR THE THROTTLES. THEY WERE RETARDED TO THE IDLE STOPS AND RETARD WAS ON THE FMA. NORMAL PWR WAS APPLIED AND A NORMAL UNEVENTFUL RECOVERY WAS ACHIEVED. I HAD TO TURN THE AUTOTHROTTLES OFF TO GET OUT OFF RETARD MODE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT IDENTED ACFT AS MD83. DURING THE LNDG PHASE; PRIOR TO THE FAR; THE AUTOTHROTTLES RETARDED CORRECTLY. WHEN THE GAR WAS INITIATED; RPTR SAYS THAT THE FMA SHOWED GAR AND AUTOTHROTTLES ADVANCED. RPTR CAPT ADMITS THAT HE USES A TECHNIQUE WHICH MAY HAVE CONTRIBUTED. WHEN IN A DSCNT; LNDG; FOR EXAMPLE; HE SPINS THE ALT SELECTOR UP TO A HIGHER ALT SO AS TO ELIMINATE ALT WARNINGS. THUS; DURING A GAR; WHEN SETTING THE GAR ALT; IT IS NECESSARY TO SPIN THE ALT KNOB BACK DOWN TO THE ASSIGNED ALT. SO; RPTR THINKS THAT MAY HAVE CONTRIBUTED; BUT DOESN'T KNOW HOW OR WHY. ALSO; RPTR FOLLOWED UP WITH DISCUSSION WITH HIS COMPANY AND OTHER PLTS AND NOBODY HAD HEARD OF A SIMILAR EVENT ON THE MD83. BECAUSE THE AUTOTHROTTLES GO TO RETARD ON LNDG WHEN 75 FT FROM THE GND; AND THAT FUNCTION IS TIED TO THE RADAR ALTIMETER; RPTR THINKS A RADAR ALTIMETER MALFUNCTION MIGHT HAVE CAUSED THE IDLE POS. SUPPORTING THAT CONTENTION WAS THE FMA SHOWING RETARD. WITH THE THROTTLES AT IDLE AND IN A CLB; THE AIRSPD DROPPED RAPIDLY AND THE FLC GOT A STICK SHAKER PRIOR TO ADVANCING THE THROTTLES.