AFTER A COUPLED APCH; A B737-700 FO FAILS TO DISCONNECT THE AUTOTHRUST DURING THE FLARE WITH THE CREW HAVING TO MAKE A GAR AT CLE; OH.

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I WAS CAPT OF FLT AND FUNCTIONING AS THE MONITORING PLT FOR THE FO WHO WAS THE PF. APCHING CLE; OUR INTENDED DEST; WE BRIEFED FOR 2 DIFFERENT RWYS AND APCHS. ATC; TWR CHANGED THE APCH AT LEAST TWICE PRIOR TO COMMENCING APCH. JUST PRIOR TO THE APCH; 2 ACFT AHEAD OF US (B727) RPTED POOR BRAKING. SO; I BRIEFED THE FO THAT WE WOULD LAND FLAPS 40 DEGS; BRAKES SET AT 3 DEGS; AND STRESSED THAT THE FO HAD TO LAND RIGHT ON THE AIRSPD LOC/GS NUMBERS IN THE LNDG ZONE. AT ABOUT 400 FT I HAD A VISUAL OF THE RWY AND CALLED 'RWY IN SIGHT.' THE FO DISCONNECTED THE AUTOPLT; BUT I FAILED TO NOTICE THAT HE LEFT THE AUTOTHROTTLES ON; WHICH IS CONTRARY TO SOP. WHILE IN THE FLARE AT ABOUT 30 FT; THE THROTTLES INCREASED TO TKOF/GAR THRUST; THE FO SLAMMED THE THROTTLES BACK TO IDLE; AND THE THROTTLES ONCE AGAIN MOVED FORWARD. I DIDN'T KNOW IF HE WAS CTLING THE THROTTLES OR THE AUTOTHRUST WAS; AS I THOUGHT HE HAD DISCONNECTED THEM WHEN HE DISENGAGED THE AUTOPLT. AFTER 1 OR 2 SECONDS; HE INITIATED A GAR; AT THE SAME TIME I COMMANDED A GAR. ON DOWNWIND WHILE BEING VECTORED FOR ANOTHER APCH; TWR WANTED TO KNOW THE REASON FOR THE GAR. SINCE I HADN'T HAD TIME TO DISCUSS IT WITH THE FO; I ASKED HIM WHY AND WHAT HAPPENED. HE SAID HE THOUGHT HE DISCONNECTED THE AUTOTHROTTLES AT ABOUT 50 FT BUT THEY WOULD NOT DISENGAGE. SO; THAT'S WHAT I RPTED. ON THE SECOND ATTEMPT; HE ASKED ME TO MAKE APCH; WHICH I DID TO A SUCCESSFUL LNDG; THE AUTOTHROTTLES DISCONNECTED NORMALLY AT ABOUT 400 FT WHEN I PUSHED THE AUTOTHROTTLE DISCONNECT BUTTONS ON THE SIDES OF EACH THROTTLE. ON DEBRIEFING; I DETERMINED THAT HE WAS UNAWARE OF THE REQUIREMENT TO DISENGAGE AUTOTHROTTLES WHEN DISENGAGING AUTOPLT. IT IS MY BELIEF THAT HE HIT THE TOGA BUTTONS BY MISTAKE WHICH COMMANDED THE THROTTLES TO GAR THRUST. ONCE WE LANDED; THE TWR ONCE AGAIN ASKED ME THE REASON FOR GAR. I SAID 'WE MAY HAVE HAD A PROB WITH THE AUTOTHROTTLES AND WITH POOR BRAKING BEING RPTED; ELECTED TO GO AROUND AND TRY AGAIN.'