AN A320 ENTERED A 'THRUST LOCK' CONDITION DURING A GO AROUND AFTER TOGA WAS NOT SELECTED. THIS CONFUSED THE CREW AS THE ACFT OVERSPED THE FLAPS AND CLB 500 FT HIGH.

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ALTDEV OF ABOUT 500 FT OCCURRED WHEN THE AUTOTHRUST SYS WENT INTO THRUST LOCK IN THE TOGA POS. WE CLBED FROM 2000-2500 FT. WE WERE IN LEVEL FLT BEING VECTORED BACK AROUND FOR ANOTHER APCH AFTER PERFORMING A GAR DUE TO AN ACFT ON THE RWY (HE WAS IN POS AND HOLDING; BUT WOULD NOT RESPOND TO TWR'S REPEATED ATTEMPTS TO CLR HIM FOR TKOF). THE GAR WAS NOT INITIATED BY POSITIONING THRUST LEVERS TO TOGA; RATHER AN OPEN CLB WAS SELECTED TO 2000 FT. WE BEGAN THE GAR FROM ABOUT 1200-1300 FT AGL. HOWEVER; WITHIN ABOUT 15 SECONDS OF INITIATING THE GAR; THE CAPT DID SELECT TOGA BRIEFLY; THEN RETURNED THE THRUST LEVERS TO THE CLB DETENT. THIS DID NOT PUT IT INTO SRS AND GND TRK MODE HOWEVER; AND WE STILL HAD THE LNDG MEMOS DISPLAYED ON THE UPPER ECAM. THAT IS WHY ANOTHER ATTEMPT WAS MADE TO GO TO TOGA (BRIEFLY; THEN RETURN TO CLB DETENT) WHILE IN LEVEL FLT. BUT IT DOES NOT EXPLAIN HOW OR WHY THE AUTOTHRUST WENT TO THRUST LOCK WHILE AT TOGA POS. IT SEEMS THAT MAYBE THE AUTOTHRUST SOMEHOW DISCONNECTED WHEN THE LEVERS WERE NOT TOGA. IN ANY CASE THE EVENT CAUSED A FLAP OVERSPD ALERT AND THE GAIN OF 500 FT. AUTOTHRUST AND AUTOPLT WERE DISCONNECTED. WE DID REACTIVATE AND CONFIRM THE APCH. IT WAS AFTER THAT WHEN THINGS RETURNED TO NORMAL. AUTOPLT AND AUTOTHRUST WERE RE-ENGAGED; AND WE COMPLETED A NORMAL LNDG. ATC MADE NO MENTION OF THE ALTDEV. THE FLAP OVERSPD WAS WRITTEN UP. THE CAPT EXPLAINED THAT IT DIDN'T MAKE SENSE TO GO TO TOGA TO CLB ABOUT 700-800 FT; AND THAT IS WHY HE FLEW IT THE WAY HE DID (VERY SMOOTH BY THE WAY). AND HE DID GO TO TOGA MOMENTARILY TO GET IT INTO THE GAR MODE; BUT IT DIDN'T HAPPEN. WE AGREED THAT MAYBE HE DIDN'T LEAVE THE THRUST LEVERS IN THE TOGA POS LONG ENOUGH FOR THE FMGC TO RECOGNIZE IT. STILL; IT DOESN'T EXPLAIN THE THRUST LOCK AT TOGA THAT WE GOT LATER ON; BUT I CAN'T HELP BUT THINK THAT NOT USING TOGA INITIALLY HAD SOMETHING TO DO WITH IT. I WILL REMEMBER THIS EPISODE THE NEXT TIME I DO A GAR WHETHER I'M PF OR PM. ALWAYS START A GAR WITH TOGA; EVEN IF IT IS ONLY MOMENTARILY AS THE MANUAL SUGGESTS (WHEN INITIATING ABOVE 1000 FT AGL).