AN A300-600 LOSES ALT WHEN THE AUTOTHROTTLE SYS MALFUNCTIONS. THE FO PF WAS UNAWARE THAT THE AUTOTHROTTLES HAD DISENGAGED AFTER AN ENG TRIM SYS WARNING ON THE ECAM.

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LEVEL FL350; FO FLYING; I WAS IN THE BACK. HE WAS GIVEN THE FOLLOWING CLRNC: 'AFTER ARERO; CLRED DIRECT DELTA INTXN AND HOLD; REDUCE TO MINIMUM SPD AT THIS TIME.' THE FO SELECTED ALT HOLD AND 240 KTS; THEN THE THROTTLES CAME TO IDLE. SHORTLY AFTER HE GOT A CHIME; MASTER CAUTION AND THE FOLLOWING ECAM 'ENG TRIM FAULT; SELECT ENG TRIM OFF' WHICH HE DID. ON RESETTING THE ENG TRIM; HE NOTICED THE AIRSPD WAS BELOW THE HOOK. HE MANUALLY PUT THE THROTTLES TO FIREWALL PWR. AS THE AIRSPD WAS STILL DECREASING; HE DISCONNECTED THE AUTOPLT AND STARTED A GRADUAL DSCNT TO GET THE AIRSPD BACK TO GREEN DOT. I RETURNED TO THE COCKPIT AS THE AIRSPD WAS APCHING GREEN DOT; WITH APPROX 700 FT ALT LOSS. I CALLED HKG ATC REQUESTING A DSCNT TO A LOWER ALT; BUT WAS TOLD TO MAINTAIN ALT. BY THIS TIME THE FO WAS CLBING BACK TO FL350. CONTRIBUTING FACTORS: 1) THE THROTTLES WERE AT IDLE WHEN THE ENG TRIM FAULT HAPPENED. 2) THE AUTOTHROTTLES TRIPPED OFF; ALSO; SO DID NOT CAPTURE THE SPD SELECTED. SUGGESTION: HAVE A STATUS ECAM; AFTER YOU RESET THE ENG TRIM; STATING THE AUTOTHROTTLES ARE OFF.