A319 CREW HAD A RUNAWAY AUTOTHRUST SYS WHICH EXCEEDED SPD LIMITATIONS OF THE ACFT AND ATC ENVIRONMENT.

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AT APPROX XA35 ON SEP/FRI/00; AN AUTOTHRUST INCIDENT OCCURRED WHILE OPERATING AIRBUS 319 SVC FROM SNA TO PIT. THIS EVENT CAUSED MY FO AND I TO EXCEED AN ACFT FLAP LIMITATION SPD OF 230 KTS AND TO INADVERTENTLY ACCELERATE TO AN APPROX AIRSPD OF 310 KTS BELOW 10000 FT. AT THE TIME OF OUR ARR IN THE PIT TERMINAL AREA; WX CONDITIONS WERE VFR DURING THE DSCNT AND APCH PHASE OF THE FLT. AS CAPT; I WAS THE PNF AND THE FO WAS THE PF. ALL AUTOPLT; AUTOTHRUST AND ACFT SYS WERE OPERATING NORMALLY AT THAT TIME. IN COMPLIANCE WITH THE INSTRUCTIONS GIVEN TO US BY PIT APCH CTL AT APPROX XA36; WE DSNDED TO AN ALT OF 4000 FT MSL AND PROCEEDED FROM WISKE INTXN ON A NE COURSE TOWARD THE PIT ARPT. APPROX 8 MI W OF THE ARPT; I INFORMED THE APCH CTLR THAT WE HAD RWY 10L IN VISUAL CONTACT. THE CTLR ACKNOWLEDGED THE XMISSION AND CLRED US FOR A VISUAL APCH TO RWY 10L. AS THE FO CONTINUED THE DSCNT TOWARD RWY 10L; HE RESTED HIS L HAND ON THE AUTOTHRUST LEVERS. OUR SPD WAS APPROX 220 KTS WITH A FLAP SETTING OF 1 DEG. APPROX 3 MI W OF THE RWY 10L FAF; IT BECAME OBVIOUS THAT WE WERE SLIGHTLY HIGH ON THE APCH FOR LNDG ON RWY 10L; DUE TO A SLIGHT QUARTERING TAILWIND AT OUR ALT. I DETERMINED THAT IT MIGHT BE NECESSARY TO REQUEST S-TURNS ON FINAL TO LOSE ALT; BUT BEFORE I COULD SPEAK TO THE APCH CTLR; THE FO INADVERTENTLY RAISED THE THRUST LEVERS OUT OF THE C/L DETENT. AT THE INSTANT I OBSERVED THIS ACTION; I ADVISED THE FO TO IMMEDIATELY RETURN THE THRUST LEVERS TO THE C/L DETENT. I ALSO NOTED THAT HE HAD BARELY RETARDED THE THRUST LEVERS 1/8 INCH BACK FROM THE C/L DETENT. THE FO RETURNED THE THRUST LEVERS TO THE C/L DETENT; BUT THE ACFT BEGAN TO ACCELERATE AS THRUST SIGNIFICANTLY INCREASED. NO ECAM MESSAGE OR MENU ACTION WAS PRESENT ON THE ENG/WARNING DISPLAY AND THE AUTOTHRUST COLUMN OF THE PRIMARY FLT DISPLAY WAS BLANK. AS THE ACFT ACCELERATED THROUGH 230 KTS; MASTER WARNING AND MASTER CAUTION LIGHTS ILLUMINATED; THE PFD OVERSPD INDICATION CAME INTO VIEW AND THE AURAL WARNING WAS INITIATED. IN RESPONSE TO THESE WARNINGS; I STATED TO THE FO; 'I HAVE THE ACFT.' I PULLED THE SELECTED SPD KNOB IN AN EFFORT TO RESET THE AUTOTHRUST SPD MODE OF THE SYS; BUT THIS ACTION PROVED UNSUCCESSFUL. AT THAT POINT; THE ACFT HAD QUICKLY ACCELERATED TO APPROX 250 KTS. I RETRACTED THE FLAPS AND SCANNED THE PFD; THE FLT CTL UNIT AND THE ECAM WARNING DISPLAY FOR ANY ENHANCED INFO WHICH WOULD EXPLAIN THE UNCOMMANDED AUTOTHRUST ACCELERATION. I ALSO ADVISED THE PIT APCH CTLR THAT WE WERE EXPERIENCING DIFFICULTIES. OUR AIRBUS 319/320/321 POH CLRLY STATES THAT ONE MUST RETARD THE THRUST LEVERS TO THE IDLE DETENT IN ORDER TO DISCONNECT THE AUTOTHROTTLE SYS. THE FO HAD ONLY REMOVED THE THRUST LEVERS FROM THE C/L DETENT AND HAD NOT RETARDED THE THRUST LEVERS MORE THAN 1/8 INCH. THE THRUST LEVERS HAD NEVER BEEN RETARDED TO THE IDLE DETENT. WITH NO ECAM WARNING MESSAGE OR PFD INFO REGARDING THE STATUS OF THE AUTOTHRUST SYS; WE WERE AT A LOSS TO EXPLAIN WHY THE ACFT WAS ACCELERATING. AT OUR SPD APCHED 300 KTS; I WAS ABOUT TO OVERRIDE THE AUTOTHRUST ACCELERATION BY RETARDING THE THRUST LEVERS. PRIOR TO ACCOMPLISHING THIS ACTION; HOWEVER; I RESET THE AUTOTHRUST P/B AND CTL OVER THE AUTOTHRUST SYS WAS RE-ESTABLISHED. AT THIS POINT IN TIME; OUR SPD HAD REACHED APPROX 300-310 KTS. BY RESETTING THE AUTOTHRUST P/B; WE WERE ABLE TO RECOVER CTL OVER THE AUTOTHRUST SYS. THRUST REDUCED TO IDLE; AND THE SELECTED SPD TARGET OF 170 KTS WAS MET. WE ADVISED PIT APCH CTL THAT WE HAD SUCCESSFULLY ADDRESSED OUR PROB AND ACCOMPLISHED A NORMAL LNDG ON RWY 10L. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE INCIDENT IS UNDER INVESTIGATION BY THE ACR; THE MANUFACTURER AND THE FAA. SIMILAR OPS HAVE BEEN UNSUCCESSFUL IN DUPLICATING THE PROB. THE RPTR DOES NOT KNOW WHAT MAINT HAS DONE TO RETURN THE ACFT TO SVC. HE IS CONCERNED THAT A DUPLICATION COULD PLACE A CREW IN A DANGEROUS POS; AND WOULD LIKE TO SEE SOME POSITIVE ACTION TAKEN.