TRI CTLR ISSUED A LOWER THAN MVA ALT AND THEN MISSED THE READBACK FROM THE FLT CREW.

Narrative

ISSUED DSCNT TO 2000 FT IN A 6000 FT MVA. ACFT READ BACK 2000 FT AND CTLR FAILED TO CATCH THE INCORRECT ALT. ONLY WHEN THE FACILITY 'TERRAIN ALERT' SOUNDED DID I SEE THE ACFT HAD DSNDED BELOW 6000 FT. PLT WAS ADVISED TO CLB; HE SAID NO PROB HE HAD GOOD VISIBILITY AND BEGAN A CLB. REASON: FAILURE TO LISTEN TO READBACK. I ASSUMED THAT I GAVE THE ACFT 6000 FT. I MARKED THE STRIP AS I TALKED '60.' ONLY AFTER REVIEWING THE TAPE DID I HEAR MYSELF SAY '2000.' WHY? I HAVE NO IDEA! OUR MVA IS 3200 FT. WE NEVER GIVE AN ACFT 2000 FT. CORRECT WAY; OR WAY TO PREVENT THIS: PAY ATTN TO EACH DETAIL. THERE WAS NO EVASIVE ACTION. THE TERRAIN WAS ABOUT 3300 FT WHEN THE FACILITY ALERT SOUNDED. THE PLT WAS NOT AWARE OF THE PENDING SITUATION. SUPPLEMENTAL INFO FROM ACN 682309: THE FO WAS THE PF AND I WAS THE CAPT PERFORMING THE PNF DUTIES. WE WERE AT 9000 FT APPROX 30 NM N OF TRI. THE APCH CTLR TOLD US TO DSND TO 2000 FT. WHILE PASSING THROUGH 4500 FT; THE CTLR ASKED WHAT ALT WE WERE AT AND WE REPLIED THAT WE WERE DSNDING THROUGH 4500 FT FOR 2000 FT. THE CTLR SAID WE WERE ONLY CLRED TO 6000 FT. WE IMMEDIATELY CLBED BACK TO 6000 FT AND CONTINUED TO LAND WITHOUT FURTHER INCIDENT. BOTH THE FO AND I BELIEVE WE WERE GIVEN THE CLRNC TO 2000 FT. WE BOTH VERIFIED IT WHEN IT WAS SET INTO THE ALT PRE-SELECTOR. HOWEVER; I FEEL IF ONE IS EVER IN DOUBT OF AN ASSIGNED ALT; ONE SHOULD ASK ATC TO CONFIRM THE INSTRUCTION OR REPEAT THE CLRNC. THIS CAN PREVENT SUCH AN EVENT FROM OCCURRING AGAIN.