MD80 CREW DSNDED BELOW THEIR CLRED ALT ON A STAR AT DFW.

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ALTDEV. GLEN ROSE ARR; EXPECTING RWY 18R; TOLD TO KEEP SPD UP (WE WERE DOING 300 KTS). CAPT NOTICED TARGET ON TCASII AT BOUT 4-5 MI; 400 FT BELOW US. JUST AFTER HE MENTIONED TFC; APCH POINTED OUT TFC AND ASKED IF WE SAW IT. AT THE TIME CAPT WAS REPLYING WE HAD TFC IN SIGHT I WAS MAKING TURN AT DELMO (FO FLYING). I HEARD APCH SAY FOLLOW THE TFC; CLRED THE VISUAL RWY 18R; MAINTAIN 210 KTS. I ROLLED OUT HDG N BEHIND THE TFC; BEGAN SLOWING TO 210 KTS; AND STARTED DSNDING OUT OF 11000 FT. I REACHED OVER TO PUT MY 'CLRED THE VISUAL' MEMORY DEVICE IN THE PMS AND SAID 'CLRED THE VISUAL.' ALT ALERT SOUNDED NEXT AS 11000 FT WAS STILL SELECTED. I RESELECTED TO 5000 FT. CAPT SAW ME DO THIS AND ASKED IF WE HAD BEEN CLRED TO 5000 FT. I REPLIED; I BELIEVE; 'WE ARE CLRED THE VISUAL; I AM SETTING THE GAR ALT FOR RWY 18R.' HE SAID WE WERE NOT CLRED THE VISUAL. OUR ALT WAS 9900 FT. HE ASKED APCH IF WE WERE CLRED THE VISUAL AND THEY DID NOT DIRECTLY ANSWER BUT CLRED US TO 6000 FT. NO MENTION WAS MADE BY APCH OF AN ALTDEV. FURTHER DISCUSSION REVEALED THE CAPT HEARD EXPECT THE VISUAL; NOT CLRED THE VISUAL. HE ALSO SAW ME ENTER MEMORY DEVICE IN PMS AND SAY SOMETHING. ALTHOUGH HE DID NOT KNOW WHAT I HAD SAID; HE STATED HE COULD NOT HEAR 80% OF WHAT I SAY. I DO NOW REMEMBER HIM MENTIONING DIFFICULTY HEARING ME EARLIER IN THE TRIP. I DO NOT RECALL WHAT HE READ BACK TO APCH AFTER THEY CLRED US TO FOLLOW THE TFC. I ASKED THE CAPT TO PLEASE SAY SO IF HE DID NOT HEAR WHAT I WAS SAYING. IN THE FUTURE; IN AN ATTEMPT TO PRECLUDE A SIMILAR CONFUSING INCIDENT -- I INTEND TO: 1) INFORM CAPT OF MY PMS MEMORY CRUTCH FOR APCH AND LNDG. 2) SPEAK UP WHEN CAPT SAYS HE CANNOT HEAR ME. I NORMALLY DO NOT SPEAK SOFTLY; BUT I UNDERSTAND MORE AND MORE AS TIME PASSES THE EFFECTS OF GROWING OLDER. 3) NOT EXPECT; AS I THINK I DID; TO HEAR CLRNC FOR A VISUAL APCH WHEN INSTRUCTED TO FOLLOW TFC. SUPPLEMENTAL INFO FROM ACN 420781: GLEN ROSE ARR FO FLYING WE WERE TOLD TO KEEP THE SPD UP TO DELMO. JUST BEFORE DELMO SLOWING TO 210 KTS WE WERE GIVEN TFC. I SAID I HAD THE TFC AND WE WERE TOLD TO MAINTAIN VISUAL WITH THE TFC AND EXPECT A VISUAL TO RWY 18R. THE FO SAID SOMETHING I DIDN'T HEAR AND I TURNED TO GET MY APCH PLATE FOR RWY 18R. A FEW SECONDS LATER I HEARD THE ALT ALERT. I ASKED THE FO IF WE HAD BEEN CLRED LOWER AND HE SAID HE THOUGHT WE WERE CLRED THE VISUAL. I ASKED APCH CTL AND HE SAID NO; BUT CLRED US TO 5000 FT. OUR ALT WAS 9900 FT AT THE TIME. NO MENTION WAS MADE BY APCH CTL OF AN ALTDEV. I SHOULD HAVE ASKED THE FO WHAT HE SAID BUT I HAD HAD TROUBLE HEARING HIM THE WHOLE TRIP AND I GUESS I JUST GOT TIRED OF ASKING HIM TO REPEAT.