B727 CREW DSNDED BELOW CLRED ALT ON THE RWY 24R ILS AT DAY.

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MY FO HAD BEEN FLYING THE PLANE WHEN WE WERE CLRED BY DAYTON APCH FOR THE ILS FOR RWY 24R. THE CTLR TOLD US TO MAINTAIN 3000 FT UNTIL ESTABLISHED; WHICH I REPEATED BACK; AND THEN I SWITCHED OVER TO TWR AS INSTRUCTED. MY FO; HOWEVER; THOUGHT HE HAD SAID 2000 FT AND AS HE WAS JOINING THE LOC; HE CONTINUED HIS DSCNT THROUGH 3000 FT. I THOUGHT HE WAS DSNDING DOWN TO THE INITIAL APCH ALT OF 2600 FT SINCE WE WERE ESTABLISHED AND DIDN'T KNOW THAT HE HAD 2000 FT SET IN HIS MIND. JUST AS I NOTICED WHAT HE WAS DOING AND SAID SOMETHING; DAYTON TWR INFORMED US THAT WE HAD SET OFF AN ALT ALERT BECAUSE WE WERE TOO LOW FOR THE APCH. MY FO HAD REACHED ABOUT 2200 FT AT THIS TIME AND WAS CORRECTING BY QUICKLY CLBING BACK UP TO 2600 FT. I STATED WE WERE AT 2600 FT AND HE CLRED US TO LAND. WE HAD GND CONTACT THE ENTIRE TIME AND WERE ABLE TO FLY AND LAND VISUALLY. A BETTER REVIEW OF THE APCH PLATE; IN PARTICULAR THE INITIAL APCH ALT; WHETHER CONDITIONS ARE VMC OR IMC; WOULD PREVENT A RECURRENCE. SUPPLEMENTAL INFO FROM ACN 484650: I WAS THE FE ON A FLT FROM JAX TO DAY. ONCE IN THE DAY AIRSPACE; WE WERE CLRED FOR THE ILS APCH TO RWY 24R AND DOWN TO 3000 FT. 3000 FT WAS SET INTO THE ALT ALERT BY THE CAPT; AS THE FO WAS FLYING THE ACFT. DURING THE DSCNT; I WAS BUSY SCANNING THE ENGINEER PANEL AND SETTING UP THE ENG XFEED TO THE PROPER CONFIGN. LOOKING BACK FORWARD AT THE INSTS; I SAW THAT WE WERE PASSING THROUGH 2500 FT MSL. I THEN CHKED THE FO'S ALT INDICATOR AND SAW THAT WE WERE WELL BELOW GS. I WAS ABOUT TO INFORM THE FO OF THIS WHEN DAYTON TWR CALLED AND SAID HE WAS GETTING A LOW ALT ALERT AND THAT WE WERE TO CLB. THE CAPT REPLIED THAT WE WERE CORRECTING TO 2600 FT. THE FO THEN ACQUIRED THE GS; AND AN UNEVENTFUL LNDG ENSUED. THE WX WAS RPTED AS 210 DEGS/4 KTS; 5 MI MIST; 6 MI SCATTERED; 47 MI SCATTERED; 80 MI OVCST; 22/21/30.15. WE BROKE OUT OF THE CLOUD COVERAGE AND ACQUIRED THE RWY VISUALLY AT APPROX 1500 FT MSL AND 5 MI.