Narrative:

We were on base leg at 4000 ft approximately 15 mi north of dfw. The controller told us to turn to 150 degrees, descend to 3000 ft, cleared ILS 18R, maintain 300 ft until established. I did that, however, after intercepting final I continued by descent toward the final approach fix (OM) altitude of 2300 ft. The problem is that the min altitude is 3000 ft until 9 DME. During the turn from base to final we received 2 taþs from the TCASII distracting both pilots to some degree. At around 2500 ft, the approach controller advised us that we had descended below approach altitude and reclred us to intercept the GS at 2500 ft if we desired. I added power and began a climb back toward 3000 ft. Approach and landing were normal from this point. In retrospect, I do not believe that we would have descended below 3000 ft had I had the flight director engaged. However, the fact that I missed the altitude points to the need for some basic instrument work. In this case, while heading 150 degrees, I became fixated on the localizer CDI. This was due, in part, to the fact that the approach controllers at dfw commonly set us up for large turns to intercept or give us the dogleg at or near CDI case break. Although, that was not the case in this situation.

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Original NASA ASRS Text

Title: FO OF ACR WDB ACFT INADVERTENTLY DSNDED BELOW APCH INTERCEPTION ALT RESULTING IN ATC INTERVENING AND REISSUING APCH CLRNC. ALTDEV OVERSHOT.

Narrative: WE WERE ON BASE LEG AT 4000 FT APPROX 15 MI N OF DFW. THE CTLR TOLD US TO TURN TO 150 DEGS, DSND TO 3000 FT, CLRED ILS 18R, MAINTAIN 300 FT UNTIL ESTABLISHED. I DID THAT, HOWEVER, AFTER INTERCEPTING FINAL I CONTINUED BY DSCNT TOWARD THE FINAL APCH FIX (OM) ALT OF 2300 FT. THE PROBLEM IS THAT THE MIN ALT IS 3000 FT UNTIL 9 DME. DURING THE TURN FROM BASE TO FINAL WE RECEIVED 2 TAþS FROM THE TCASII DISTRACTING BOTH PLTS TO SOME DEGREE. AT AROUND 2500 FT, THE APCH CTLR ADVISED US THAT WE HAD DSNDED BELOW APCH ALT AND RECLRED US TO INTERCEPT THE GS AT 2500 FT IF WE DESIRED. I ADDED PWR AND BEGAN A CLB BACK TOWARD 3000 FT. APCH AND LNDG WERE NORMAL FROM THIS POINT. IN RETROSPECT, I DO NOT BELIEVE THAT WE WOULD HAVE DSNDED BELOW 3000 FT HAD I HAD THE FLT DIRECTOR ENGAGED. HOWEVER, THE FACT THAT I MISSED THE ALT POINTS TO THE NEED FOR SOME BASIC INST WORK. IN THIS CASE, WHILE HDG 150 DEGS, I BECAME FIXATED ON THE LOC CDI. THIS WAS DUE, IN PART, TO THE FACT THAT THE APCH CTLRS AT DFW COMMONLY SET US UP FOR LARGE TURNS TO INTERCEPT OR GIVE US THE DOGLEG AT OR NEAR CDI CASE BREAK. ALTHOUGH, THAT WAS NOT THE CASE IN THIS SITUATION.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.