Narrative:

While on approach control vectors to ILS runway 23R at rdu, we were cleared to descend to 4000 MSL on a heading to intercept the 23R localizer. We had brief visuals with a widebody transport 'heavy' 5 mi ahead and had been advised of his position and that we were following a 'heavy.' the scattered rain showers and clouds precluded maintaining a constant visual with the widebody transport and the airport. We intercepted and tracked in on the localizer at 4000 ft until the GS intercepted our glide path. I then wrongly initiated a descent on the GS while maintaining a traffic watch for the widebody transport and the airport. At approximately 3500 ft, the approach controller asked our altitude and remarked we should've been at 4000 ft. He asked if we had a visual on the airport (we did now) and we were cleared for a quick 'visual approach.' once on the ground, after an uneventful visual approach and landing, I talked to the approach supervisor about the event. The controller was vectoring a small commuter aircraft under the localizer to 23L and needed us at 4000 ft for traffic separation. If we had been told of this traffic, I doubt I would've descended as readily on the GS out of habit. The comment/explanation would have reinforced the 'maintain 4000 ft' clearance. Also, at the point of the controller's query of our altitude, the ILS GS would have been full deflection scale down if we had maintained the 4000 ft clearance. This situation was not desirable, and too close in for a comfortable ILS approach without visually identing the airport for a visual approach. This event could've been prevented by my diligently following the altitude clearance, and a brief explanation from the controller would have reinforced the altitude clearance.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION.

Narrative: WHILE ON APCH CTL VECTORS TO ILS RWY 23R AT RDU, WE WERE CLRED TO DSND TO 4000 MSL ON A HDG TO INTERCEPT THE 23R LOC. WE HAD BRIEF VISUALS WITH A WDB 'HVY' 5 MI AHEAD AND HAD BEEN ADVISED OF HIS POS AND THAT WE WERE FOLLOWING A 'HVY.' THE SCATTERED RAIN SHOWERS AND CLOUDS PRECLUDED MAINTAINING A CONSTANT VISUAL WITH THE WDB AND THE ARPT. WE INTERCEPTED AND TRACKED IN ON THE LOC AT 4000 FT UNTIL THE GS INTERCEPTED OUR GLIDE PATH. I THEN WRONGLY INITIATED A DSCNT ON THE GS WHILE MAINTAINING A TFC WATCH FOR THE WDB AND THE ARPT. AT APPROX 3500 FT, THE APCH CTLR ASKED OUR ALT AND REMARKED WE SHOULD'VE BEEN AT 4000 FT. HE ASKED IF WE HAD A VISUAL ON THE ARPT (WE DID NOW) AND WE WERE CLRED FOR A QUICK 'VISUAL APCH.' ONCE ON THE GND, AFTER AN UNEVENTFUL VISUAL APCH AND LNDG, I TALKED TO THE APCH SUPVR ABOUT THE EVENT. THE CTLR WAS VECTORING A SMALL COMMUTER ACFT UNDER THE LOC TO 23L AND NEEDED US AT 4000 FT FOR TFC SEPARATION. IF WE HAD BEEN TOLD OF THIS TFC, I DOUBT I WOULD'VE DSNDED AS READILY ON THE GS OUT OF HABIT. THE COMMENT/EXPLANATION WOULD HAVE REINFORCED THE 'MAINTAIN 4000 FT' CLRNC. ALSO, AT THE POINT OF THE CTLR'S QUERY OF OUR ALT, THE ILS GS WOULD HAVE BEEN FULL DEFLECTION SCALE DOWN IF WE HAD MAINTAINED THE 4000 FT CLRNC. THIS SITUATION WAS NOT DESIRABLE, AND TOO CLOSE IN FOR A COMFORTABLE ILS APCH WITHOUT VISUALLY IDENTING THE ARPT FOR A VISUAL APCH. THIS EVENT COULD'VE BEEN PREVENTED BY MY DILIGENTLY FOLLOWING THE ALT CLRNC, AND A BRIEF EXPLANATION FROM THE CTLR WOULD HAVE REINFORCED THE ALT CLRNC.

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.