Narrative:

Aircraft was on approach to ILS runway 19 at sdf airport. First officer was flying the approach with the autoplt coupled at the time. Aircraft was established on the localizer to runway 19 and descending to final approach fix altitude of 2800 ft and cleared for the approach by ATC. At approximately 3000 ft, the aircraft entered heavy rain. Simultaneously, ATC advised that the visibility at the airport had decreased to 1/2 mi and that the last 2 aircraft to make the approach had gone around. Captain elected to abandon the approach and advised ATC that we would level off. He then disengaged the altitude select while the first officer attempted to arrest the descent with the autoplt pitch wheel. However, the aircraft continued to descend, and the captain noticed that the autoplt had apparently captured the GS and was tracking it downward. At about this time ATC advised that the aircraft immediately ahead of ours had accomplished a successful landing and wanted to know if we desired to continue the approach. The captain responded negatively and attempted to disengage the approach coupler by selecting the autoplt command switch from 'ILS' to 'heading.' first officer again attempted to use pitch wheel to arrest descent, but the aircraft failed to respond and continued descent. At 2500 ft MSL, first officer disconnected autoplt and manually initiated a climb back to 3000 ft. The aircraft was vectored back for an uneventful approach and landing. Poor WX, multiple communications, and an unresponsive autoplt contributed to the altitude excursion. A more rapid disengagement of the autoplt may have prevented the excursion.

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Original NASA ASRS Text

Title: AUTOCOUPLED ILS APCH FLC DIFFICULTY ABANDONING APCH FOR GAR.

Narrative: ACFT WAS ON APCH TO ILS RWY 19 AT SDF ARPT. FO WAS FLYING THE APCH WITH THE AUTOPLT COUPLED AT THE TIME. ACFT WAS ESTABLISHED ON THE LOC TO RWY 19 AND DSNDING TO FINAL APCH FIX ALT OF 2800 FT AND CLRED FOR THE APCH BY ATC. AT APPROX 3000 FT, THE ACFT ENTERED HVY RAIN. SIMULTANEOUSLY, ATC ADVISED THAT THE VISIBILITY AT THE ARPT HAD DECREASED TO 1/2 MI AND THAT THE LAST 2 ACFT TO MAKE THE APCH HAD GONE AROUND. CAPT ELECTED TO ABANDON THE APCH AND ADVISED ATC THAT WE WOULD LEVEL OFF. HE THEN DISENGAGED THE ALT SELECT WHILE THE FO ATTEMPTED TO ARREST THE DSCNT WITH THE AUTOPLT PITCH WHEEL. HOWEVER, THE ACFT CONTINUED TO DSND, AND THE CAPT NOTICED THAT THE AUTOPLT HAD APPARENTLY CAPTURED THE GS AND WAS TRACKING IT DOWNWARD. AT ABOUT THIS TIME ATC ADVISED THAT THE ACFT IMMEDIATELY AHEAD OF OURS HAD ACCOMPLISHED A SUCCESSFUL LNDG AND WANTED TO KNOW IF WE DESIRED TO CONTINUE THE APCH. THE CAPT RESPONDED NEGATIVELY AND ATTEMPTED TO DISENGAGE THE APCH COUPLER BY SELECTING THE AUTOPLT COMMAND SWITCH FROM 'ILS' TO 'HDG.' FO AGAIN ATTEMPTED TO USE PITCH WHEEL TO ARREST DSCNT, BUT THE ACFT FAILED TO RESPOND AND CONTINUED DSCNT. AT 2500 FT MSL, FO DISCONNECTED AUTOPLT AND MANUALLY INITIATED A CLB BACK TO 3000 FT. THE ACFT WAS VECTORED BACK FOR AN UNEVENTFUL APCH AND LNDG. POOR WX, MULTIPLE COMS, AND AN UNRESPONSIVE AUTOPLT CONTRIBUTED TO THE ALT EXCURSION. A MORE RAPID DISENGAGEMENT OF THE AUTOPLT MAY HAVE PREVENTED THE EXCURSION.

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.