Narrative:

We were conducting a flight in IMC toward nantucket airport and had been cleared for the instrument approach just outside the marker. This was a familiarization instrument flight, and I was in the right seat while the instrument pilot was instructed in the use of the coupled autoplt. We engaged the flight director system into the approach mode, and it quickly coupled. The GS bars immediately centered, and the aircraft began a descent. We failed to monitor the other instruments, and we received a low-altitude warning from the tower as we broke out. Visibility was 7 mi, and we continued the approach at 500 ft with the airport in sight at 2 mi, landing without incident. The GS bars on the flight director had come down and 'stuck' and had commanded a descent far greater than the approach GS path. Our discussions of, and during the approach, detracted us from a full scan (#2 navigation with GS, distance equipment, etc) which would have alerted us to the failed instrument. Best technique would have been to immediately go to the missed approach and, ultimately, reconfigure once the error was detected, despite the visual ground contact and our initial surprise at being distant from the map. Errors: equipment malfunction, failure to adequately continue instrument scan and xref all sources, failure to commit to immediate missed approach. Correction: continuous xchk of all instruments under all circumstances, chkpoint references throughout the approach, consider missed approach as the first response to any significant deviation in the approach.

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Original NASA ASRS Text

Title: A PA32 CFI WAS NOTIFIED BY ACK TWR, DURING AN ILS IN IMC, THAT THEY WERE TOO LOW.

Narrative: WE WERE CONDUCTING A FLT IN IMC TOWARD NANTUCKET ARPT AND HAD BEEN CLRED FOR THE INSTRUMENT APCH JUST OUTSIDE THE MARKER. THIS WAS A FAMILIARIZATION INSTRUMENT FLT, AND I WAS IN THE R SEAT WHILE THE INSTRUMENT PLT WAS INSTRUCTED IN THE USE OF THE COUPLED AUTOPLT. WE ENGAGED THE FLT DIRECTOR SYS INTO THE APCH MODE, AND IT QUICKLY COUPLED. THE GS BARS IMMEDIATELY CTRED, AND THE ACFT BEGAN A DSCNT. WE FAILED TO MONITOR THE OTHER INSTRUMENTS, AND WE RECEIVED A LOW-ALT WARNING FROM THE TWR AS WE BROKE OUT. VISIBILITY WAS 7 MI, AND WE CONTINUED THE APCH AT 500 FT WITH THE ARPT IN SIGHT AT 2 MI, LNDG WITHOUT INCIDENT. THE GS BARS ON THE FLT DIRECTOR HAD COME DOWN AND 'STUCK' AND HAD COMMANDED A DSCNT FAR GREATER THAN THE APCH GS PATH. OUR DISCUSSIONS OF, AND DURING THE APCH, DETRACTED US FROM A FULL SCAN (#2 NAV WITH GS, DISTANCE EQUIP, ETC) WHICH WOULD HAVE ALERTED US TO THE FAILED INSTRUMENT. BEST TECHNIQUE WOULD HAVE BEEN TO IMMEDIATELY GO TO THE MISSED APCH AND, ULTIMATELY, RECONFIGURE ONCE THE ERROR WAS DETECTED, DESPITE THE VISUAL GND CONTACT AND OUR INITIAL SURPRISE AT BEING DISTANT FROM THE MAP. ERRORS: EQUIP MALFUNCTION, FAILURE TO ADEQUATELY CONTINUE INSTRUMENT SCAN AND XREF ALL SOURCES, FAILURE TO COMMIT TO IMMEDIATE MISSED APCH. CORRECTION: CONTINUOUS XCHK OF ALL INSTRUMENTS UNDER ALL CIRCUMSTANCES, CHKPOINT REFS THROUGHOUT THE APCH, CONSIDER MISSED APCH AS THE FIRST RESPONSE TO ANY SIGNIFICANT DEV IN THE APCH.

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.