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.