DURING VOR DME APCH RPTR CAPT INADVERTENTLY HAD THE DME HOLD FUNCTION ENGAGED AND; THINKING HE WAS PAST THE FAF; COMMENCED DSCNT TO MINIMUMS. THE FO ALERTED HIM AND HE NOTED THE INCORRECT POS OF THE DME; CLBED BACK TO FAF ALT AND CONTINUED APCH.

Narrative

WHILE FLYING RADAR VECTORS FOR THE VOR/DME RWY 17 APCH TO MHT; I HAD THE KNS-660 RNAV SELECTED AS MY SOURCE FOR NAV INFO. THIS ALLOWED GENERATION OF A MOVING MAP DISPLAY ON THE WX RADAR SCREEN; WHICH SHOWED THE FINAL APCH COURSE AND NAVAIDS IN RELATION TO ACFT POS AND WX RETURNS. THE FO'S SOURCE FOR NAV INFO WAS NAV RADIO #2; WHICH HAD BEEN TUNED TO CONCORD VOR. AS WE DSNDED TOWARD OUR ASSIGNED ALT OF 2000 FT AND NEARED THE APCH COURSE; THE FO NOTED THAT WE WOULD BE VECTORED ONTO THE APCH OUTSIDE OF FERNS INTXN; THE FAF. MY RNAV INFO AGREED WITH THIS. PRIOR TO INTERCEPTING THE APCH COURSE; I SELECTED NAV RADIO #1; WHICH HAD ALSO BEEN TUNED TO CONCORD VOR; AS MY NAV SOURCE. UPON INTERCEPTING; I SLOWED THE ACFT; CALLED FOR GEAR AND FLAP EXTENSION; THEN GLANCED AT MY DME. THINKING I WOULD HAVE ADEQUATE TIME TO CONFIGURE THE ACFT PRIOR TO FERNS; I WAS SURPRISED TO FIND MY DME INDICATING 13.5; OR 1.5 MI PAST FERNS. I IMMEDIATELY INITIATED DSCNT TOWARD 1200 FT; OUR NEXT STEP-DOWN ALT. SHORTLY; THE FO INDICATED SOMETHING WAS NOT RIGHT. I RECALL GLANCING AGAIN AT MY DME READOUT AND REALIZING IT DID NOT MAKE SENSE. I THEN DISCOVERED MY DME HOLD FEATURE WAS ENGAGED AND IMMEDIATELY SELECTED IT OFF. MY DME THEN READ APPROX 10.5 AND I REALIZED WE HAD NOT YET REACHED FERNS. I IMMEDIATELY INITIATED A CLB BACK TO 2000 FT; NOTING THE RADAR ALTIMETER INDICATION OF APPROX 1000 FT. WE HAD DSNDED SEVERAL HUNDRED FT BELOW THE MINIMUM ALT OF 2000 FT. WE CONFIRMED THAT BOTH DME READOUTS WERE IN AGREEMENT AND CONTINUED THE APCH UNEVENTFULLY. FACTORS INVOLVED INCLUDE THE FOLLOWING: 1) MY DME INITIALLY GAVE AN INCORRECT FIGURE DUE TO INADVERTENT SELECTION OF THE DME HOLD FEATURE. THE VALUE DISPLAYED SEEMS CONSISTENT WITH A READOUT FROM MHT VOR; ALTHOUGH THE LAST TIME I RECALL ENGAGING THE HOLD FEATURE WAS FOR LGA DURING AN APCH ON THE PREVIOUS FLT. THE ONLY VISUAL INDICATION THAT DME HOLD IS SELECTED ON IS A SMALL GREEN ANNUNCIATOR ABOVE THE ATTITUDE INDICATOR. MORSE CODE IDENTS WERE CHKED FOR THE VOR; BUT NOT THE DME; PRIOR TO THE APCH. 2) THE FO LATER SAID HE REALIZED THAT DSCNT WAS INITIATED AT A DME DISTANCE DIFFERENT FROM WHAT HE THOUGHT IT SHOULD HAVE BEEN. HE INITIALLY THOUGHT THAT HE MIGHT HAVE BEEN IN ERROR AND REFRAINED FROM SAYING ANYTHING UNTIL HE HAD RECHKED HIS APCH CHART AND NAV SETUP. 3) WHEN THE CAPT'S NAV SOURCE IS SWITCHED FROM THE RNAV TO NAV RADIO #1; THE RNAV COMPUTER RESORTS TO A DEAD RECKONING MODE (NAV RADIO #1 NO LONGER SUPPLIES REQUIRED INFO TO THE RNAV COMPUTER); AND BOTH THE INFO ON THE RNAV SCREEN AND THE MOVING MAP DISPLAY BECOME UNUSABLE. FOR THIS REASON; I USUALLY DELAY SELECTION OF NAV RADIO #1 AS MY NAV SOURCE UNTIL JUST PRIOR TO INTERCEPT OF A FINAL APCH COURSE. IN THIS CASE; THIS PROC MAY HAVE PREVENTED THE CREW FROM HAVING ENOUGH TIME TO ADEQUATELY XCHK DME READOUTS AS WE INITIATED THE APCH. 4) DISTRACTIONS WERE PRESENT; INCLUDING TURB AND RADAR WX RETURNS IN THE VICINITY OF THE APCH COURSE. 5) THE CREW WAS FATIGUED DUE TO A VERY LATE FINISH THE PREVIOUS NIGHT AS A RESULT OF A DIVERSION AND SUBSEQUENT GND TRANSPORTATION BACK TO BASE.