PA31 PLT DESCENDS EARLY ON A NON PRECISION APCH DUE TO MALFUNCTIONING DME.

Narrative

WHILE INITIATING A VOR RWY 33 APCH INTO THE ARPT; A FALSE DISTANCE MEASURING EQUIP READING RESULTED IN A PREMATURE DSCNT FROM THE FINAL APCH ALT OF 3100 FT TOWARDS THE APCH MINIMUM OF 1940 FT. BEGINNING WITH THE CRUISE PORTION OF THE FLT; I RECEIVED THE ATIS INFO WHICH INFORMED ME TO EXPECT VECTORS TO THE VOR RWY 33 APCH. I BRIEFED MY APCH CHART ACCORDINGLY; AND THEN TUNED AND IDENTED BOTH NAV SYS. BOTH OF THE RADIOS AND VOR'S TUNED SATISFACTORILY; WITH NO FLAGS ON THE DME. BOTH VOR'S WERE WITHIN THE 30 DAY CHK LIMITS; SHOWING LESS THAN A 4 DEG DISCREPANCY; AS SHOWN IN THE LOGBOOK ENTRY. ON THE INITIAL APCH INTO THE ARPT; I WAS TOLD TO EXPECT THE VISUAL APCH TO RWY 33; GIVEN A HDG OF 140 DEGS; AND A DSCNT TO 4000 FT. ABEAM THE FIELD; I WAS GIVEN PERMISSION TO DSND TO 3100 FT FOR THE VISUAL APCH WITH THE POSSIBILITY OF VECTORS FOR THE VOR RWY 33 APCH SHOULD I NOT MAKE CONTACT WITH THE ARPT. THE ACFT PRECEDING ME TO THE ARPT WAS CLRED FOR THE VISUAL APCH; WHICH HE ACCOMPLISHED. AS I WAS PASSING THE FIELD; I WAS IN VISUAL CONTACT WITH THE GND HAVING SEVERAL MI VISIBILITY WITH A SCATTERED TO BROKEN LAYER BELOW; BUT WAS UNABLE TO MAKE VISUAL CONTACT WITH THE ARPT. I WAS THEN GIVEN INSTRUCTIONS TO CONTINUE MY PRESENT HDG AND EXPECT THE VOR APCH TO RWY 33. I WAS THEN GIVEN A HDG OF 090 DEGS AND I COMPLIED; ALL THE WHILE MAINTAINING 3100 FT. AS I CONTINUED ON THE 090 DEG HDG I WAS GIVEN A CLOSE-IN VECTOR OF L TO 360 DEGS TO THE FINAL APCH COURSE. I CONTINUED ON; STILL IN MARGINAL VFR CONDITIONS; AND WAS THEN CLRED FOR THE VOR RWY 33 APCH INTO THE ARPT; MAINTAINING 3100 FT UNTIL ESTABLISHED. AS I WAS TURNING TO THE 360 DEG INTERCEPT HDG I REALIZED THAT; ACCORDING TO MY #1 VOR INDICATOR; I WAS IMMEDIATELY FLYING INTO THE APCH COURSE. DUE TO THE CLOSE IN VECTOR I MADE AN EXPEDITED TURN BACK TO THE L; CONTINUED INBOUND; AND JOINED THE APCH COURSE; NOT HAVING EXCEEDED FULL SCALE DEFLECTION AT ANY POINT. THE APCH CTLR SOON ASKED ME IF I WAS ESTABLISHED ON THE FINAL APCH COURSE AND; SINCE MY #1 VOR NEEDLE WAS CTRING; I RESPONDED AFFIRMATIVELY. IT WAS AT THAT POINT THAT I TOOK A READING FROM MY DISTANCE MEASURING EQUIP; WHICH WAS SHOWING A DISTANCE OF 9.9 MI FROM THE VOR. THIS DISTANCE IS INSIDE OF THE 10 MI FAF AT NEISH; THE LET-DOWN POINT TO THE MINIMUM DSCNT ALT. THERE WERE NO FLAGS OR WARNINGS SHOWING THAT THIS INFO WAS UNRELIABLE; SO I BEGAN MY DSCNT FROM 3100 FT TOWARDS THE MINIMUM DSCNT ALT OF 1940 FT. NOW ESTABLISHED; I WAS BEGINNING THE FINAL APCH/LNDG CHKLIST WHEN THE CTLR INFORMED ME THAT I WAS L OF COURSE. UPON INSPECTION; I DETERMINED THAT THE #1 VOR WAS STILL CLOSE TO CTR; BUT THE INDICATIONS OF THE #2 VOR COINCIDED MORE WITH THE INFO GIVEN BY THE CTLR. IT WAS AT THIS TIME THAT THE CTLR ISSUED A LOW ALT WARNING/ALERT. UPON HEARING THIS WARNING I IMMEDIATELY CHKED MY ALTIMETER; WHICH WAS GIVING ME A READING OF APPROX 2700 FT MSL. NOT REALIZING THAT THE DME WAS NOT FUNCTIONING PROPERLY AND BELIEVING THAT I WAS INSIDE OF THE FAF; I ASSUMED THAT THE CTLR WAS SPEAKING OF A DSCNT BELOW THE MINIMUM DSCNT ALT WITHIN THE FAF. I IMMEDIATELY STOPPED MY DSCNT AND LEVELED AT MY CURRENT ALT OF 2700 FT MSL. I THEN VERIFIED FROM MY APCH CHARTS THAT I HAD CORRECTLY CHOSEN THE MDA OF 1940 FT MSL; WHICH IS THE CORRECT ALT ONCE WITHIN 10 DME OF THE VOR. I RESPONDED TO THE CTLR SAYING THAT; BASED ON THE INFO I WAS RECEIVING FROM THE COCKPIT; I WAS WITHIN THE LIMITS OF THE APCH. THERE WAS STILL NO FLAGGED INDICATION OF MALFUNCTIONING EQUIP SO I CONTINUED INBOUND WHILE TRYING TO FIGURE OUT THE DISCREPANCY BTWN THE CTLR'S AND MY INFO. I CONTINUED TO HOLD MY ALT AND BEGAN CORRECTING FOR THE ERROR IN THE #1 VOR AND COMING TO CTR BY NAVING OFF OF THE MORE ACCURATE #2 VOR. THE CTLR THEN ISSUED A FINAL LOW ALT WARNING AND ADDED THAT I WAS CLRED TO MAINTAIN 3100 FT UNTIL THE FAF. IT WAS AT THIS POINT THAT I REALIZED I HAD AN EQUIP PROB; WAS RECEIVING FAULTY DME READINGS; AND WAS STILL OUTSIDE OF NEISH. I RESPONDED TO THE CTLR THAT I WAS SWITCHING RADIOS; INAN ATTEMPT TO REGAIN A RELIABLE DME SIGNAL; WHILE INITIATING A CLB. I THEN SWITCHED THE DME RECEIVER FROM THE #1 TO THE #2 NAV RADIO; AND THEN TAPPED THE MECHANICAL DME HEAD. AT THIS POINT; I NOTICED THE DME ROLLERS BEGAN TO SPIN FROM A STILL PRESENT 9.9 MI INDICATION AND CAME BACK INTO OP. BY THE TIME THAT THE MALFUNCTION WAS DISCOVERED AND RECTIFIED; I WAS WITHIN THE PREVIOUSLY INDICATED 10 MI FAF. I WAS THEN INFORMED THAT I WAS INSIDE OF NEISH AND GIVEN CLRNC TO SWITCH OVER TO THE TWR FREQ. I CONTINUED THE APCH; ON COURSE; DSNDING INTO AND THROUGH THE CLOUD LAYER AND LNDG WITH NO FURTHER COMPLICATIONS. A VFR REPOSITION BACK TO THE MAINT BASE WAS ACCOMPLISHED THE FOLLOWING MORNING WITH THE DME LABELED INOP; REFED UNDER THE MEL. ONCE BACK AT THE BASE OF OPS; I WAS ABLE TO DUPLICATE THE PROB IN FRONT OF OTHERS; AND THE DME WAS SIGNED INOP BY A LICENSED MECH. THE PROB APPEARED TO BE AN INTERMITTENT ISSUE; RELATED TO EITHER POOR NAVIGATIONAL RADIO SIGNAL STRENGTH WITH A MALFUNCTIONING FLAG OR MECHANICAL BINDING OF THE ROLLERS AND FLAGS. THE VOR'S WERE ALSO CHKED FOR ACCURACY FINDING BOTH TO BE WITHIN TOLERANCES; ALBEIT WITH THE #1 VOR AT THE LIMITS. THOSE READINGS SHOWING AN ACCURACY CHANGE FROM THE PREVIOUS 3 WK OLD LOGBOOK ENTRY.