Narrative:

While initiating a VOR runway 33 approach into the airport; a false distance measuring equipment reading resulted in a premature descent from the final approach altitude of 3100 ft towards the approach minimum of 1940 ft. Beginning with the cruise portion of the flight; I received the ATIS information which informed me to expect vectors to the VOR runway 33 approach. I briefed my approach chart accordingly; and then tuned and idented both navigation system. Both of the radios and VOR's tuned satisfactorily; with no flags on the DME. Both VOR's were within the 30 day check limits; showing less than a 4 degree discrepancy; as shown in the logbook entry. On the initial approach into the airport; I was told to expect the visual approach to runway 33; given a heading of 140 degrees; and a descent to 4000 ft. Abeam the field; I was given permission to descend to 3100 ft for the visual approach with the possibility of vectors for the VOR runway 33 approach should I not make contact with the airport. The aircraft preceding me to the airport was cleared for the visual approach; which he accomplished. As I was passing the field; I was in visual contact with the ground having several mi visibility with a scattered to broken layer below; but was unable to make visual contact with the airport. I was then given instructions to continue my present heading and expect the VOR approach to runway 33. I was then given a heading of 090 degrees and I complied; all the while maintaining 3100 ft. As I continued on the 090 degree heading I was given a close-in vector of left to 360 degrees to the final approach course. I continued on; still in marginal VFR conditions; and was then cleared for the VOR runway 33 approach into the airport; maintaining 3100 ft until established. As I was turning to the 360 degree intercept heading I realized that; according to my #1 VOR indicator; I was immediately flying into the approach course. Due to the close in vector I made an expedited turn back to the left; continued inbound; and joined the approach course; not having exceeded full scale deflection at any point. The approach controller soon asked me if I was established on the final approach 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 equipment; 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 descent altitude. There were no flags or warnings showing that this information was unreliable; so I began my descent from 3100 ft towards the minimum descent altitude of 1940 ft. Now established; I was beginning the final approach/landing checklist when the controller informed me that I was left of course. Upon inspection; I determined that the #1 VOR was still close to center; but the indications of the #2 VOR coincided more with the information given by the controller. It was at this time that the controller issued a low altitude warning/alert. Upon hearing this warning I immediately checked my altimeter; which was giving me a reading of approximately 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 controller was speaking of a descent below the minimum descent altitude within the FAF. I immediately stopped my descent and leveled at my current altitude of 2700 ft MSL. I then verified from my approach charts that I had correctly chosen the MDA of 1940 ft MSL; which is the correct altitude once within 10 DME of the VOR. I responded to the controller saying that; based on the information I was receiving from the cockpit; I was within the limits of the approach. There was still no flagged indication of malfunctioning equipment so I continued inbound while trying to figure out the discrepancy between the controller's and my information. I continued to hold my altitude and began correcting for the error in the #1 VOR and coming to center by navigating off of the more accurate #2 VOR. The controller then issued a final low altitude warning and added that I was cleared to maintain 3100 ft until the FAF. It was at this point that I realized I had an equipment problem; was receiving faulty DME readings; and was still outside of neish. I responded to the controller that I was switching radios; inan attempt to regain a reliable DME signal; while initiating a climb. I then switched the DME receiver from the #1 to the #2 navigation 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 operation. 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 clearance to switch over to the tower frequency. I continued the approach; on course; descending into and through the cloud layer and landing with no further complications. A VFR reposition back to the maintenance base was accomplished the following morning with the DME labeled inoperative; referenced under the MEL. Once back at the base of operations; I was able to duplicate the problem in front of others; and the DME was signed inoperative by a licensed mechanic. The problem 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 checked 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 week old logbook entry.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of January 2009 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.