Narrative:

During an IFR departure from bzn along V86 about 12 NM west of bzn VOR, the right engine RPM began overspding due to a propeller governor malfunction. Retarded the right throttle and propeller control to reduce the RPM and requested an immediate return to bzn from ZLC. I was cleared direct to manni LOM at 11000'. I started a right turn toward manni, tuned in the localizer for runway 12, and idented it. During the turn I was unable to maintain 11000' since the right engine was producing very low power. I was just about to request a lower altitude or revised clearance from ZLC when I noticed that the localizer needle on the HSI was about half scale deflection to the left of center. Since I believed that I was intercepting the localizer, I requested clearance for the ILS 12 at bzn and was so cleared. I was within about 5 mi of manni so I began descending to the G/south intercept altitude. Within a minute I descended out of the higher overcast and started looking for the airport. I soon discovered that although I was heading directly for the airport, I was well to the right of the localizer despite the nearly centered localizer needle. I visually aligned the aircraft with runway 12 and made a normal landing. After landing, I noticed that the navigation portion of the HSI was flagged, the localizer needle was inoperative, but the localizer identify was being received loud and clear. I tuned in the bzn VOR (on the airport) and confirmed that the navigation portion of the HSI was inoperative. Back in 2/88, after the fatal crash of an small transport while executing the ILS 27 at hln, I did some in-flight experiments with the same airplane (and avionics) as in the above incident at bzn. I found that when tuning to the hln localizer frequency in places where the localizer signal could not be received (behind mountains and at low altitudes in valleys near helena) the HSI localizer needle would move to a position slightly left of center or actually centered, with no flag showing even though the navigation signal was unreliable or nonexistent and the identify was not received. To relate this to the bzn incident, I believe that the propeller overspd (and attendant alternator overspd) led to a voltage spike which damaged the HSI and interrupted the signal to the course indicator. The needle then moved to the slightly left of center position which led me to believe that I was intercepting the localizer, when in fact I was not. I'm not certain whether or not the flag was showing the first time I scanned the HSI (and decided I was intercepting), but there's no doubt that under the stress of dealing with the right engine malfunction and the unplanned return to bzn (I was flying single pilot). I failed to fully use the other navigation information available to me (VOR, ADF). A further lesson here is that anytime an overvoltage electrical condition occurs, sensitive avionics equipment should be carefully scrutinized for any unusual or erroneous indications.

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Original NASA ASRS Text

Title: ATX SMT DESCENT TO GLIDE SLOPE INTERCEPT ALT BEFORE LOCALIZER INTERCEPT AND WITHOUT POSITIVE SPATIAL ORIENTATION.

Narrative: DURING AN IFR DEP FROM BZN ALONG V86 ABOUT 12 NM W OF BZN VOR, THE RIGHT ENG RPM BEGAN OVERSPDING DUE TO A PROP GOVERNOR MALFUNCTION. RETARDED THE RIGHT THROTTLE AND PROP CTL TO REDUCE THE RPM AND REQUESTED AN IMMEDIATE RETURN TO BZN FROM ZLC. I WAS CLRED DIRECT TO MANNI LOM AT 11000'. I STARTED A RIGHT TURN TOWARD MANNI, TUNED IN THE LOC FOR RWY 12, AND IDENTED IT. DURING THE TURN I WAS UNABLE TO MAINTAIN 11000' SINCE THE RIGHT ENG WAS PRODUCING VERY LOW PWR. I WAS JUST ABOUT TO REQUEST A LOWER ALT OR REVISED CLRNC FROM ZLC WHEN I NOTICED THAT THE LOC NEEDLE ON THE HSI WAS ABOUT HALF SCALE DEFLECTION TO THE LEFT OF CENTER. SINCE I BELIEVED THAT I WAS INTERCEPTING THE LOC, I REQUESTED CLRNC FOR THE ILS 12 AT BZN AND WAS SO CLRED. I WAS WITHIN ABOUT 5 MI OF MANNI SO I BEGAN DSNDING TO THE G/S INTERCEPT ALT. WITHIN A MINUTE I DSNDED OUT OF THE HIGHER OVCST AND STARTED LOOKING FOR THE ARPT. I SOON DISCOVERED THAT ALTHOUGH I WAS HDG DIRECTLY FOR THE ARPT, I WAS WELL TO THE RIGHT OF THE LOC DESPITE THE NEARLY CENTERED LOC NEEDLE. I VISUALLY ALIGNED THE ACFT WITH RWY 12 AND MADE A NORMAL LNDG. AFTER LNDG, I NOTICED THAT THE NAV PORTION OF THE HSI WAS FLAGGED, THE LOC NEEDLE WAS INOP, BUT THE LOC IDENT WAS BEING RECEIVED LOUD AND CLEAR. I TUNED IN THE BZN VOR (ON THE ARPT) AND CONFIRMED THAT THE NAV PORTION OF THE HSI WAS INOP. BACK IN 2/88, AFTER THE FATAL CRASH OF AN SMT WHILE EXECUTING THE ILS 27 AT HLN, I DID SOME INFLT EXPERIMENTS WITH THE SAME AIRPLANE (AND AVIONICS) AS IN THE ABOVE INCIDENT AT BZN. I FOUND THAT WHEN TUNING TO THE HLN LOC FREQ IN PLACES WHERE THE LOC SIGNAL COULD NOT BE RECEIVED (BEHIND MOUNTAINS AND AT LOW ALTS IN VALLEYS NEAR HELENA) THE HSI LOC NEEDLE WOULD MOVE TO A POS SLIGHTLY LEFT OF CENTER OR ACTUALLY CENTERED, WITH NO FLAG SHOWING EVEN THOUGH THE NAV SIGNAL WAS UNRELIABLE OR NONEXISTENT AND THE IDENT WAS NOT RECEIVED. TO RELATE THIS TO THE BZN INCIDENT, I BELIEVE THAT THE PROP OVERSPD (AND ATTENDANT ALTERNATOR OVERSPD) LED TO A VOLTAGE SPIKE WHICH DAMAGED THE HSI AND INTERRUPTED THE SIGNAL TO THE COURSE INDICATOR. THE NEEDLE THEN MOVED TO THE SLIGHTLY LEFT OF CENTER POS WHICH LED ME TO BELIEVE THAT I WAS INTERCEPTING THE LOC, WHEN IN FACT I WAS NOT. I'M NOT CERTAIN WHETHER OR NOT THE FLAG WAS SHOWING THE FIRST TIME I SCANNED THE HSI (AND DECIDED I WAS INTERCEPTING), BUT THERE'S NO DOUBT THAT UNDER THE STRESS OF DEALING WITH THE RIGHT ENG MALFUNCTION AND THE UNPLANNED RETURN TO BZN (I WAS FLYING SINGLE PLT). I FAILED TO FULLY USE THE OTHER NAV INFO AVAILABLE TO ME (VOR, ADF). A FURTHER LESSON HERE IS THAT ANYTIME AN OVERVOLTAGE ELECTRICAL CONDITION OCCURS, SENSITIVE AVIONICS EQUIP SHOULD BE CAREFULLY SCRUTINIZED FOR ANY UNUSUAL OR ERRONEOUS INDICATIONS.

Data retrieved from NASA's ASRS site as of August 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.