Narrative:

I filed an IFR flight plan from ues to dri via RNAV. The day and route of flight was cavu. We were cleared, 'radar vectors pmm direct dfi.' a prior FSS briefing, supplemented by a 'wxmation' computer report gave no NOTAMS applicable to this flight. About 40 NM southeast of pmm, the waypoint address of dfi which is R-57.8 degrees D-40.5 NM off of fwa 117.8 was activated in a RNAV unit with no response. Shortly thereafter, ATC turned me over to fwa approach control and I asked if they were having problems with 117.8 omni. The controller thought it might be off the air, but that he would check. We were receiving the morse idents. The controller came back saying it (117.8) had been shut down, but was now back on line. I told him about the waypoint address problem. Subsequently, the controller told me the omni was on the air, but not the DME. Shortly thereafter the address and waypoint came up and the needle showed me left of course (east) and a substantial correction in heading was made. The address went out and a few mins (perhaps 3) later it came back on showing us right (west) of course. Because we were close to dfi (about 17 NM) a very substantial (about 30 degree) correction was made, but we still arrived at dfi. From a distance standpoint, we were probably 7-10 mi west of dfi with the needle showing a very slight deflection to the left (east). No big problem in cavu conditions. We cancelled IFR, entered a standard landing pattern and landed on runway 30 at dfi. I report this incident because it is the first time in many hours of RNAV flying I have ever gotten wrong information on a waypoint address. I have gotten no information, have had the information go at at inopportune time, but never received the signal and identify and RNAV readings only to find that a response to that information did not bring you to the appropriate geographical spot. I don't even understand how this is mechanically (technically) possible with the little I know about how the computerized RNAV equipment works. Except for reconfirming the appropriate radial and distance comprising the address of the airport of intended landing, I know of no other verification method to assure an accurate waypoint reading. I might add that a quick entry of dfi into my other navigation unit gave an accurate heading and distance display to the airport. If the experts you have on staff can figure out 1) what happened here and 2) what corrective or cross-check a pilot may practically use under 'real' instrument conditions, or 3) what care can be taken during ground maintenance of VORTAC equipment to prevent erroneous signals, then this report will have served to improve safety in the air.

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Original NASA ASRS Text

Title: PLT OF SMA FOUND HIMSELF OFF COURSE ACCOUNT UNRELIABILITY OF HIS NAVIGATION INSTRUMENTS.

Narrative: I FILED AN IFR FLT PLAN FROM UES TO DRI VIA RNAV. THE DAY AND ROUTE OF FLT WAS CAVU. WE WERE CLRED, 'RADAR VECTORS PMM DIRECT DFI.' A PRIOR FSS BRIEFING, SUPPLEMENTED BY A 'WXMATION' COMPUTER RPT GAVE NO NOTAMS APPLICABLE TO THIS FLT. ABOUT 40 NM SE OF PMM, THE WAYPOINT ADDRESS OF DFI WHICH IS R-57.8 DEGS D-40.5 NM OFF OF FWA 117.8 WAS ACTIVATED IN A RNAV UNIT WITH NO RESPONSE. SHORTLY THEREAFTER, ATC TURNED ME OVER TO FWA APCH CTL AND I ASKED IF THEY WERE HAVING PROBS WITH 117.8 OMNI. THE CTLR THOUGHT IT MIGHT BE OFF THE AIR, BUT THAT HE WOULD CHK. WE WERE RECEIVING THE MORSE IDENTS. THE CTLR CAME BACK SAYING IT (117.8) HAD BEEN SHUT DOWN, BUT WAS NOW BACK ON LINE. I TOLD HIM ABOUT THE WAYPOINT ADDRESS PROBLEM. SUBSEQUENTLY, THE CTLR TOLD ME THE OMNI WAS ON THE AIR, BUT NOT THE DME. SHORTLY THEREAFTER THE ADDRESS AND WAYPOINT CAME UP AND THE NEEDLE SHOWED ME LEFT OF COURSE (E) AND A SUBSTANTIAL CORRECTION IN HDG WAS MADE. THE ADDRESS WENT OUT AND A FEW MINS (PERHAPS 3) LATER IT CAME BACK ON SHOWING US RIGHT (W) OF COURSE. BECAUSE WE WERE CLOSE TO DFI (ABOUT 17 NM) A VERY SUBSTANTIAL (ABOUT 30 DEG) CORRECTION WAS MADE, BUT WE STILL ARRIVED AT DFI. FROM A DISTANCE STANDPOINT, WE WERE PROBABLY 7-10 MI W OF DFI WITH THE NEEDLE SHOWING A VERY SLIGHT DEFLECTION TO THE LEFT (E). NO BIG PROB IN CAVU CONDITIONS. WE CANCELLED IFR, ENTERED A STANDARD LNDG PATTERN AND LANDED ON RWY 30 AT DFI. I RPT THIS INCIDENT BECAUSE IT IS THE FIRST TIME IN MANY HRS OF RNAV FLYING I HAVE EVER GOTTEN WRONG INFO ON A WAYPOINT ADDRESS. I HAVE GOTTEN NO INFO, HAVE HAD THE INFO GO AT AT INOPPORTUNE TIME, BUT NEVER RECEIVED THE SIGNAL AND IDENT AND RNAV READINGS ONLY TO FIND THAT A RESPONSE TO THAT INFO DID NOT BRING YOU TO THE APPROPRIATE GEOGRAPHICAL SPOT. I DON'T EVEN UNDERSTAND HOW THIS IS MECHANICALLY (TECHNICALLY) POSSIBLE WITH THE LITTLE I KNOW ABOUT HOW THE COMPUTERIZED RNAV EQUIP WORKS. EXCEPT FOR RECONFIRMING THE APPROPRIATE RADIAL AND DISTANCE COMPRISING THE ADDRESS OF THE ARPT OF INTENDED LNDG, I KNOW OF NO OTHER VERIFICATION METHOD TO ASSURE AN ACCURATE WAYPOINT READING. I MIGHT ADD THAT A QUICK ENTRY OF DFI INTO MY OTHER NAV UNIT GAVE AN ACCURATE HDG AND DISTANCE DISPLAY TO THE ARPT. IF THE EXPERTS YOU HAVE ON STAFF CAN FIGURE OUT 1) WHAT HAPPENED HERE AND 2) WHAT CORRECTIVE OR CROSS-CHK A PLT MAY PRACTICALLY USE UNDER 'REAL' INSTRUMENT CONDITIONS, OR 3) WHAT CARE CAN BE TAKEN DURING GND MAINT OF VORTAC EQUIP TO PREVENT ERRONEOUS SIGNALS, THEN THIS RPT WILL HAVE SERVED TO IMPROVE SAFETY IN THE AIR.

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.