Narrative:

Problem occurred with GPS unit in aircraft. Unit is an IFR certified unit. The unit incorrectly plots my position. IFR flight plan was filed from crq to ont. The unit had been used earlier on a flight to crq from yum by myself and had not shown any position errors. I was being vectored to petis radio beacon. Although the beacon had been tuned in and idented, I had also loaded the beacon into the GPS unit prior to departure. Using the moving map enhances situational awareness considerably. I was instructed to fly a heading, and when able proceed direct to petis NDB. The heading assigned precisely concurred with the GPS readout to petis. I continued on this heading although the NDB indications showed petis 40 degrees to the left of my nose. The map indication indicated petis to be on my course. The controller requested I descend to 4200 ft and then turn back to petis. At this time, the display was now showing petis in that direction. This concurred with the NDB indication. An ILS continued with no further problem. No comment was made by the controllers. On the inbound course on the localizer, the track, moving map display, and localizer indications all were in agreement. I was unable to address the situation with the controller due to controller workload. To assess the situation was time consuming as I was setting navaids up for the approach and attempting to disseminate the correct information. On the flight departing ont to my base airport, I decided to constantly check GPS position against my 2 VOR receivers and a hand-held GPS which I always carry as a backup. The flight was in VFR conditions. The indications from the IFR certified unit showed a 10 degree track error from the display. The VOR and portable GPS indications all were in agreement. Upon arrival at my home base I faxed a request to my avionics shop as to a course of action to be taken. I will no longer use this unit until it has been checked by the manufacturer. The errors are not constant or consistent. Many times the indications are extremely precise, but this cannot be guaranteed. Summary: reliance on the GPS navigation system could have presented a greater problem if I was in IMC in mountainous conditions. It is far too easy to take this next-generation equipment for granted. A simple 'off' flag does not rear its head as in the VOR. In this situation, 2 navigation instruments were being used without the accurate failure device that we have in our VOR system.

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Original NASA ASRS Text

Title: PLT OF C441 HAS GPS NAV PROB ON IFR FLT PLAN. ATC VECTORS TO CORRECT COURSE AND PLT USES XCHK OF VORS TO REMAIN ON APCH COURSE.

Narrative: PROB OCCURRED WITH GPS UNIT IN ACFT. UNIT IS AN IFR CERTIFIED UNIT. THE UNIT INCORRECTLY PLOTS MY POS. IFR FLT PLAN WAS FILED FROM CRQ TO ONT. THE UNIT HAD BEEN USED EARLIER ON A FLT TO CRQ FROM YUM BY MYSELF AND HAD NOT SHOWN ANY POS ERRORS. I WAS BEING VECTORED TO PETIS RADIO BEACON. ALTHOUGH THE BEACON HAD BEEN TUNED IN AND IDENTED, I HAD ALSO LOADED THE BEACON INTO THE GPS UNIT PRIOR TO DEP. USING THE MOVING MAP ENHANCES SITUATIONAL AWARENESS CONSIDERABLY. I WAS INSTRUCTED TO FLY A HDG, AND WHEN ABLE PROCEED DIRECT TO PETIS NDB. THE HDG ASSIGNED PRECISELY CONCURRED WITH THE GPS READOUT TO PETIS. I CONTINUED ON THIS HDG ALTHOUGH THE NDB INDICATIONS SHOWED PETIS 40 DEGS TO THE L OF MY NOSE. THE MAP INDICATION INDICATED PETIS TO BE ON MY COURSE. THE CTLR REQUESTED I DSND TO 4200 FT AND THEN TURN BACK TO PETIS. AT THIS TIME, THE DISPLAY WAS NOW SHOWING PETIS IN THAT DIRECTION. THIS CONCURRED WITH THE NDB INDICATION. AN ILS CONTINUED WITH NO FURTHER PROB. NO COMMENT WAS MADE BY THE CTLRS. ON THE INBOUND COURSE ON THE LOC, THE TRACK, MOVING MAP DISPLAY, AND LOC INDICATIONS ALL WERE IN AGREEMENT. I WAS UNABLE TO ADDRESS THE SIT WITH THE CTLR DUE TO CTLR WORKLOAD. TO ASSESS THE SIT WAS TIME CONSUMING AS I WAS SETTING NAVAIDS UP FOR THE APCH AND ATTEMPTING TO DISSEMINATE THE CORRECT INFO. ON THE FLT DEPARTING ONT TO MY BASE ARPT, I DECIDED TO CONSTANTLY CHK GPS POS AGAINST MY 2 VOR RECEIVERS AND A HAND-HELD GPS WHICH I ALWAYS CARRY AS A BACKUP. THE FLT WAS IN VFR CONDITIONS. THE INDICATIONS FROM THE IFR CERTIFIED UNIT SHOWED A 10 DEG TRACK ERROR FROM THE DISPLAY. THE VOR AND PORTABLE GPS INDICATIONS ALL WERE IN AGREEMENT. UPON ARR AT MY HOME BASE I FAXED A REQUEST TO MY AVIONICS SHOP AS TO A COURSE OF ACTION TO BE TAKEN. I WILL NO LONGER USE THIS UNIT UNTIL IT HAS BEEN CHKED BY THE MANUFACTURER. THE ERRORS ARE NOT CONSTANT OR CONSISTENT. MANY TIMES THE INDICATIONS ARE EXTREMELY PRECISE, BUT THIS CANNOT BE GUARANTEED. SUMMARY: RELIANCE ON THE GPS NAV SYS COULD HAVE PRESENTED A GREATER PROB IF I WAS IN IMC IN MOUNTAINOUS CONDITIONS. IT IS FAR TOO EASY TO TAKE THIS NEXT-GENERATION EQUIP FOR GRANTED. A SIMPLE 'OFF' FLAG DOES NOT REAR ITS HEAD AS IN THE VOR. IN THIS SIT, 2 NAV INSTS WERE BEING USED WITHOUT THE ACCURATE FAILURE DEVICE THAT WE HAVE IN OUR VOR SYS.

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