Narrative:

After passing trm VOR the next waypoint was blh VOR. Aircraft commenced left turn. RNAV computer showed blh was next waypoint, but disagreed with CDI as to the course. After determining correct course an immediate right was commenced. ZLA called and asked our heading. Someone else responded. They called again and said to turn to 060 degrees which we acknowledged. Center told an air carrier Z flight to turn right to some heading. We told center we had the air carrier Z flight in sight. He was below and to our left. The RNAV problem was written up in aircraft log book. I was requested to call ZLA upon arrival. I talked to center manager. He said he had to file a pilot deviation due to reduced sep (3.8 mi 1300'). Callback conversation with reporter revealed the following: FAA is investigating the incident. This is the fourth track deviation in this area by this air carrier involving this same type aircraft. 2 of the problems were the result of erroneous latitude/longitude information supplied by the commercial chart company, but that does not seem to be the problem in this incident. Track updates are made periodically to the aircraft's software and RNAV programming is done by track # followed by waypoint verification by the flight crew. All company procedures were followed in the incident and the aircraft subsys accepted the program but did not execute as expected. Supplemental information from acn 155731: the tendency to trust this RNAV system is very strong. The time required to recognize a problem exists takes a few seconds to start the correction process. A chart has to be picked up and the route found so that the proper corrections can be made.

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Original NASA ASRS Text

Title: ACR MLG TRACK HEADING DEVIATION ON CANNED FLT PLAN USING RNAV.

Narrative: AFTER PASSING TRM VOR THE NEXT WAYPOINT WAS BLH VOR. ACFT COMMENCED LEFT TURN. RNAV COMPUTER SHOWED BLH WAS NEXT WAYPOINT, BUT DISAGREED WITH CDI AS TO THE COURSE. AFTER DETERMINING CORRECT COURSE AN IMMEDIATE RIGHT WAS COMMENCED. ZLA CALLED AND ASKED OUR HDG. SOMEONE ELSE RESPONDED. THEY CALLED AGAIN AND SAID TO TURN TO 060 DEGS WHICH WE ACKNOWLEDGED. CENTER TOLD AN ACR Z FLT TO TURN RIGHT TO SOME HDG. WE TOLD CENTER WE HAD THE ACR Z FLT IN SIGHT. HE WAS BELOW AND TO OUR LEFT. THE RNAV PROB WAS WRITTEN UP IN ACFT LOG BOOK. I WAS REQUESTED TO CALL ZLA UPON ARR. I TALKED TO CENTER MGR. HE SAID HE HAD TO FILE A PLT DEVIATION DUE TO REDUCED SEP (3.8 MI 1300'). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: FAA IS INVESTIGATING THE INCIDENT. THIS IS THE FOURTH TRACK DEVIATION IN THIS AREA BY THIS ACR INVOLVING THIS SAME TYPE ACFT. 2 OF THE PROBS WERE THE RESULT OF ERRONEOUS LAT/LONGITUDE INFO SUPPLIED BY THE COMMERCIAL CHART COMPANY, BUT THAT DOES NOT SEEM TO BE THE PROB IN THIS INCIDENT. TRACK UPDATES ARE MADE PERIODICALLY TO THE ACFT'S SOFTWARE AND RNAV PROGRAMMING IS DONE BY TRACK # FOLLOWED BY WAYPOINT VERIFICATION BY THE FLT CREW. ALL COMPANY PROCS WERE FOLLOWED IN THE INCIDENT AND THE ACFT SUBSYS ACCEPTED THE PROGRAM BUT DID NOT EXECUTE AS EXPECTED. SUPPLEMENTAL INFO FROM ACN 155731: THE TENDENCY TO TRUST THIS RNAV SYS IS VERY STRONG. THE TIME REQUIRED TO RECOGNIZE A PROB EXISTS TAKES A FEW SECS TO START THE CORRECTION PROCESS. A CHART HAS TO BE PICKED UP AND THE RTE FOUND SO THAT THE PROPER CORRECTIONS CAN BE MADE.

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.