Narrative:

Course deviation on LNAV departure. First officer was the PF on the flight to lax. He was on day 2 of his uoe, and I was the check airman flying in the right seat. The flight was being observed by FAA inspector. We were cleared via the idale 2 LNAV departure, which first officer thoroughly briefed and route was closed and verified correct in the mcdu. The flight pushed and was off. Company LNAV departure procedures were followed to the letter, and we leveled at 7000 ft MSL. Between lodzy and idale, departure control cleared us to boach. First officer asked me to enter boach in the mcdu, and I did. I asked him if he wanted me to execute boach. He verified boach was in the proper position and said to execute. I executed boach and it was highlighted in the mcdu, and a magenta line formed on the cfis display from present position to boach, and the aircraft began a left turn to boach. Shortly after that, both mcdu's blanked and flashed back on. Boach was no longer highlighted. Several route discontinuity blocks appeared on the legs page, and boach was no longer displayed on the EFIS display. The aircraft sequenced its bank and continued turning left toward an unknown fix. I told first officer that 'the box had dropped boach, and we needed to turn right.' first officer selected heading mode on the MCP and began an immediate right turn. At this time, departure control asked 'company number where are you going?' I answered, we were in a right turn towards hector. We could no longer identify boach, and hector was the next fix. Departure control instructed us to go direct boach. By this time, first officer had re-established boach in the mcdu and executed it and engaged LNAV. We proceeded towards boach, but the EFIS display now showed a magenta line to boach, hector and back boach. First officer corrected the problem, and no further problems occurred. Inspector indicated that it appeared that we had a software problem. We proceeded to lax and landed. During the debrief of the flight, inspector, again, stated that he was of the opinion that the software had failed, and it appeared to him that we were doing everything possible to correct the problem. He stated that he thought first officer's situational awareness could have been better, but he reacted as rapidly as possible, and he would be available to verify the facts of our problem. Inspector signed first officer's uoe forms, and told us to continue our uoe. I called the number requested of us to call, and was answered with a recorded message from 'name.' I left a brief message of the situation. I then called chief pilot and relayed the events. I made a write-up in the aircraft logbook, and lax maintenance performed several tests and signed off the aircraft for service. I contacted company ATC specialist and relayed the events to him. First officer and I both made phone calls to company safety committee and answered the questions. In retrospect, I feel we were reacting to identing a problem and correcting the problem as fast as humanly possible. I feel I should have immediately asked for a vector due to an LNAV failure. I am confident the problem was not induced by pilot action. I have never seen this kind of problem before, and have not had any problems with any LNAV departures. I think first officer did a very nice job in the briefing and execution of our LNAV departure procedures. This was the second day of his uoe and under the circumstances, I think he did an admirable job. Result: a growing distrust of LNAV departures. Recommendation: extreme vigilance of all LNAV procedures and immediate declarations of LNAV failure to ATC and request vectors.

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Original NASA ASRS Text

Title: DEV FROM CLRED RNAV (DP) RTE DUE TO MALFUNCTION OF SINGLE FMC SYS.

Narrative: COURSE DEV ON LNAV DEP. FO WAS THE PF ON THE FLT TO LAX. HE WAS ON DAY 2 OF HIS UOE, AND I WAS THE CHK AIRMAN FLYING IN THE R SEAT. THE FLT WAS BEING OBSERVED BY FAA INSPECTOR. WE WERE CLRED VIA THE IDALE 2 LNAV DEP, WHICH FO THOROUGHLY BRIEFED AND RTE WAS CLOSED AND VERIFIED CORRECT IN THE MCDU. THE FLT PUSHED AND WAS OFF. COMPANY LNAV DEP PROCS WERE FOLLOWED TO THE LETTER, AND WE LEVELED AT 7000 FT MSL. BTWN LODZY AND IDALE, DEP CTL CLRED US TO BOACH. FO ASKED ME TO ENTER BOACH IN THE MCDU, AND I DID. I ASKED HIM IF HE WANTED ME TO EXECUTE BOACH. HE VERIFIED BOACH WAS IN THE PROPER POS AND SAID TO EXECUTE. I EXECUTED BOACH AND IT WAS HIGHLIGHTED IN THE MCDU, AND A MAGENTA LINE FORMED ON THE CFIS DISPLAY FROM PRESENT POS TO BOACH, AND THE ACFT BEGAN A L TURN TO BOACH. SHORTLY AFTER THAT, BOTH MCDU'S BLANKED AND FLASHED BACK ON. BOACH WAS NO LONGER HIGHLIGHTED. SEVERAL RTE DISCONTINUITY BLOCKS APPEARED ON THE LEGS PAGE, AND BOACH WAS NO LONGER DISPLAYED ON THE EFIS DISPLAY. THE ACFT SEQUENCED ITS BANK AND CONTINUED TURNING L TOWARD AN UNKNOWN FIX. I TOLD FO THAT 'THE BOX HAD DROPPED BOACH, AND WE NEEDED TO TURN R.' FO SELECTED HDG MODE ON THE MCP AND BEGAN AN IMMEDIATE R TURN. AT THIS TIME, DEP CTL ASKED 'COMPANY NUMBER WHERE ARE YOU GOING?' I ANSWERED, WE WERE IN A R TURN TOWARDS HECTOR. WE COULD NO LONGER IDENT BOACH, AND HECTOR WAS THE NEXT FIX. DEP CTL INSTRUCTED US TO GO DIRECT BOACH. BY THIS TIME, FO HAD RE-ESTABLISHED BOACH IN THE MCDU AND EXECUTED IT AND ENGAGED LNAV. WE PROCEEDED TOWARDS BOACH, BUT THE EFIS DISPLAY NOW SHOWED A MAGENTA LINE TO BOACH, HECTOR AND BACK BOACH. FO CORRECTED THE PROB, AND NO FURTHER PROBS OCCURRED. INSPECTOR INDICATED THAT IT APPEARED THAT WE HAD A SOFTWARE PROB. WE PROCEEDED TO LAX AND LANDED. DURING THE DEBRIEF OF THE FLT, INSPECTOR, AGAIN, STATED THAT HE WAS OF THE OPINION THAT THE SOFTWARE HAD FAILED, AND IT APPEARED TO HIM THAT WE WERE DOING EVERYTHING POSSIBLE TO CORRECT THE PROB. HE STATED THAT HE THOUGHT FO'S SITUATIONAL AWARENESS COULD HAVE BEEN BETTER, BUT HE REACTED AS RAPIDLY AS POSSIBLE, AND HE WOULD BE AVAILABLE TO VERIFY THE FACTS OF OUR PROB. INSPECTOR SIGNED FO'S UOE FORMS, AND TOLD US TO CONTINUE OUR UOE. I CALLED THE NUMBER REQUESTED OF US TO CALL, AND WAS ANSWERED WITH A RECORDED MESSAGE FROM 'NAME.' I LEFT A BRIEF MESSAGE OF THE SIT. I THEN CALLED CHIEF PLT AND RELAYED THE EVENTS. I MADE A WRITE-UP IN THE ACFT LOGBOOK, AND LAX MAINT PERFORMED SEVERAL TESTS AND SIGNED OFF THE ACFT FOR SVC. I CONTACTED COMPANY ATC SPECIALIST AND RELAYED THE EVENTS TO HIM. FO AND I BOTH MADE PHONE CALLS TO COMPANY SAFETY COMMITTEE AND ANSWERED THE QUESTIONS. IN RETROSPECT, I FEEL WE WERE REACTING TO IDENTING A PROB AND CORRECTING THE PROB AS FAST AS HUMANLY POSSIBLE. I FEEL I SHOULD HAVE IMMEDIATELY ASKED FOR A VECTOR DUE TO AN LNAV FAILURE. I AM CONFIDENT THE PROB WAS NOT INDUCED BY PLT ACTION. I HAVE NEVER SEEN THIS KIND OF PROB BEFORE, AND HAVE NOT HAD ANY PROBS WITH ANY LNAV DEPS. I THINK FO DID A VERY NICE JOB IN THE BRIEFING AND EXECUTION OF OUR LNAV DEP PROCS. THIS WAS THE SECOND DAY OF HIS UOE AND UNDER THE CIRCUMSTANCES, I THINK HE DID AN ADMIRABLE JOB. RESULT: A GROWING DISTRUST OF LNAV DEPS. RECOMMENDATION: EXTREME VIGILANCE OF ALL LNAV PROCS AND IMMEDIATE DECLARATIONS OF LNAV FAILURE TO ATC AND REQUEST VECTORS.

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.