Narrative:

Our flight was cleared for takeoff from ont, runway 26L. I, the first officer, was the PF. Our clearance had been via the pom 6 departure to ehf and then as filed to smf. It was a normal takeoff, we were following the depicted SID procedure. As we were turning to intercept pom radial 114 degrees on the climb, departure control cleared us direct to bucck intersection and issued a frequency change to ZLA. As I leveled the wings, I engaged the autoplt, line selected direct to bucck on the CDU and last, I engaged LNAV on the MCP -- at least I thought I did. When I depressed the LNAV button on this -700 MCP, I saw its internal green light illuminate. My normal flow is that after selecting a function on the autoplt MCP, I corroborate on the flight mode annunciator that what I have selected on the MCP has actually engaged. A few mins later, ZLA asked us if we were on a heading, because he showed us between 2 and 3 mi deviating left of course. We immediately recognized our error and turned right to the next fix (ehf). Center then cleared us direct to ehf and moments later to czq. ATC made no further comments. Events like the previous can be prevented by utilizing all available resources, always, to insure that the aircraft performs as intended. Lesson learned is that from now on, I will adopt the habit of asking the PNF to confirm what I do on the CDU. 2 heads and 4 eyes are better than 1 and 2. Supplemental information from acn 572428: he then reached up and selected LNAV on the autoplt control panel. While doing so, I watched his finger hit the button, and as soon as I saw a green light, my eyes came off the panel too quickly to notice that the light did not stay on. Furthermore, neither of us confirmed LNAV engagement on the AFDS. I had the 294 degree radial dialed with pom VOR selected, and because we were heading in the right direction to start with, the course track on the csi was pretty much centered. ZLA called and advised they were showing us a couple of mi off course. This was our 4TH and last crew pairing of the month, and I felt our coordination and communication had been very positive all month. We tried to figure out why we both missed the navigation error and couldn't put our finger on anything specific. Apparently, we both bit on the initial flickering of the green LNAV light and didn't follow up by checking the AFDS. There were no outside distrs at that time, no traffic callouts, no cockpit dings. Fatigue was not a factor. The only thing left, I suppose, would be complacency, and I don't know why. At that point in space and time, we were both up and locked, thinking everything was ok. Lesson learned, or should I say often repeated, keep the xchk going!

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Original NASA ASRS Text

Title: HDG TRACK DEV BY A B737-500 FLC WHEN THEY FAILED TO DETECT AN FMS LIGHT THAT DID NOT STAY ON WHILE ON THE POM 6 DEP FROM ONT, CA.

Narrative: OUR FLT WAS CLRED FOR TKOF FROM ONT, RWY 26L. I, THE FO, WAS THE PF. OUR CLRNC HAD BEEN VIA THE POM 6 DEP TO EHF AND THEN AS FILED TO SMF. IT WAS A NORMAL TKOF, WE WERE FOLLOWING THE DEPICTED SID PROC. AS WE WERE TURNING TO INTERCEPT POM RADIAL 114 DEGS ON THE CLB, DEP CTL CLRED US DIRECT TO BUCCK INTXN AND ISSUED A FREQ CHANGE TO ZLA. AS I LEVELED THE WINGS, I ENGAGED THE AUTOPLT, LINE SELECTED DIRECT TO BUCCK ON THE CDU AND LAST, I ENGAGED LNAV ON THE MCP -- AT LEAST I THOUGHT I DID. WHEN I DEPRESSED THE LNAV BUTTON ON THIS -700 MCP, I SAW ITS INTERNAL GREEN LIGHT ILLUMINATE. MY NORMAL FLOW IS THAT AFTER SELECTING A FUNCTION ON THE AUTOPLT MCP, I CORROBORATE ON THE FLT MODE ANNUNCIATOR THAT WHAT I HAVE SELECTED ON THE MCP HAS ACTUALLY ENGAGED. A FEW MINS LATER, ZLA ASKED US IF WE WERE ON A HDG, BECAUSE HE SHOWED US BTWN 2 AND 3 MI DEVIATING L OF COURSE. WE IMMEDIATELY RECOGNIZED OUR ERROR AND TURNED R TO THE NEXT FIX (EHF). CTR THEN CLRED US DIRECT TO EHF AND MOMENTS LATER TO CZQ. ATC MADE NO FURTHER COMMENTS. EVENTS LIKE THE PREVIOUS CAN BE PREVENTED BY UTILIZING ALL AVAILABLE RESOURCES, ALWAYS, TO INSURE THAT THE ACFT PERFORMS AS INTENDED. LESSON LEARNED IS THAT FROM NOW ON, I WILL ADOPT THE HABIT OF ASKING THE PNF TO CONFIRM WHAT I DO ON THE CDU. 2 HEADS AND 4 EYES ARE BETTER THAN 1 AND 2. SUPPLEMENTAL INFO FROM ACN 572428: HE THEN REACHED UP AND SELECTED LNAV ON THE AUTOPLT CTL PANEL. WHILE DOING SO, I WATCHED HIS FINGER HIT THE BUTTON, AND AS SOON AS I SAW A GREEN LIGHT, MY EYES CAME OFF THE PANEL TOO QUICKLY TO NOTICE THAT THE LIGHT DID NOT STAY ON. FURTHERMORE, NEITHER OF US CONFIRMED LNAV ENGAGEMENT ON THE AFDS. I HAD THE 294 DEG RADIAL DIALED WITH POM VOR SELECTED, AND BECAUSE WE WERE HDG IN THE RIGHT DIRECTION TO START WITH, THE COURSE TRACK ON THE CSI WAS PRETTY MUCH CTRED. ZLA CALLED AND ADVISED THEY WERE SHOWING US A COUPLE OF MI OFF COURSE. THIS WAS OUR 4TH AND LAST CREW PAIRING OF THE MONTH, AND I FELT OUR COORD AND COM HAD BEEN VERY POSITIVE ALL MONTH. WE TRIED TO FIGURE OUT WHY WE BOTH MISSED THE NAV ERROR AND COULDN'T PUT OUR FINGER ON ANYTHING SPECIFIC. APPARENTLY, WE BOTH BIT ON THE INITIAL FLICKERING OF THE GREEN LNAV LIGHT AND DIDN'T FOLLOW UP BY CHKING THE AFDS. THERE WERE NO OUTSIDE DISTRS AT THAT TIME, NO TFC CALLOUTS, NO COCKPIT DINGS. FATIGUE WAS NOT A FACTOR. THE ONLY THING LEFT, I SUPPOSE, WOULD BE COMPLACENCY, AND I DON'T KNOW WHY. AT THAT POINT IN SPACE AND TIME, WE WERE BOTH UP AND LOCKED, THINKING EVERYTHING WAS OK. LESSON LEARNED, OR SHOULD I SAY OFTEN REPEATED, KEEP THE XCHK GOING!

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.