Narrative:

We departed ont from runway 8L with the first officer hand flying using the flight director in VNAV and LNAV with the initial commands being a heading to 250 degrees. Prior to intercepting the 143 degree radial of pom per the SID and FMC routing, we were cleared by socal departure 'direct to pom and the remainder of the ont 2, comply with restrs.' I selected 'direct intercept' to pom and we proceeded in LNAV. Just prior to reaching pom we heard a clearance for another aircraft to 'turn immediately to a 230 degree heading,' but heard no mention of the location of traffic nor any mention from the controller that our position may have caused the necessity for another aircraft to deviate. No further mention either to another aircraft or ourselves was made, so we resumed our climb and leveled at 9000 ft MSL prior to reaching the gaski intersection. Upon noting 'altitude hold' annunciated on the eadi, I selected the cleared altitude of 14000 ft MSL into the altitude window of the MCP and after passing gaski, VNAV was selected and a climb initiated. The departure controller issued us a clearance to 'turn 20 degrees left and intercept the pom 323 degree radial' and added that we 'weren't at pom.' both of our FMC map tracks showed us on the pom 323 degree radial slightly past gaski on course to kelse. Doubtful of the controller's clearance, I typed pom into the fix page of the CDU and also typed in the 323 degree radial which overlapped the track displayed on both ehsi's. After confirming our position in this manner, the position of the aircraft was displayed to the left (west) of course and I queried the controller if he 'wanted us to intercept the pom 323 degree radial, but I was showing us to the left of course.' the controller responded that we were showing to the 'right of course' and that we must have selected pdz instead of pom. At this time, the first officer selected his VOR into the manual mode on the 323 degree radial of pom (which as previously mentioned, had been both preselected and confirmed on the ground prior to our departure). True to the controller's indications, the VOR course showed us to be to the right (east) of course. We both selected the manual mode of our vors and flew the remainder of the departure in the manual mode without further incident. Upon reselecting the map mode on my ehsi, I noticed in the l-hand corner 2 indications that the IRU position were about 1.7 to 1.8 mi off the FMC position. As the flight continued toward sfo, the discrepancy between the IRU's and the FMC closed until it was about .4 to .5 mi and all LNAV functions operated normally. Obviously there was some error in the position indication of our FMC map track. I have not yet been able to reach a conclusion about the reason for the discrepancy in the FMC position on this particular flight, and have never before encountered an incident like this in nearly 4000 hours of glass aircraft experience. I have experienced minor map shifts, but never such a discrepancy that wasn't immediately verifiable. In retrospect, my first action should have been to immediately select my VOR to manual and ascertain the position of the aircraft from pom rather than placing pom into the fix page and essentially confirming an erroneous position with more incorrect information.

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Original NASA ASRS Text

Title: THE ACR FLC OF A B737-300 HAS BOTH IRU'S DRIFT OFF DURING THE DEP PROCESS AND THEY GET OFF COURSE BEFORE THEY COULD SELF-CORRECT. NEITHER OF THE FLC WAS MONITORING THE RAW DATA FROM THE DEP VORS.

Narrative: WE DEPARTED ONT FROM RWY 8L WITH THE FO HAND FLYING USING THE FLT DIRECTOR IN VNAV AND LNAV WITH THE INITIAL COMMANDS BEING A HDG TO 250 DEGS. PRIOR TO INTERCEPTING THE 143 DEG RADIAL OF POM PER THE SID AND FMC ROUTING, WE WERE CLRED BY SOCAL DEP 'DIRECT TO POM AND THE REMAINDER OF THE ONT 2, COMPLY WITH RESTRS.' I SELECTED 'DIRECT INTERCEPT' TO POM AND WE PROCEEDED IN LNAV. JUST PRIOR TO REACHING POM WE HEARD A CLRNC FOR ANOTHER ACFT TO 'TURN IMMEDIATELY TO A 230 DEG HDG,' BUT HEARD NO MENTION OF THE LOCATION OF TFC NOR ANY MENTION FROM THE CTLR THAT OUR POS MAY HAVE CAUSED THE NECESSITY FOR ANOTHER ACFT TO DEVIATE. NO FURTHER MENTION EITHER TO ANOTHER ACFT OR OURSELVES WAS MADE, SO WE RESUMED OUR CLB AND LEVELED AT 9000 FT MSL PRIOR TO REACHING THE GASKI INTXN. UPON NOTING 'ALT HOLD' ANNUNCIATED ON THE EADI, I SELECTED THE CLRED ALT OF 14000 FT MSL INTO THE ALT WINDOW OF THE MCP AND AFTER PASSING GASKI, VNAV WAS SELECTED AND A CLB INITIATED. THE DEP CTLR ISSUED US A CLRNC TO 'TURN 20 DEGS L AND INTERCEPT THE POM 323 DEG RADIAL' AND ADDED THAT WE 'WEREN'T AT POM.' BOTH OF OUR FMC MAP TRACKS SHOWED US ON THE POM 323 DEG RADIAL SLIGHTLY PAST GASKI ON COURSE TO KELSE. DOUBTFUL OF THE CTLR'S CLRNC, I TYPED POM INTO THE FIX PAGE OF THE CDU AND ALSO TYPED IN THE 323 DEG RADIAL WHICH OVERLAPPED THE TRACK DISPLAYED ON BOTH EHSI'S. AFTER CONFIRMING OUR POS IN THIS MANNER, THE POS OF THE ACFT WAS DISPLAYED TO THE L (W) OF COURSE AND I QUERIED THE CTLR IF HE 'WANTED US TO INTERCEPT THE POM 323 DEG RADIAL, BUT I WAS SHOWING US TO THE L OF COURSE.' THE CTLR RESPONDED THAT WE WERE SHOWING TO THE 'R OF COURSE' AND THAT WE MUST HAVE SELECTED PDZ INSTEAD OF POM. AT THIS TIME, THE FO SELECTED HIS VOR INTO THE MANUAL MODE ON THE 323 DEG RADIAL OF POM (WHICH AS PREVIOUSLY MENTIONED, HAD BEEN BOTH PRESELECTED AND CONFIRMED ON THE GND PRIOR TO OUR DEP). TRUE TO THE CTLR'S INDICATIONS, THE VOR COURSE SHOWED US TO BE TO THE R (E) OF COURSE. WE BOTH SELECTED THE MANUAL MODE OF OUR VORS AND FLEW THE REMAINDER OF THE DEP IN THE MANUAL MODE WITHOUT FURTHER INCIDENT. UPON RESELECTING THE MAP MODE ON MY EHSI, I NOTICED IN THE L-HAND CORNER 2 INDICATIONS THAT THE IRU POS WERE ABOUT 1.7 TO 1.8 MI OFF THE FMC POS. AS THE FLT CONTINUED TOWARD SFO, THE DISCREPANCY BTWN THE IRU'S AND THE FMC CLOSED UNTIL IT WAS ABOUT .4 TO .5 MI AND ALL LNAV FUNCTIONS OPERATED NORMALLY. OBVIOUSLY THERE WAS SOME ERROR IN THE POS INDICATION OF OUR FMC MAP TRACK. I HAVE NOT YET BEEN ABLE TO REACH A CONCLUSION ABOUT THE REASON FOR THE DISCREPANCY IN THE FMC POS ON THIS PARTICULAR FLT, AND HAVE NEVER BEFORE ENCOUNTERED AN INCIDENT LIKE THIS IN NEARLY 4000 HRS OF GLASS ACFT EXPERIENCE. I HAVE EXPERIENCED MINOR MAP SHIFTS, BUT NEVER SUCH A DISCREPANCY THAT WASN'T IMMEDIATELY VERIFIABLE. IN RETROSPECT, MY FIRST ACTION SHOULD HAVE BEEN TO IMMEDIATELY SELECT MY VOR TO MANUAL AND ASCERTAIN THE POS OF THE ACFT FROM POM RATHER THAN PLACING POM INTO THE FIX PAGE AND ESSENTIALLY CONFIRMING AN ERRONEOUS POS WITH MORE INCORRECT INFO.

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.