Narrative:

We were on the amoga-G SID at the paris cdg airport in france. We took off in VMC on runway 9 and were to fly the SID, which required a straight out climb on radial 88 off the cdg VOR until an NDB/DME fix at which we had to turn left to depart the terminal area. Originally, our flight plan called for an amoga-H departure off runway 8L, for which our GPS computers were programmed. As we received clearance from paris on the ground to expect runway 9 for departure, I reprogrammed the departure SID in the GPS computers. I hand flew the departure with reference to my GPS/flight director, while the captain had his flight director set to VOR with the cdg VOR course of 88 degrees selected. For some reason, my flight director commanded me to fly a right turn to intercept a different radial than required by the SID for the runway we used (runway 9). I interpreted this command as a slight course correction to stay on the 88 degree radial/heading. I initiated this slight turn, realized soon, however, that my flight director/GPS showed a desired track of 109 degrees and called for a continuing course correction/turn. I looked at the captain's course bar which used the VOR navigation mode and realized that it commanded a turn to the left instead. That is when I recognized my GPS course to be in error and I corrected the turn by banking to the left to return to the original track of 88 degrees. That is when the departure controller called and asked for an immediate turn to 360 degrees because we were about to enter the departure corridor for runways 8L&right. I turned the aircraft to the new heading while continuing our climb. There were no other aircraft in our vicinity which would have caused a traffic conflict, and our TCASII system never indicated any traffic close to our position. The controller said that, due to the unauthorized course correction to the left, he was required to file a traffic report of this incident. When we evaluated the situation, we came to the conclusion that, even though I had reprogrammed the GPS computers to the new departure SID, my GPS, for some unknown reason, still directed me to intercept the initial outbound course for runway 8L. We did not come up with any explanation as to why the GPS had not accepted my program change. Even though I corrected my outbound course after realizing the error, I should have caught this initial turn command as being incorrect sooner and continue on runway heading, using raw VOR data from the captain's navigation system, until the reason for my GPS error could be determined. Luckily, no traffic conflict had been created and the situation did not become dangerous.

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Original NASA ASRS Text

Title: DC10-30 SUSTAINS TRACK AND HDG DEV AT CDG ON DEP.

Narrative: WE WERE ON THE AMOGA-G SID AT THE PARIS CDG ARPT IN FRANCE. WE TOOK OFF IN VMC ON RWY 9 AND WERE TO FLY THE SID, WHICH REQUIRED A STRAIGHT OUT CLB ON RADIAL 88 OFF THE CDG VOR UNTIL AN NDB/DME FIX AT WHICH WE HAD TO TURN L TO DEPART THE TERMINAL AREA. ORIGINALLY, OUR FLT PLAN CALLED FOR AN AMOGA-H DEP OFF RWY 8L, FOR WHICH OUR GPS COMPUTERS WERE PROGRAMMED. AS WE RECEIVED CLRNC FROM PARIS ON THE GND TO EXPECT RWY 9 FOR DEP, I REPROGRAMMED THE DEP SID IN THE GPS COMPUTERS. I HAND FLEW THE DEP WITH REF TO MY GPS/FLT DIRECTOR, WHILE THE CAPT HAD HIS FLT DIRECTOR SET TO VOR WITH THE CDG VOR COURSE OF 88 DEGS SELECTED. FOR SOME REASON, MY FLT DIRECTOR COMMANDED ME TO FLY A R TURN TO INTERCEPT A DIFFERENT RADIAL THAN REQUIRED BY THE SID FOR THE RWY WE USED (RWY 9). I INTERPED THIS COMMAND AS A SLIGHT COURSE CORRECTION TO STAY ON THE 88 DEG RADIAL/HDG. I INITIATED THIS SLIGHT TURN, REALIZED SOON, HOWEVER, THAT MY FLT DIRECTOR/GPS SHOWED A DESIRED TRACK OF 109 DEGS AND CALLED FOR A CONTINUING COURSE CORRECTION/TURN. I LOOKED AT THE CAPT'S COURSE BAR WHICH USED THE VOR NAV MODE AND REALIZED THAT IT COMMANDED A TURN TO THE L INSTEAD. THAT IS WHEN I RECOGNIZED MY GPS COURSE TO BE IN ERROR AND I CORRECTED THE TURN BY BANKING TO THE L TO RETURN TO THE ORIGINAL TRACK OF 88 DEGS. THAT IS WHEN THE DEP CTLR CALLED AND ASKED FOR AN IMMEDIATE TURN TO 360 DEGS BECAUSE WE WERE ABOUT TO ENTER THE DEP CORRIDOR FOR RWYS 8L&R. I TURNED THE ACFT TO THE NEW HDG WHILE CONTINUING OUR CLB. THERE WERE NO OTHER ACFT IN OUR VICINITY WHICH WOULD HAVE CAUSED A TFC CONFLICT, AND OUR TCASII SYS NEVER INDICATED ANY TFC CLOSE TO OUR POS. THE CTLR SAID THAT, DUE TO THE UNAUTH COURSE CORRECTION TO THE L, HE WAS REQUIRED TO FILE A TFC RPT OF THIS INCIDENT. WHEN WE EVALUATED THE SIT, WE CAME TO THE CONCLUSION THAT, EVEN THOUGH I HAD REPROGRAMMED THE GPS COMPUTERS TO THE NEW DEP SID, MY GPS, FOR SOME UNKNOWN REASON, STILL DIRECTED ME TO INTERCEPT THE INITIAL OUTBOUND COURSE FOR RWY 8L. WE DID NOT COME UP WITH ANY EXPLANATION AS TO WHY THE GPS HAD NOT ACCEPTED MY PROGRAM CHANGE. EVEN THOUGH I CORRECTED MY OUTBOUND COURSE AFTER REALIZING THE ERROR, I SHOULD HAVE CAUGHT THIS INITIAL TURN COMMAND AS BEING INCORRECT SOONER AND CONTINUE ON RWY HDG, USING RAW VOR DATA FROM THE CAPT'S NAV SYS, UNTIL THE REASON FOR MY GPS ERROR COULD BE DETERMINED. LUCKILY, NO TFC CONFLICT HAD BEEN CREATED AND THE SIT DID NOT BECOME DANGEROUS.

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.