Narrative:

Prior to takeoff copenhagen, captain and I discussed the odin 1C departure procedure and mentioned that the kastrup VOR was at the far end of runway 22R and the DME would wind down before increasing again to 2.0 for our right turn to 285 degree heading. After takeoff, the gear was raised and LNAV armed. Passing about .5 DME the captain noted the magenta line was behind the aircraft and I should start my right turn to 285 degrees. I initiated my turn slowly, pointing out that we were not yet 2.0 DME. Confused, I stopped midway in my turn. Captain contacted departure control and was given a 260 degree heading and told to continue climb to 8000 ft. Having realized our early turn, he apologized for our diversion from the departure profile. The controller stated he 'would speak to the tower about it.' further departure instructions were given, and nothing else was said about the deviation. Lesson learned: believe the raw data (DME) especially just after takeoff when the FMC is still trying to update itself. Callback conversation with reporter revealed the following information: on domestic flight, flcs generally delay arming LNAV until 2000-3000 ft, whereas in foreign countries they are instructed to arm LNAV just after takeoff. But, reporter says, foreign SID's and STAR's invariably call for quicker heading and/or altitude changes than domestic operations. Thus, when arming LNAV immediately after takeoff, it is likely that the FMC will still be updating and be inaccurate. Reporter follows raw data only on the initial SID procedure in order to give the FMC time to update. He also says that FMC's vary from aircraft to aircraft, some update quicker than others. However, all will be accurate within a few mi after takeoff.

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Original NASA ASRS Text

Title: FMC UPDATING WHEN LNAV ARMED. FLC CONFUSION.

Narrative: PRIOR TO TKOF COPENHAGEN, CAPT AND I DISCUSSED THE ODIN 1C DEP PROC AND MENTIONED THAT THE KASTRUP VOR WAS AT THE FAR END OF RWY 22R AND THE DME WOULD WIND DOWN BEFORE INCREASING AGAIN TO 2.0 FOR OUR R TURN TO 285 DEG HDG. AFTER TKOF, THE GEAR WAS RAISED AND LNAV ARMED. PASSING ABOUT .5 DME THE CAPT NOTED THE MAGENTA LINE WAS BEHIND THE ACFT AND I SHOULD START MY R TURN TO 285 DEGS. I INITIATED MY TURN SLOWLY, POINTING OUT THAT WE WERE NOT YET 2.0 DME. CONFUSED, I STOPPED MIDWAY IN MY TURN. CAPT CONTACTED DEP CTL AND WAS GIVEN A 260 DEG HDG AND TOLD TO CONTINUE CLB TO 8000 FT. HAVING REALIZED OUR EARLY TURN, HE APOLOGIZED FOR OUR DIVERSION FROM THE DEP PROFILE. THE CTLR STATED HE 'WOULD SPEAK TO THE TWR ABOUT IT.' FURTHER DEP INSTRUCTIONS WERE GIVEN, AND NOTHING ELSE WAS SAID ABOUT THE DEV. LESSON LEARNED: BELIEVE THE RAW DATA (DME) ESPECIALLY JUST AFTER TKOF WHEN THE FMC IS STILL TRYING TO UPDATE ITSELF. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ON DOMESTIC FLT, FLCS GENERALLY DELAY ARMING LNAV UNTIL 2000-3000 FT, WHEREAS IN FOREIGN COUNTRIES THEY ARE INSTRUCTED TO ARM LNAV JUST AFTER TKOF. BUT, RPTR SAYS, FOREIGN SID'S AND STAR'S INVARIABLY CALL FOR QUICKER HDG AND/OR ALT CHANGES THAN DOMESTIC OPS. THUS, WHEN ARMING LNAV IMMEDIATELY AFTER TKOF, IT IS LIKELY THAT THE FMC WILL STILL BE UPDATING AND BE INACCURATE. RPTR FOLLOWS RAW DATA ONLY ON THE INITIAL SID PROC IN ORDER TO GIVE THE FMC TIME TO UPDATE. HE ALSO SAYS THAT FMC'S VARY FROM ACFT TO ACFT, SOME UPDATE QUICKER THAN OTHERS. HOWEVER, ALL WILL BE ACCURATE WITHIN A FEW MI AFTER TKOF.

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.