Narrative:

We were departing london heathrow on the wobun 2F (wob 2F) SID. While on the 301 degree bearing towards bur NDB, the next turn occurs at 7 DME from the lon VOR when you intercept the 360 degree bearing from bur NDB. I was hand flying the airplane using LNAV at the time. At D7 lon, we began the right turn as the SID directs to intercept the 360 degree bearing from bur NDB. With a normal rate of turn following the flight director, the turn took us a small amount through the course to the west so we had to correct back northeasterly to intercept the 360 degree bearing. London control then gave us a vector to fly heading 015 degrees. This slight fly-through of the course was not very significant at all, and london control never indicated there was any problem. The reason I wanted to report this was because I know that it is extremely important to follow these sids precisely. In this situation, the turn should probably commence a little prior to the D7 lon, and wind could be a factor too. Taking these things into account, the turn could be made to roll out right on the 360 degree bearing from bur NDB. It seems as though the LNAV wants to begin the turn right at D7 lon, and this could roll you out a little west of course depending on wind. Planning ahead and keeping wind and your speed in consideration will help to maintain the proper SID routing (track). One thing I did not mention previously was that prior to D7 lon, london control had given us an unrestr speed for the climb. This too contributed to the slight drift-through.

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Original NASA ASRS Text

Title: B767 CREW HAD TRACK DEV DEPARTING EGLL ON THE WOBUN 2 (WOB 2F) SID.

Narrative: WE WERE DEPARTING LONDON HEATHROW ON THE WOBUN 2F (WOB 2F) SID. WHILE ON THE 301 DEG BEARING TOWARDS BUR NDB, THE NEXT TURN OCCURS AT 7 DME FROM THE LON VOR WHEN YOU INTERCEPT THE 360 DEG BEARING FROM BUR NDB. I WAS HAND FLYING THE AIRPLANE USING LNAV AT THE TIME. AT D7 LON, WE BEGAN THE R TURN AS THE SID DIRECTS TO INTERCEPT THE 360 DEG BEARING FROM BUR NDB. WITH A NORMAL RATE OF TURN FOLLOWING THE FLT DIRECTOR, THE TURN TOOK US A SMALL AMOUNT THROUGH THE COURSE TO THE W SO WE HAD TO CORRECT BACK NORTHEASTERLY TO INTERCEPT THE 360 DEG BEARING. LONDON CTL THEN GAVE US A VECTOR TO FLY HDG 015 DEGS. THIS SLIGHT FLY-THROUGH OF THE COURSE WAS NOT VERY SIGNIFICANT AT ALL, AND LONDON CTL NEVER INDICATED THERE WAS ANY PROB. THE REASON I WANTED TO RPT THIS WAS BECAUSE I KNOW THAT IT IS EXTREMELY IMPORTANT TO FOLLOW THESE SIDS PRECISELY. IN THIS SIT, THE TURN SHOULD PROBABLY COMMENCE A LITTLE PRIOR TO THE D7 LON, AND WIND COULD BE A FACTOR TOO. TAKING THESE THINGS INTO ACCOUNT, THE TURN COULD BE MADE TO ROLL OUT RIGHT ON THE 360 DEG BEARING FROM BUR NDB. IT SEEMS AS THOUGH THE LNAV WANTS TO BEGIN THE TURN R AT D7 LON, AND THIS COULD ROLL YOU OUT A LITTLE W OF COURSE DEPENDING ON WIND. PLANNING AHEAD AND KEEPING WIND AND YOUR SPD IN CONSIDERATION WILL HELP TO MAINTAIN THE PROPER SID ROUTING (TRACK). ONE THING I DID NOT MENTION PREVIOUSLY WAS THAT PRIOR TO D7 LON, LONDON CTL HAD GIVEN US AN UNRESTR SPD FOR THE CLB. THIS TOO CONTRIBUTED TO THE SLIGHT DRIFT-THROUGH.

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.