Narrative:

We were filed and cleared to fly the cowby two RNAV departure off runway 7L at las. After takeoff and shortly after LNAV was selected on the MCP; the flight directors started to command a slow turn to the south. I thought this seemed odd; but we were approaching the first waypoint and I thought that due to our closeness to the point and winds aloft that the system was just making some quick corrections to get back on the 'black line.' not long after checking in with departure control they cleared us direct to cowby. Just after clearing us; departure control came back and advised us that we were south of course and gave us a vector toward cowby. After seeing how the flight directors acted earlier; I advised departure control that we were having aircraft navigation system problems and would check out our system. I asked departure control if this had caused any problems and they said no and that our company ahead of us had done the same thing and that they thought it must be the winds. The winds shouldn't have made any difference; but we couldn't find any obvious problems and after xchking our data to cowby with departure control; everything seemed to work fine for the remainder of the flight. The obvious explanation would be that we entered the wrong runway for takeoff. I know this wasn't the problem because while checking the route on the ground; I found that the first officer had entered runway 7R for departure. I advised him that we would get runway 7L and I watched him make the changes and we both checked the legs page to see that the fixes were right. The only thing that I can think of is that the data in the FMC for waste and/or bakrr was wrong. This could also explain why our company ahead of us had the same problem. Since we rarely use runway 7's in las; we might have had some bad data that's been in the system that we are just finding out about. This is the only explanation I can think of (unless I missed something). This has always been my worst fear in flying RNAV departures in this aircraft. Without a map display; you have no real practical way to quality control the FMC on the ground or in the air; and with both pilots in navigation; there is no real-time way to xchk the system in-flight. Plus; RNAV departures have minimal radial/DME information to work with. The problem was made worse by the fact that this was the first time I had flown this departure off runway 7L at las; so I had no real feel for what the departure was going to be like.

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Original NASA ASRS Text

Title: AN ACR CAPT DEVIATED FROM THE PUBLISHED TRACK ON A LAS RNAV SID.

Narrative: WE WERE FILED AND CLRED TO FLY THE COWBY TWO RNAV DEP OFF RWY 7L AT LAS. AFTER TKOF AND SHORTLY AFTER LNAV WAS SELECTED ON THE MCP; THE FLT DIRECTORS STARTED TO COMMAND A SLOW TURN TO THE S. I THOUGHT THIS SEEMED ODD; BUT WE WERE APCHING THE FIRST WAYPOINT AND I THOUGHT THAT DUE TO OUR CLOSENESS TO THE POINT AND WINDS ALOFT THAT THE SYS WAS JUST MAKING SOME QUICK CORRECTIONS TO GET BACK ON THE 'BLACK LINE.' NOT LONG AFTER CHKING IN WITH DEP CTL THEY CLRED US DIRECT TO COWBY. JUST AFTER CLRING US; DEP CTL CAME BACK AND ADVISED US THAT WE WERE S OF COURSE AND GAVE US A VECTOR TOWARD COWBY. AFTER SEEING HOW THE FLT DIRECTORS ACTED EARLIER; I ADVISED DEP CTL THAT WE WERE HAVING ACFT NAV SYS PROBS AND WOULD CHK OUT OUR SYS. I ASKED DEP CTL IF THIS HAD CAUSED ANY PROBS AND THEY SAID NO AND THAT OUR COMPANY AHEAD OF US HAD DONE THE SAME THING AND THAT THEY THOUGHT IT MUST BE THE WINDS. THE WINDS SHOULDN'T HAVE MADE ANY DIFFERENCE; BUT WE COULDN'T FIND ANY OBVIOUS PROBS AND AFTER XCHKING OUR DATA TO COWBY WITH DEP CTL; EVERYTHING SEEMED TO WORK FINE FOR THE REMAINDER OF THE FLT. THE OBVIOUS EXPLANATION WOULD BE THAT WE ENTERED THE WRONG RWY FOR TKOF. I KNOW THIS WASN'T THE PROB BECAUSE WHILE CHKING THE RTE ON THE GND; I FOUND THAT THE FO HAD ENTERED RWY 7R FOR DEP. I ADVISED HIM THAT WE WOULD GET RWY 7L AND I WATCHED HIM MAKE THE CHANGES AND WE BOTH CHKED THE LEGS PAGE TO SEE THAT THE FIXES WERE RIGHT. THE ONLY THING THAT I CAN THINK OF IS THAT THE DATA IN THE FMC FOR WASTE AND/OR BAKRR WAS WRONG. THIS COULD ALSO EXPLAIN WHY OUR COMPANY AHEAD OF US HAD THE SAME PROB. SINCE WE RARELY USE RWY 7'S IN LAS; WE MIGHT HAVE HAD SOME BAD DATA THAT'S BEEN IN THE SYS THAT WE ARE JUST FINDING OUT ABOUT. THIS IS THE ONLY EXPLANATION I CAN THINK OF (UNLESS I MISSED SOMETHING). THIS HAS ALWAYS BEEN MY WORST FEAR IN FLYING RNAV DEPS IN THIS ACFT. WITHOUT A MAP DISPLAY; YOU HAVE NO REAL PRACTICAL WAY TO QUALITY CTL THE FMC ON THE GND OR IN THE AIR; AND WITH BOTH PLTS IN NAV; THERE IS NO REAL-TIME WAY TO XCHK THE SYS INFLT. PLUS; RNAV DEPS HAVE MINIMAL RADIAL/DME INFO TO WORK WITH. THE PROB WAS MADE WORSE BY THE FACT THAT THIS WAS THE FIRST TIME I HAD FLOWN THIS DEP OFF RWY 7L AT LAS; SO I HAD NO REAL FEEL FOR WHAT THE DEP WAS GOING TO BE LIKE.

Data retrieved from NASA's ASRS site as of January 2009 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.