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.