DEPENDING ON THE RNAV SYS; AN ACR CREW HAS A TRACK DEV ON THE IDALE RNAV 2 DEP FROM LAS; NV. THAT IS QUICKLY CORRECTED BY THE DEP CTLR.

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WE DEPARTED LAS ON THE IDALE 2 DEP; AND USING THE LNAV MODE; ENDED UP GOING DIRECTLY TOWARD OAL AFTER IDALE INSTEAD OF THE DEP RTE TO SHEAD INTXN; BIKKN INTXN; THEN OAL VOR. AS WE MADE THE TURN TOWARD OAL; ATC QUERIED US AND GAVE US A HDG OF 240 DEGS. HE ASKED WHERE WE WERE HEADED. WE THEN LOOKED AT THE DEP IN THE BOX AND SAW THAT IT WASN'T IN THERE ANYMORE. WE HAD IT IN THE 'RE' WHEN WE CHKED IT AT THE GATE AND THINK THAT IT MUST HAVE DROPPED OUT WHEN WE DID THE RWY UPDATE; WE DID NOT CHK IT AGAIN AFTER THE UPDATE. THIS IS THE FIRST TIME THAT THE DEP SID WASN'T FLOWN; AS IT WAS SUPPOSED TO BE FLOWN; AND SINCE I AM PRETTY NEW WITH THE B757 COMING; OFF THE B727. I HAVE ALWAYS FOLLOWED SID MYSELF AND NOT TRUSTED THE BOX TO FLY IT BY ITSELF. I WAS ALWAYS TOLD TO 'RELAX; IT WILL DO FINE.' I STARTED TO COUNT ON THE AUTOPLT TO FLY IT AS IT WAS SUPPOSED TO AND GOT A LITTLE LAX. LESSON LEARNED! ALWAYS BACK IT UP AND DON'T RELAX. GARBAGE IN; GARBAGE OUT. IF THE RTE ISN'T IN THERE OR IT DROPS OUT; YOU'RE NOT GOING TO FLY WHAT YOU'RE THINKING YOU'LL FLY. SUPPLEMENTAL INFO FROM ACN 572915: IN THE TURN FROM IDALE DIRECT TO OAL; THE DEP CTLR CORRECTED OUR HDG (ABOUT 30 DEGS) TO RE-ESTABLISH US ON THE IDALE 2 DEP. I SPOKE WITH THE DEP SUPVR WHEN WE ARRIVED IN SFO AND TOLD HIM THE SERIES OF EVENTS FROM OUR VIEWPOINT.