AN UNDETECTED AUTOFLT MALFUNCTION ON THE SKEBR RNAV 1 ARR TO LAS; NV. RESULTS IN A TRACK DEV.

Narrative

ON THE SKEBR 1 ARR; ATC REQUESTED 250 KTS FOR SPACING INTO LAS. WHILE ON THE DSCNT PROFILE; WE ENCOUNTERED WAKE TURB FROM THE PRECEDING ACFT. WHEN WE PASSED CLARR INTXN; THE ACFT WAS TRACKING THE ARR. WHEN WE CROSSED OVER THE SKEBR INTXN THE ACFT DID NOT FOLLOW THE TURN AND CONTINUED ON THE 031 DEG HDG. WE NOTIFIED ATC AND TOOK THE CORRECTIVE ACTION TO REINTERCEPT THE ARR. UPON ARR AT THE GATE; THE CAPT AND I DISCUSSED THE PROB WITH MAINT AND CAME TO THE SAME CONCLUSION THAT THE LNAV HAD MALFUNCTIONED. IN RETROSPECT; FATIGUE FROM THE 7 LEGS AND 10+ HR DUTY DAY AND THE UNCOMMANDED ROLL FROM THE WAKE TURB MAY HAVE DISENGAGED THE LNAV WITHOUT EITHER THE CAPT OR I REALIZING IT. IN THE FUTURE; I WILL INCORPORATE A XCHK WITH AFDS TO ENSURE THE AUTOPLT IS STILL FUNCTIONING PROPERLY AFTER ANY TYPE OF TURB THAT IS ENCOUNTERED. SUPPLEMENTAL INFO FROM ACN 561130: CONTRIBUTING FACTORS: WAKE TURB/SUBSEQUENT DISCOVERY THE ACFT WAS HAVING FMS AND AUTOPLT PROBS SUBSEQUENT WRITE-UPS AND SAME PROB NEXT DAY WRITE-UP.