ACR ACFT WITH AUTOMATED COCKPIT TURNS EARLY ON SID. CTLR CATCHES ERROR.

Narrative

COPLT FOR LGT BUCKEYE 9 SID RWY 8L AT KPHX. ACFT HAD JUST GOTTEN AIRBORNE; WAS ON AUTOPLT AND IN MANAGED (COMPUTER) NAV. PROBLEM: ACFT TURNED 4 NM E OF KPHX (RWY) VICE 4 NM E OF PXR (VOR) TO 190 DEG HDG AS REQUIRED BY SID. OBVIOUS MISTAKE; CTLR QUESTIONED TURN; IMMEDIATE INVESTIGATION BY PLTS SAW ERROR AND MADE APOLOGY. CTLR REPLIED 'NO PROBLEM' IN THE FUTURE USE PXR VICE PHX FOR REFING TURN TO S. REST OF SID FLOWN WITHOUT INCIDENT. IN RECONSTRUCTION OF WHY COMPUTER NAV MADE TURN; ONLY CONCLUSION DRAWN WAS THAT DATABASE WHEN SELECTING BUCKEYE 9 SID AT KPHX; PROVIDED ALL POINTS AND COURSE LEGS BUT PXR NAV POINT WAS NOT IN FLT MGMNT CTL UNIT FLT PLAN PAGE AS A POINT. REASON FOR OMISSION IS UNKNOWN. NAV DISPLAY 'LOOKED' CORRECT (PLAN FORM) WHEN VISUALLY COMPARED TO COMMERCIAL CHART. IN FACT; TURN OCCURRED SO QUICK THAT PROPER REFING TO VOR (RAW DATA) WAS NOT DONE. ACFT FLEW WHAT WAS IN THE COMPUTER BUT WHAT WAS IN THE COMPUTER (FLT PLAN) WAS WRONG. AGAIN REASON FOR OMISSION OF PXR POINT IS UNKNOWN. FUNDAMENTALLY; LGT AIR CREWS ONLY HAVE TO BE ABSOLUTELY 100% SURE THAT EACH LEG OF FMC FLT PLAN IS CORRECT FROM 'POINT TO NEXT POINT.' NO SAFETY OF FLT CONCERN AROSE FROM THIS SITUATION.