Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 9200; Flight Crew Type 5000
Human-Machine Interface; Distraction; Confusion; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Aircraft
Prior to departure from LAX on Runway 24L; we both confirmed the routing on the FMS which included the correct runway and all fixes during climb and cruise to our destination.  After departure I called for 'NAV' at 400 feet AGL as SOP.  At that time; that caused a slight movement of my heading/course pointer on the flight director however that is typical during a change from heading to NAV; especially with the plane's particular FMS.  In this instance however; the command continued to call for a left turn at a 15 degree bank.  Because it was windy; for a couple of seconds I thought that perhaps it was a wind correction glitch after selecting NAV however; because it continued to call for a left turn on such a simple; strait out departure; I looked at the ND which showed the course simply turning left; as if to fly back to the LAX VOR; basically a 180 degree turn. I took the aircraft out of NAV and back to 'heading' at which time we turned back to the proper heading for this departure.  During this event the aircraft turned approximately 15 to 20 degrees left of course for about 20 to 40 seconds.  It was enough that after turning back on heading; LAX Departure asked us our heading; which had already been re-established at 250 degrees.  The departure for 25R calls for a left turn to 235 degrees after passing the SMO 160 radial; so I believe that LAX Departure controllers assume that any left turn on departure off 24L is an FMS loading error however; in this instance; that is not the case.  In fact; our FMS directed the left turn as soon as I selected NAV; at 400 feet; well before the point at which a left turn would be directed for a departure off of 25R.  During and after the flight; I again verified that the FMS was loaded with the LOOP 6 and not just a direct route to the LAX VOR after departure.  It was loaded with LOOP 6 as was confirmed earlier; therefore I don't know why it commanded a left turn during a strait out 'vector' phase of a departure. My procedure departing LAX with this particular FMS will be to simply stay on 'heading mode' to fly the initial phase of departure since the FMS appears to be unreliable during the straight out vector phase of this simple departure.
An air carrier departed from LAX Runway 24L on the LOOP 6 and when NAV was selected at 400 feet the aircraft attempted to turn left as if making a left turn back to the LAX VOR.