B737-800
Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Other / Unknown
Aircraft Equipment Problem Less Severe; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Aircraft; Human Factors
We were transitioning from the arrival to vectors for the RNAV GPS 8L. The FO (First Officer) was the PF (Pilot Flying); I was the PM (Pilot Monitoring). We have been using this approach extensively during these lower traffic times; so we were both quite familiar with the procedure. We were on an assigned downwind heading; and had been cleared appropriately to succeeding lower altitudes. We were given an initial southerly base heading and descent clearance to 1;500 ft. The final turn was to heading 120; join on that heading; outside the KACDE FAF; and cleared for a visual approach to RWY 8L. As is normal on the 737; cleared for the approach triggers the litany of engaging 'LNAV; VNAV; confirm VNAV PTH; set minimums.' My FO had the FMS programmed appropriately [and] I had extended the final approach line into KACDE on the 092 inbound course - everything was absolutely normal. We were; as best I recall; perhaps 2 miles north of the final approach magenta displayed inbound course. The FO selected LNAV per the normal cockpit flow and litany; and the airplane immediately rolled up into a 30 degree right bank and started - apparently - trying to intersect the final line aggressively - near a 90 degree intercept! This was totally unexpected - everything was correctly programmed; the approach was familiar; and we both expected - at most - a gentle right turn to join the final - NOT this dramatic 30 degree turn from the autopilot! By the time we got it sorted; the autopilot rolled back into a 30 degree left turn to join the final; but by the time the final track was successfully established; we had blown through the final for 8L; 8R; and were probably abeam the Control Tower at our point of furthest south exceedance. We finally got established on the course line and landed uneventfully; but the huge S turn across the parallel final track was completely unacceptable.I observed traffic for Runway 8R on the TCAS; they appeared to be perhaps 2 miles behind us on the parallel approach. Despite our blundering into their final; the TCAS did not alert. The Approach Controller restated to us the assigned 120 heading to join; then told us to turn left immediately as the autopilot was rolling into the dramatic left correcting turn. As the Pilot Monitoring; I should have intervened MUCH sooner and directed the FO to return to heading select and re-attempt the intercept ON the assigned heading; or just disconnected the A/P (Auto-Pilot) and hand flown the correct intercept/track. I attribute my slow reaction to attempting to UNDERSTAND what was unfolding with the automation; instead of abandoning that and correctly RESPONDING to and correcting the actual flight path. The startle effect was very much in play; as that turn to that approach from that heading using LNAV is a completely routine maneuver that I've seen work correctly a dozen times! As has been highlighted many times; the role of PM is crucial. My expectation bias was complete - I was very much relying on and expecting a normal autopilot function!It becomes much too easy to just expect everything to work appropriately. I know I'm not the first to say it; but the PM role is demanding and requires vigilance. Just because the system has worked correctly 100 times before is no assurance that it will work correctly on the 101st go. I will re-focus once again on the foremost task - ASSURING correct aircraft path control; despite distractions of automation; ATC; weather; or whatever other variables come into play!
B737 Captain reported that during approach to landing; the autopilot initiated an unexpected turn to intercept the course at a large intercept angle.