B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Workload; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Human Factors; Aircraft
Just off the STAR; we were being vectored by ATC for an anticipated approach to RWY 16R SEA from the east. We received a clearance to proceed direct AGANE. We programmed the FMC; EXEC; LNAV and direct we went. About 3 NM from AGANE we queried ATC whether they wanted us to proceed inbound on the localizer; or hold at AGANE...they replied 'standby.' LNAV just began turning the aircraft toward the next waypoint inbound on the approach; 'CELAK' when ATC gave us the approach clearance 'turn left 190; cross CELAK at or above 4;000; cleared for the ILS 16R'. We complied; pushed HDG SEL on the MCP; set heading 190; armed the APPCH mode and also armed the second [autopilot] 'B' for the CAT III ILS as requested and cleared.Reading the FMA on the PFD; VOR/LOC was captured immediately and we proceeded inbound...but something looked wrong. On the MAP display we noticed we were tracking east of course; approaching BUGNE we were about 0.3 east of course; raw data confirmed this; as well as a visual to the airport ahead. I recognized this as a 'Standoff' and disengaged automation; turned off the Flight Directors...then back on; rebuilt the approach and automation just outside BUGNE. The Glide Slope captured at this point and our descent rate slowed...so did our approach speed; the Autothrottles did not get re-engaged during the 'rebuild' of the approach and automation. Speed got down to 127 knots referencing a target speed set at what I believe was 139 knots. Power was manually set; Autothrottles were re-engaged. We were stabilized by the Glideslope Intercept Altitude (GSIA) 'FINKA' and had a successful CAT III Autoland rollout.This was a training flight with a new First Officer.ATC's late approach clearance combined with FMC LNAV functions (and interface with APPCH mode/immediate capture) caused a control wheel position that I believe put the aircraft into an automated 'Standoff Mode.' The recognition and subsequent rebuilding of the automation and timing of all the above resulted in the Autothrottles not being re-engaged.
A B737 Captain reported deviating from cleared track when the aircraft entered a 'standoff' condition.