B737-700
Flight Crew Last 90 Days 115
Workload; Troubleshooting; Time Pressure; Situational Awareness; Distraction; Confusion
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action; Flight Crew Became Reoriented
ATC Equipment / Nav Facility / Buildings
We were cleared the ILS 28R SFO about 14 miles out; tracking the Localizer in LNAV and descending in LVL CHNG. I was the Pilot Flying (PF) and selected Approach mode on the MCP since we were perfectly aligned on the LOC and glideslope. This usually makes the transition to the ILS seamless. As soon as I selected Approach mode; the aircraft made a hard right 30 degree bank turn for absolutely no apparent reason. I immediately overpowered the autopilot and corrected back left; kicking the autopilot to Control Wheel Steering (CWS) roll and pitch. The LOC went full scale deflection in the process. I elected to just LNAV to CEPIN to re-intercept; now wary of the LOC. After re-intercepting in LNAV and verifying we had the correct ILS frequency tuned; I cautiously selected Approach mode again just outside of AXMUL and the aircraft intercepted the LOC and GS without incident. I am doubtful this was an aircraft equipment malfunction; as I have had a similar incident with the SFO 28L LOC and (to a lesser extent) other Localizers throughout the entire country. I've noticed many ILS's around the country are very rough when intercepting the LOC and/or GS. For this reason I use LNAV/VNAV as much as is allowed in our Aircraft operation Manual (AOM).
A B737-700 pilot engaged the APP Mode on the SFO 28R ILS when tracking the Localizer in LNAV about 14NM out at which time the aircraft made a hard right 30 degree turn for no apparent reason. The APP Mode was reselected outside AXMUL without incident. An airport equipment anomaly is suspected.