A B737-700 pilot engaged the app mode on the sfo 28R ILS when tracking the localizer in LNAV about 1...

Narrative:

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 localizer 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 localizer went full scale deflection in the process. I elected to just LNAV to cepin to re-intercept; now wary of the localizer. 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 localizer and GS without incident. I am doubtful this was an aircraft equipment malfunction; as I have had a similar incident with the sfo 28L localizer 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 localizer and/or GS. For this reason I use LNAV/VNAV as much as is allowed in our aircraft operation manual (aom).

Original NASA ASRS Text

Title: 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.

Narrative: 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).

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1265365
Time
Date	201505
Local Time Of Day	1801-2400
Place
Locale Reference	SFO.Airport
State Reference	CA
Environment
Flight Conditions	Mixed
Light	Night
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	ILS/VOR
Person 1
Function	Captain Pilot Flying
Experience	Flight Crew Last 90 Days 115
Events
Anomaly	Deviation - Procedural Clearance Deviation - Track / Heading All Types

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.