An MD90 flight crew overrode the autopilot when it failed to capture the localizer.

Attributes
ACN	880697
Time
Date	201003
Local Time Of Day	1201-1800
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	MD-90 Series (DC-9-90) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Approach Coupler
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 110 Flight Crew Type 500
Person 2
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Flight Engineer Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 163 Flight Crew Total 8300 Flight Crew Type 4563
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Track / Heading All Types

Narrative:

We were cleared to intercept the localizer at 5000 ft. Aircraft began the left turn from 130 heading and continued turning through 60 degrees. It was evident that the autopilot was not stopping the turn so the first officer disconnected the autopilot and began a turn to the right to re-intercept. During the turn back to the right the aircraft reached 5300 ft and we corrected back to 5000 ft. The wind at the time was 50 KTS from a 140 heading. The aircraft was hand flown until established approximately 1 mile outside of the FAF. The autopilot was reengaged; captured and operated normally to minimums when it was then disconnected for an uneventful landing. After we parked at the gate and the aircraft was secured; we made a logbook entry and notified maintenance about the event. Also; we called TRACON and confirmed that no loss of separation occurred.

Original NASA ASRS Text

Title: An MD90 flight crew overrode the autopilot when it failed to capture the localizer.

Narrative: We were cleared to intercept the localizer at 5000 FT. Aircraft began the left turn from 130 heading and continued turning through 60 degrees. It was evident that the autopilot was not stopping the turn so the First Officer disconnected the autopilot and began a turn to the right to re-intercept. During the turn back to the right the aircraft reached 5300 FT and we corrected back to 5000 FT. The wind at the time was 50 KTS from a 140 heading. The aircraft was hand flown until established approximately 1 mile outside of the FAF. The autopilot was reengaged; captured and operated normally to minimums when it was then disconnected for an uneventful landing. After we parked at the gate and the aircraft was secured; we made a logbook entry and notified maintenance about the event. Also; we called TRACON and confirmed that NO loss of separation occurred.

Data retrieved from NASA's ASRS site as of April 2012 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.