Narrative:

We were on a vector for the visual approach. It was VMC on the field; there were lots of building rain showers north painting on the radar; but causing no adverse conditions to arriving aircraft. We were at 3000 ft MSL on a 220 heading to join the localizer and expecting to break out of clouds shortly after the initial approach fix. The captain was the pilot flying and had heading selected on the MCP/FMA and VOR/localizer armed on both the MCP and FMA. Neither the flight director nor autopilot captured the localizer. The captain initiated a turn back to the localizer simultaneously with approach giving us a call to turn to a 150 heading to intercept xxl. Both the FD and autopilot again failed to capture the final. As approach called to tell us we were again going through final; we asked for a vector out to a box pattern to rejoin and approach complied. We rechecked our set up; but found nothing out of the ordinary. As we were re-vectored; the same thing occurred. The captain disconnected the autopilot and hand-flew the ILS to an uneventful full stop. As expected; we broke out about 2200 ft MSL. On the final; we noticed the captain's FMA indicated heading selected (green) VOR/localizer armed (white); and G/south (glideslope) captured (green). The first officer's FMA indicated both VOR/localizer and G/south captured (green). Additionally; while on final; we could not manually disconnect heading on the MCP. It was very strange. Maintenance determined the captain's ILS was bad and MEL'd it. Personally; I am not sure this was the problem. There was definitely a computer glitch somewhere. I was very happy with our coordination on final as well as the decision to depart the approach and get a box pattern to figure out what was going on.

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Original NASA ASRS Text

Title: A B737-700's was programmed to capture an ILS from the CMD heading mode. The FMA and MCP indicated VOR/LOC armed but the autopilot would not transition from the HDG mode to the ILS Capture mode.

Narrative: We were on a vector for the visual approach. It was VMC on the field; there were lots of building rain showers north painting on the radar; but causing no adverse conditions to arriving aircraft. We were at 3000 FT MSL on a 220 heading to join the Localizer and expecting to break out of clouds shortly after the initial approach fix. The Captain was the pilot flying and had heading selected on the MCP/FMA and VOR/LOC armed on both the MCP and FMA. Neither the Flight Director nor autopilot captured the Localizer. The Captain initiated a turn back to the Localizer simultaneously with Approach giving us a call to turn to a 150 heading to intercept XXL. Both the FD and autopilot again failed to capture the final. As Approach called to tell us we were again going through final; we asked for a vector out to a box pattern to rejoin and Approach complied. We rechecked our set up; but found nothing out of the ordinary. As we were re-vectored; the same thing occurred. The Captain disconnected the autopilot and hand-flew the ILS to an uneventful full stop. As expected; we broke out about 2200 FT MSL. On the final; we noticed the Captain's FMA indicated Heading selected (green) VOR/LOC armed (white); and G/S (glideslope) captured (green). The First Officer's FMA indicated both VOR/LOC and G/S captured (green). Additionally; while on final; we could not manually disconnect heading on the MCP. It was very strange. Maintenance determined the Captain's ILS was bad and MEL'd it. Personally; I am not sure this was the problem. There was definitely a computer glitch somewhere. I was very happy with our coordination on final as well as the decision to depart the approach and get a box pattern to figure out what was going on.

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.