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.
Synopsis
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.
Second reporter narrative
We were on a heading of 220 degrees to intercept the Localizer at 3000 FT and were cleared for the approach. The 'A' autopilot CMD (Configuration Management Directive) was on; heading mode was engaged; and VOR/LOC was armed. I called localizer alive; but noticed the FD and autopilot failed to intercept the Localizer. We started a left turn for the Localizer; and then Approach Control gave us a heading of 150 degrees to intercept the Localizer. Again; the 'A' autopilot CMD was on; heading mode was engaged; and VOR/LOC was armed. The FD and A/P failed to intercept the Localizer. We requested a vector to intercept the Localizer again. The Captain's FMA showed VOR/LOC and G/S armed. The Localizer never captured; but G/S did. The First Officer's FMA showed VOR/LOC and G/S captured. I could not manually deselect Heading on the MCP. I disengaged the autopilot and hand-flew the visual approach. Disengaging the autopilot and hand-flying the approach was the proper thing to do. Also; after determining the First Officer's FD was operating normally; if autopilot was needed; we could have used the autopilot
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.