Date: 201508
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: IMC
Light: Night
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Final Approach
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ; Tower ZZZ
Aircraft component: Autoflight System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Human factors: Communication Breakdown; Troubleshooting; Situational Awareness; Distraction; Confusion; Workload
Second reporter function: Captain; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Aircraft Equipment Problem Dissipated; Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Company Policy; Procedure; Weather
Primary problem: Procedure

Narrative: After reaching cruise altitude VNAV disengaged and the aircraft reverted to Control Wheel Steering (CWS) pitch mode. We investigated the problem by referring to the QRH; the MEL and through crew discussion. Unable to find a solution we contacted dispatch through SATCOM and got patched in to maintenance. After discussion with operations we decided to continue to our destination. The airport was using the west runways which only have RNAV approaches available so we began to discuss completing an approach with the use of the vertical speed mode and the Pilot Direction Indicator (PDI). On the descent we lost LNAV and the aircraft reverted to CWS roll mode. We were unable to reconnect LNAV or VNAV and decided to request an ILS to runway 6R with a slight tailwind. Approaching to the airport both LNAV and VNAV became active again. While on arrival approach control asked if we would be able to fly the ILS to 6L to check the glideslope. We accepted and briefed the ILS to 6L. Just a few miles from the turn to final; we were cleared for the localizer to 6L which was unexpected. We flew through the localizer course and corrected back on. I noticed we were high on the PDI and determined that we needed to start down. I requested a lower altitude be set in the MCP and deactivated the autopilot to start down. Instead of flying a VNAV glidepath I attempted to 'dive and drive' to the MDA. Having not briefed the localizer approach we both felt uncomfortable with continuing any further and executed a missed approach. We received vectors to an RNAV Y to 24R since we had re-established LNAV and VNAV and successfully landed from that approach.We never should have put ourselves in a position of flying an approach that we were not adequately prepared for. The combination of aircraft system malfunctions; a late flight and an unusual clearance led us down a path that could have resulted in an undesired aircraft state. I believe that we recognized this and recovered adequately. We could easily have avoided this event if we had flown the approach that was originally briefed or requested time to set up for and brief the localizer approach then utilized LNAV and VNAV procedures.


Second narrative: It was very uncomfortable; and a bit of a 'Cluster'; however; we were able to get the aircraft out of Takeoff Go-Around (TOGA) by cycling the Flight Director switches. We got cleaned up and now became number 5 behind all the traffic on the 24s.We should have never accepted the clearance to 6L. This really created more of a compound problem on top of a problem. We were not sure if ATC was aware of the mis-information when giving us 6L.


Synopsis: A B737NG with LNAV/VNAV anomalies was cleared for an ILS following a storm but the ILS was inoperative. The crew continued on the LOC but executed a go-around because of nearby terrain in night IMC. The aircraft autoflight returned to normal so a RNAV was flown to the opposite runway.