A B737NG with LNAV/VNAV anomalies was cleared for an ILS following a storm but the ILS was inoperati...

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.

Original NASA ASRS Text

Title: 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.

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.

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	1288371
Time
Date	201508
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Make Model Name	B737 Next Generation Undifferentiated
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Component
Aircraft Component	Autoflight System
Person 1
Function	First Officer Pilot Flying
Experience	Flight Crew Last 90 Days 240 Flight Crew Total 7000 Flight Crew Type 1800
Person 2
Function	Captain Pilot Not Flying
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy Deviation - Speed All Types Inflight Event / Encounter Unstabilized Approach Inflight Event / Encounter Weather / Turbulence

A B737NG with LNAV/VNAV anomalies was cleared for an ILS following a storm but the ILS was inoperative. The crew continued on the localizer 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.