Narrative:

Captain was flying; first officer was pilot not flying. No MEL's; runway and departure verified; runway updated in FMS. Through 400 ft; LNAV requested by pilot flying and selected/confirmed by pilot not flying; flight directors indicated properly. At 1;000 ft AGL; pilot flying commanded climb thrust; speed 220; flaps 1. Pilot not flying complied. Shortly out of 1;000 ft; pilot flying selected 'a' channel auto pilot. The auto pilot engaged and the flight director (FD) continued to give good information and the aircraft began its intercept turn. At approximately 1;500 ft the following occurred: FD biased out of view; auto pilot cws (central warning system) engaged; auto pilot P/rst amber light illuminated. Aircraft continued turning and climbing in cws mode; pilot flying made corrections. Pilot not flying made immediate call to tower and advised '(callsign) is negative RNAV.' tower gave vectors to continue on the departure. The aircraft cleaned up for climb out by the pilot not flying and after take-off checklist run. The pilot flying reset the FD switches and autopilot and was able to regain RNAV capability. Tower was advised and they gave vectors to intercept/rejoin the departure. Tower was advised as to why we reported RNAV loss. Report is being filed due to FD and auto pilot malfunctioning/dropping off line during an RNAV departure for no apparent reason. Weather was good and winds were light. RNAV capabilities of this aircraft are unreliable.the cause is unknown. All systems were set up properly and engaged; giving proper guidance prior to malfunctioning. Suggest that the 737 fleet be removed from RNAV capable status until a flight management system and autopilot that is reliable can be installed.

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Original NASA ASRS Text

Title: B737-400 First Officer reported auto pilot and flight director malfunctions passing 1;500 FT during an RNAV departure. Immediate vectors are requested from ATC.

Narrative: Captain was flying; First Officer was pilot not flying. No MEL's; runway and departure verified; runway updated in FMS. Through 400 FT; LNAV requested by pilot flying and selected/confirmed by pilot not flying; Flight Directors indicated properly. At 1;000 FT AGL; pilot flying commanded Climb Thrust; Speed 220; Flaps 1. Pilot not flying complied. Shortly out of 1;000 FT; pilot flying selected 'A' channel auto pilot. The auto pilot engaged and the Flight Director (FD) continued to give good information and the aircraft began its intercept turn. At approximately 1;500 FT the following occurred: FD biased out of view; auto pilot CWS (Central Warning System) engaged; auto pilot P/RST amber light illuminated. Aircraft continued turning and climbing in CWS mode; pilot flying made corrections. Pilot not flying made immediate call to Tower and advised '(Callsign) is Negative RNAV.' Tower gave vectors to continue on the departure. The aircraft cleaned up for climb out by the pilot not flying and after take-off checklist run. The pilot flying reset the FD switches and autopilot and was able to regain RNAV capability. Tower was advised and they gave vectors to intercept/rejoin the departure. Tower was advised as to why we reported RNAV loss. Report is being filed due to FD and auto pilot malfunctioning/dropping off line during an RNAV departure for no apparent reason. Weather was good and winds were light. RNAV capabilities of this aircraft are unreliable.The cause is unknown. All systems were set up properly and engaged; giving proper guidance prior to malfunctioning. Suggest that the 737 fleet be removed from RNAV capable status until a Flight Management System and autopilot that is reliable can be installed.

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.