B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Engineer; Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 210; Flight Crew Total 15000; Flight Crew Type 5000
Aircraft Equipment Problem Less Severe
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
On downwind for PRM approach to Runway 28 ATL descending through 5;500 FT for 4;000 FT both flight directors failed simultaneously and autopilot disengaged. First Officer (PF) assumed manual (un-flight-directed) control and leveled aircraft at 4;000 FT. Tried to cycle F/Ds with no joy. Communicated with ATC 'unable PRM.' Vectored south; then west; then north over the top of airport at 5;000 FT on vectors for basic ILS to Runway 26R. During vectors attempted to regain FDs and/or A/Ps. During troubleshooting; crew noted heading bug on First Officer's PFD frozen on 090 degree heading and would not respond to changes made to heading on MCP. In addition; First Officer's PFD displayed commanded airspeed was frozen at 230 KTS and would not respond to changes made to airspeed on MCP. During ATC assigned climb from 4;000 FT to 5;000 FT MCP altitude window reset itself to 50;000 FT several times; requiring multiple resets back to 5;000 FT. Noted Captain heading bug and airspeed responding normally to MCP inputs. First Officer did outstanding job of hand flying aircraft through multiple; compound malfunction environment; during high workload in high density ATC environment. Control of aircraft transferred to Captain. Reengaged both FDs; with Captain as master; and reengaged A/P A. Flew rest of approach with A/P A to visual approach passing 1;500 FT AGL. First Officer's FD command bars redisplayed as landing gear was lowered and flaps were selected to 15. Suspect some combination of FCC B and/or MCP failure. This would be an excellent scenario for recurrent training this year with our current emphasis on automation download and hand flying skill maintenance. Would need to see maintenance action regarding this malfunction and corrective action before recommending preventive measures or further corrective action.
B737-800 Captain reported multiple autoflight anomalies on approach; perhaps related to FCC failure.