B737 flight crew reported encountering significant auto flight system malfunctions during climb out and returned to the departure airport for an overweight landing.
Synopsis
B737 flight crew reported encountering significant auto flight system malfunctions during climb out and returned to the departure airport for an overweight landing.
Narrative
During climb out of ZZZ at approximately 16000 ft.; we started to experience issues with the auto flight system. The first issue noted was a loss of VNAV and a Control Display Panel (CDU) scratch pad message VNAV DISCONNECT; VNAV would not re-engage so we used LVL CHG for a pitch mode. The A autopilot was engaged and continued troubleshooting of the VNAV issue started; shortly thereafter the autopilot disengaged by itself and a loss of the pitch portion of the Flight Director (FD) was noticed. Neither autopilot would reengage. The Mode Control Panel (MCP) now became completely unusable; airspeed; heading; and ALT windows were completely frozen. ATC was advised of loss of RVSM capability and a lower altitude requested. A phone patch with Dispatch and Maintenance Control was initiated and a decision to return to ZZZ made; where the weather was VFR/VMC and light winds. With the loss of MCP; paper notes were used to keep track of ATC assigned headings; altitude; and airspeeds. A clearance from ATC was obtained; Flight Attendants notified; and passengers informed. Diversion checklist was referenced and all necessary data for landing in ZZZ was obtained. Due to the deteriorating nature of our auto flight systems; the decision to make an overweight landing; 8000 lbs; was made. A raw data ILS/visual approach was briefed; ATC was advised; due to the overweight landing; a normal approach and landing was made. After clearing the runway; out of an abundance of caution; we had Airport Rescue and Firefighting (ARFF) personnel inspect the brakes and tires. No defects noted and the aircraft was taxied to the gate and the passengers were deplaned via the jet bridge. Maintenance met the aircraft and debriefed; Dispatch verbally debriefed; and Chief Pilot debriefed.
Second reporter narrative
Climbing out of ZZZ on the ZZZZZ departure; we were cleared to climb to FL 230. Crossing 16000 ft. we noticed the VNAV Flight Mode Annunciator (FMA) blanked and a VNAV disconnect message was displayed on the FMC. As we tried to figure out what the issue was; the Pilot flying (PF) tried to engage the autopilot which immediately disconnected. We were subsequently cleared to climb to FL 290. We set 29000 ft. in the altitude window and then the numbers changed; displaying 50000 ft. At this point we attempted to regain our Mode Control Panel (MCP) and flight guidance by recycling the flight directors and the LNAV and VNAV functions. Nothing came back. At this point only our heading bug functioned properly. The speed window would change the speed on the Electronic Attitude Director Indicator (EADI) but not in the window. We discussed being unable to enter RVSM airspace with our current malfunctioning systems and developed a course of action given our current situation. We coordinated with ZZZ to level off outside of RVSM airspace while we troubleshoot further. The Captain discussed the situation with Dispatch and Maintenance Control; after being unable to rectify any of the failed equipment. We decided it would be best to return to ZZZ given the favorable clear weather conditions. Once we came up with our plan of action; the Captain let the passengers and Flight Attendants know what the situation was and that we would be returning to ZZZ. We considered burning down fuel to get below max landing weight; but given the deteriorating nature of our flight systems and knowing it would take a while to burn down the 8000 lbs. of fuel to get below it. We decided it would be best to get the plane back on the ground. We coordinated our clearance with ZZZ Center to return to ZZZ and land via the visual approach to Runway XXL. When we finally changed to ZZZ Approach Control we advised them we would be doing an overweight landing; but no assistance was required. The approach and landing went smoothly and safely and the responding fire trucks did a wheel inspection to ensure no indications of hot brakes. The rest of the taxi was uneventful.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.