Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Type 2789
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Crossing Restriction Not Met
Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Aircraft
Filed for PHLBO arrival into EWR. Kept getting FMC complaints about steep descent after DQO. Used to getting complaints after SOMTO; but not DQO? Anyway; tried adding some various speed restriction to DQO and SOMTO to see if it would stop the reoccurring message. Normally fixes SOMTO. In this case no speeds seemed to satisfy the FMC; so settled back to what I would normally use for speeds of 300 at DQO and 270 at SOMTO; and accepted FMC repeatedly giving steep descent after DQO message.Both Captain and I were watching automation and very actively calling out FMAs throughout the flight; which probably allowed us to catch the sudden steep descent in reasonable time.We were cleared to descend via the PHLBO and had 8000 altitude set in MCP and in normal VNAV descent. Everything had been fine for two flights on this airplane and was fine on this descent until DQO. I was in descent mode set at 310 knots; I had a slow down to 300 set at DQO as previously mentioned. As we approached DQO DECEL point at what was supposed to be a level off at 20;000; the flight director suddenly went nose down and the aircraft started to dive. The VNAV suddenly switched to what I recall as being over one thousand feet above glide path? The Captain caught the action first and disconnected autopilot. He gave the aircraft back to me and we easily recovered the aircraft to a level off at DQO at roughly 19;200 feet. DQO was supposed to be 200A. The computer suddenly seemed to catch back up and recover a normal VNAV path for the STAR. We reengaged automation while watching closely and continued on the arrival; approach and landing. The next weird part was when we passed MERSR; four points passed DQO; the FMC came back; again; with a steep descent after DQO message.
B737 First Officer reported a malfunctioning coupled auto flight system during approach resulted in an altitude deviation.