B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 169; Flight Crew Total 2352; Flight Crew Type 2352
Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Critical; Deviation - Altitude Overshoot; Deviation - Altitude Crossing Restriction Not Met; Deviation - Speed All Types; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem
Aircraft
We were on the arrival into ZZZZ and I was about half way through my approach brief. ZZZZ Center gave us a descend via about 40 miles outside of top of descent. I put the airplane into a descend now mode from our cruise altitude of FL330 and the airplane started down at 1;000 feet per min in VNAV. The first fix to cross was ZZZZZ at FL300 and 280 kts. Autopilot and autothrottle were engaged. Once everything looked kosher; I resumed my brief. I remember at some point looking up and seeing the airplane at FL315 descending in VNAV while I was finishing my brief. At some point shortly after; I felt the airplane pitching over rapidly. I noticed that the Vertical Deviation Scale on the VNAV display said we were 6;500 feet above the path. The airplane began diving for the path. I reached up to the FCP to begin fixing the issue by trying to engage V/S and shallowing out the descent. It didn't engage and that is when I noticed the airplane was in 'control wheel pitch.' The airplane was still rapidly descending increasing its airspeed. I extended the flight spoilers; disengaged the autopilot and auto-throttle; and began hand flying the airplane to a desirable state. The airplane had over sped by my estimate of around 10 kts; but everything happened so quickly that it was hard to judge. I arrested the descent around FL290 about 5 miles prior to ZZZZZ; which needed to be crossed at FL300. The Vertical Deviation Scale still said we were thousands of feet above the path. How is that possible when we were prior to and below the altitude of the first crossing restriction? ZZZZZ had FL300 'big' correctly in the FMC. I hand flew the airplane back up to FL300 and 280 kts. The Vertical Deviation Scale for the VNAV display had disappeared from sight and then reappeared to show right on path. The Captain and I were left scratching our heads. I asked for everything to be re-engaged as well as VNAV. The airplane was happy now and performed flawlessly all the way into ZZZZ. ZZZZ Center said nothing and didn't seem to even notice. The Captain wrote up the overspeed and we alerted station ops that there was a maintenance write up upon arrival. My only conclusion is that the airplane's VNAV got disoriented or confused for a bit. It's always possible for human error; but I can't put my finger on a mistake that was made that would've told the airplane to do that.
B737 First Officer reported that the autopilot made an uncommanded descent while on approach that resulted in an overspeed.