Date: 202002
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Workload
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Speed All Types
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC Overrode Automation; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: VNAV/Autopilot uses aggressive pitch changes to chase the VNAV profile and disregards airspeed target at top of descent.  Aircraft would overspeed if not for pilot intervention.  This occurs at (nearly) EVERY top of descent on seemingly EVERY flight.  In this case; we had a strong jet-stream tailwind component.  Descent winds were programmed and accurate.This is not a one-time event.  Every pilot experiences this and everyone complains about it.  We need to use speed brakes for drag at nearly every top of descent.  We often need to kick off the autopilot and/or switch to 'CWS' for a smoother transition - while pulling back to induce drag; break the rate of descent and reduce indicated airspeed/Mach.  This poor-quality system causes distraction(s) and increases workload.  It creates a poor quality 'ride' hence is bad for customer experience and provides substandard hospitality.  This is a hazard to flight attendant safety.  This all occurs nearly simultaneously with the increased workload associated with changing altitudes; ATC radio handoffs and traffic callouts/TCAS events.  More often than not; the auto-throttles simultaneously kick in and advance during steep descent; necessitating further pilot intervention to override this obvious unnecessary and distracting action. Sometimes; the FMC scratch-pad message 'drag required' will present itself - even as the autothrottles are advancing and the autopilot is dropping the nose!This cycle repeats itself as the aircraft transitions altitudes and experiences changing atmospherics; most obviously changing wind speed and/or direction.  The bewilderment at the automation is a regular passenger in the cockpit.In summary; 'what (the xxxx) is it doing?' is not just a statement of startled bewilderment; it is evidence of a gross failure of the VNAV/autopilot/auto-throttle system.  Preventive Measures: ['Use ALT INTV or descend now and begin an early; hopefully smooth interception of the descent profile.  Reprogram the VNAV.  Hire better engineers.  Update the FMC database.  Perform a systems analysis to discover the fault(s).  Redesign the Speed-Pitch-Autothrottle (VNAV/Auto-Pilot/Auto-throttle) System for better functionality.  Can we increase the size of the descent wind database? Teach the auto-throttles to be more aggressive and retard in anticipation of a nose down with increasing airspeed.  Introduce anticipation circuitry into the control system.  Can we get a little A.I. and put some learning-software on the jets? Analyze speed brake utilization data to get factual data of wasted fuel (cost impact).  Compare to other/better automation.  Improve descent atmospheric database.  Make the automation work better.




Synopsis: B737-700 Captain reported that the VNAV/Autopilot system used aggressive pitch changes to follow the VNAV profile; while disregarding the target airspeed at top of descent. The Captain stated multiple issues result from this and offered recommendations for addressing them.