Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Altitude Overshoot; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action
Aircraft
After initiating descent in 'VNAV' using Descend Now function; out of FL390 the aircraft began to accelerate passing FL340. Speed was pulled to flight detent stopping acceleration at Mach .80 momentarily. The aircraft then rapidly accelerated to Mach .83 triggering the clacker. I disconnected autopilot and pitched to slow aircraft. Mach .78/290 [kts.] was pre-selected in descent profile. Descent winds were pre-programmed with a healthy tailwind margin. At time of the overspeed there was less than a ten knot tailwind component. This is a 737MAX aircraft. At the time of initial acceleration we were in VNAV; on speed; and on profile. At the time of overspeed we actually dipped below glide path before I disconnected the autopilot. Unknown cause. Suggest trend analysis and software evaluation to solve aircraft prioritizing glide path over airspeed limitations in VNAV descent mode.
B737 MAX pilot reported speed deviations during autopilot-controlled descent.