B757-200
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 43; Flight Crew Total 10500; Flight Crew Type 1500
Situational Awareness
Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC Overrode Automation
Weather; Human Factors
Our clearance was to cross 40 east of ZZZ at 15000 ft. Autopilot was engaged; VNAV; on path; speed 325kts. Our radar indicated no precipitation before entering clouds at 22;000 ft. Upon entering the clouds the airspeed began to rise; so I extended speed brakes; airspeed slowed to below the commanded speed and the autothrottles increased to maintain this speed; at which point I stowed the speedbrakes; causing the A/S to increase rapidly. At this point I engaged V/S and airspeed continued to rise. I then disengaged the autopilot and initiated a climb to reduce the speed; but not before we had exceeded VMO/MMO by 25-30 KTS. With convective activity in the area I should have chosen a lower airspeed for our descent. All the weather that we were painting on our radar was 5-10 miles west; leading me to believe that we were OK with the chosen speed. I was unprepared for the speed in which this situation developed (15-20 Sec). After reflection on the event; I believe I should have chosen flight level change instead of V/S or better yet disengage the autopilot altogether. I faced a situation where the autothrottles were commanding more thrust when what I needed was idle thrust.
B757-200 Captain reported that during descent with the autopilot engaged the aircraft exceeded Vmo (maximum operating limit speed).