Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Took Evasive Action
Weather
While cruising at 0.82 Mach at FL350 with an FMC calculated optimum altitude of FL370; aircraft experienced unforecasted severe turbulence with the autothrottles immediately becoming disconnected and the airspeed immediately accelerated to approximately 320 kts. 0.85 Mach (5 kts. over VMO Max speed). As Pilot Flying; I manually managed the throttles to avoid any further overspeeds and requested that Pilot Monitoring coordinate for a descent to FL330. This request was made and we received a 'Standby' message. [Two minutes later]; the airspeed fluctuations became more severe and the autopilot had trouble maintaining altitude. I then advised the Pilot Monitoring that I was initiating a descent to FL330 and to advise Gander that were doing so prior to receiving ATC clearance. The PM sent this message to Gander that were descending due to severe turbulence and as we were descending through FL33.5; Gander approved our previous request for descent to FL330.
The plane was in cruise at FL350; the aircraft entered abruptly into severe turbulence without warning. The auto-throttles disconnected and the airspeed accelerated into the barber pole (approximately 3-5 kts. overspeed) for approximately 2-3 seconds. Followed by an abrupt deceleration and continuous vibrations of the aircraft. Previous cruising airspeed was .82M. The PF (Pilot Flying) was manually controlling the throttles to the best of his abilities. We then asked for FL330 via CPDLC. The aircraft was becoming extremely hard to control at an appropriate airspeed and altitude. We received a Standby message. [Two minutes later]; the aircraft began to enter constant severe turbulence and the decision was made to descend immediately to FL330 for the safety of flight. I advised Gander Radio that the decent to FL330 had been initiated due to the severe turbulence. We received an approval to descend to FL300 at approximately FL350. No other deviations were noted. We called Gander Radio to issue a Pirep and contacted the Dispatcher or subsequent Pireps and reports.
MD-11 Flight Crew reported encountering severe turbulence which resulted in evasive action for safety of flight.