B777-200
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Took Evasive Action
Weather
After reviewing [weather briefing program] and discussing with our Flight Dispatcher; we decided to file for FL 210 from ZZZ until east of ZZZ1; at which time we had agreed to climb to FL 390. [Program] showed moderate turbulence or worse for all altitudes from FL 210 to FL 430 along the route until east of ZZZ1.During climb out with TRACON we were informed that Center reported there was no reason to stay at FL 210 because there was only LT CHP with OCNL MDT CHP; and all flights were flying 'normal' altitudes. Based on that report we requested FL 310. We queried each controller about rides and we were told same as before. At [time] we got a report from Dispatch about MDT Turbulence in an area along our route. After plotting it in RTE 2; we determined that the area was about 25 minutes ahead of us; and that we would be thru it in about 5-10 minutes.We queried Center about ride reports and were given nothing different than before; LT CHP; OCNL MDT. I then called the Purser and inquired about the service and advised that I was turning the seat belt sign ON; and to hurry up and expect continuous Light Turbulence with possible occasional Moderate Turbulence in about 20 minutes for up to 10 minutes in duration.At approximately [25 minutes later]; we encountered severe turbulence at FL310. We immediately requested lower altitude. Altitude; attitude; or airspeed was uncontrollable; and the turbulence was so intense it was impossible to even press any buttons on the MCP. I disconnected the A/P and initiated the descent to FL 210 manually as authorized by ATC. Turbulence decreased when passing FL 290 in the descent; and shortly after I was able to reconnect the A/P.There were no injuries on board; but a lot of spilled liquids and food in the aft galley. I am not sure why the Flight Attendants in the aft part of the A/C had not been aware of the possible turbulence.We had the proper plan to fly at FL 210 until east of ZZZ1. In retrospect; I wish we had stayed with that plan. However; since both TRACON and Center insisted it was Lt CHP only; with occasional MDT CHP we felt conditions may have improved. I don't understand why Center did not have the same SIGMET we had from Dispatch. Even though we asked every new controller about the rides ahead; we actually were never sure how far ahead that controller was looking; or if he/she had any information about sectors ahead. Coordinate via SAT phone with Dispatch when these changes to the plan come up. We did inform Dispatch about our new altitude and received new flight plans from her; but a discussion on the phone instead of text messaging may have been more productive.
Filed for FL 210 after conversation with Dispatcher; and [weather briefing] analysis. On climb out with Center; controller informed us that there was no reason to level at FL210; as there were no turbulence reports; and only smooth rides ahead. We requested climb to FL310. In retrospect; asking the controller how far ahead she was looking regarding turbulence would have been appropriate. With Center; and subsequent sectors; we queried them as to rides at our altitude. We received a copy of a SIGMET from Dispatch with a report of MDT turbulence ahead (with coordinates that we plotted) between FL 250-320. We informed the FA's (Flight Attendants) 20 mins prior regarding possible turbulence; and again queried controllers regarding ride reports. The answer that at most we could expect light to mod occasional chop. [29 minutes later]; we encountered severe turbulence. We requested lower from ATC. Initially; the aircraft would not descend. CA (Captain) disconnected autopilot. Aircraft almost oversped twice; and AP could not be reconnected. The controller had issued us a clearance to FL 210; but we could not reach the knob on the MCP; the turbulence was so severe. As we descended through FL 290; turbulence abated. Perhaps more coordination between SIGMETS; and ATC; because clearly the SIGMET was correct.
Flight Crew reported encountering severe turbulence during flight.