Date: 202108
Local time of day: 1801-2400
Location: ZLA.ARTCC, CA
Flight conditions: VMC
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZLA
ATC / advisory: Center ZLA
Second aircraft: B787 Dreamliner Undifferentiated or Other Model
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Wake Vortex Encounter; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Regained Aircraft Control
Contributing factors: Environment - Non Weather Related; Procedure
Primary problem: Ambiguous

Narrative: Upset/wake turbulence event. Approximately five minutes prior to this event First Officer and I had completed our arrival brief and I had given a PA to the passengers that the seat belt sign would be coming on in the next 10 minutes. Ahead of us was a B787 at [our altitude]; FL360. [The B787] was directed by ATC to descend and maintain FL340. Within a few minutes we were assigned the same clearance. Shortly after leveling at FL340; our aircraft entered a rapid; shuddering turbulence; followed by a rapid roll to the right. We both immediately put our hands on the controls and recited the upset recovery call outs.  We disconnected the autopilot; though the autopilot may have disconnected by itself; and autothrottle. We both fly with our FPV (Flight Path Vector) up so we could see immediately; intuitively that we weren't stalled; but our push was slight; not even half a G; which added to our effective counter roll rate. Prior to our response to the upset; the autopilot had already put in aileron control forces counter to the roll; but the aircraft was still rolling to the right. As we took control; the aircraft reached approximately 40 degrees before the aircraft responded to our control inputs; then rolled left to approximately 20 degrees. Based on the fact we were at FL340 we left the thrust where it was positioned; possibly a slight reduction; as the energy state was good and a large thrust change was not necessary. Our altitude loss was minimal and the descent was very shallow. The recovery point at which we managed our flight path divergence started at approximately FL337; only a 300 FT loss.  Rudder may have been used - Indicated by both an overshoot; subsequent bank to the left and a report from the flight attendants that we not only rolled; but yawed. We advised ATC that we had encountered wake turbulence which had caused our descent. They offered us direct HLYWD moving us from the wake of the preceding aircraft. We then checked on the Flight Attendants and passengers to ensure there were no injuries. The seat belt sign had been off and there was a passenger in the lav; but no injuries were reported. The First Officer and I debriefed the event but; because ATC started our descent onto the Arrival; I failed to make a PA to the passengers.  After landing; I waited at the flight deck door to thank the passengers. I saw that the event was more impactful than I had realized and a PA would have gone a long way to reassure the passengers and Flight Attendants.

Callback: Reporter stated he was surprised at the strength of the wake when he was 8 to 9 miles in trail of the B787.



Synopsis: B737-800 Captain reported an upset at FL340 descending into LAX in trail of a B787.