Date: 201902
Location: ZZZ.ARTCC, US
Flight conditions: Mixed
Weather elements / visibility: Cloudy; Turbulence
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Contributing factors: Weather
Primary problem: Weather

Narrative: In the vicinity of ZZZ VOR; ATC advised us that there was moderate precipitation and turbulence ahead; and recommended that we climb to FL340. During the climb; another pilot mentioned that the turbulence was worse at FL340. ATC asked us if we wanted to level at FL320. We did. After a few minutes; and a frequency change; the turbulence was getting much worse. We asked ATC if there were any ride reports up ahead. They said that FL340 only reported light turbulence. We climbed to FL340. After another few minutes the turbulence got much worse; and we noticed the following deviations: Airspeed from M.71 to M.79 Altitude +/- 400 feet; and bank angles of 45 degrees. Further; the autopilot disconnected LNAV; and reverted to CWS (Control Wheel Steering). I slowed to LRC (Long Range Cruise) which put us halfway between low speed warning and high speed warning on the airspeed indicator. We asked for; and received a block altitude of FL330 to FL350 and explained that we were unable to maintain altitude. The cloud deck lowered (we had been IMC at first;) and the turbulence subsided. ATC asked us how the turbulence was; and we reported that it was moderate and occasionally severe. We wrote the encounter up in the logbook; and asked Dispatch if there was anything else we needed to do or if we should file a written report. They asked if we were okay; and told us that the logbook write up should be adequate. We continued on to [destination] without further incident.


Second narrative: While in cruise flight we had been operating in moderate turbulence for most of the flight. Somewhere in the vicinity of ZZZ while at FL340 the intensity of the turbulence increased sharply. The plane lost about 400 feet of altitude. While the flying pilot was working to return the plane to cruise I requested a block altitude and it was approved. The plane instantly went from straight and level flight to almost a 45 degree left turn. The flying pilot returned to straight and level flight only to have the plane instantly go back to a 45 degree bank turn. Once the wings were level again the turbulence quickly resided and transitioned to smooth flight with Center. We were IMC during this event. The other altitude deviations from FL340 were smaller than the initial loss but were inside our assigned block altitude. The winds were from the SW at 150+ knots. There was no noted severe turbulence in the route of flight that morning. The event took place very quickly and was over also very quickly.


Synopsis: 737 flight crew reported experiencing severe turbulence for a brief period of time.