Air carrier flight crew reported an inflight upset resulted from an encounter with severe turbulence in the vicinity of LWL VOR.
Synopsis
Air carrier flight crew reported an inflight upset resulted from an encounter with severe turbulence in the vicinity of LWL VOR.
Narrative
Encountered severe turbulence at FL400 enroute from ZZZ-SMF. We had battled wake turbulence since crossing the Rockies (+/- 10 knots; +/- 100 ft) but turbulence was mostly light chop. ATC reported turbulence worse lower unless we could get below FL290. It was mostly smooth leading up to the event; but it was also in close proximity to a north-south jet stream. Near LWL VOR we experienced what felt like the start of mountain wave but then progressed rapidly to severe turbulence for maybe 4-8 seconds. I pulled the throttles back slightly and then deployed the speed brakes to prevent an overspeed. The aircraft then fell and turned causing the autopilot to drop into CWS. The horizontal component of the flight director went out; but the vertical guidance remained. I completed the recovery in CWS and called for heading select to restore the flight director and return to course. The Captain was unable to get heading select to engage. I then called for LNAV and it successfully engaged. Total altitude loss was less than 300 ft and no aircraft were in the vicinity. Bank was not severe and less than 30 degrees throughout. I don't recall the position of the seatbelt sign as the Captain was needing to jockey it since the Rockies (I thought I heard the seatbelt ding at the start of the event; but there was definitely some startle effect). The FAs had just been released after a good period of relative calm. Weather data looked good for the area and ATC said to expect only light chop. No injuries were reported.
Second reporter narrative
Flying along at cruise altitude mostly smooth rides dealing with small mountain waves. Then out of the blue we encounter severe turbulence. We did not overspeed the aircraft but the autopilot kicked off and we lost about 300ft of altitude and came 30 degrees off course. Autopilot and flight directors would not come back at first after about 20 seconds we were able to get everything re-engaged Event lasted 6-10 seconds. Seat belt sign was on but flight attendants were up. No injuries reported in the back.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.