B747-400
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Confusion; Distraction; Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Requested ATC Assistance / Clarification; Flight Crew Regained Aircraft Control
Aircraft; Weather
During cruise; we encountered severe turbulence. This resulted in VNAV and LNAV disengaging and the aircraft unable to maintain altitude. After the loss of VNAV; the vertical mode reverted to Vertical Speed and was holding about a 1200 to 1400 foot per minute climb. We got an overspeed warning that lasted about one second while the aircraft was in this climb. The altitude deviation was about 500 to 600 feet off the assigned altitude. The automation was disengaged and we returned to the assigned altitude. ATC queried us about our altitude via CPDLC and we notified them about our turbulence encounter. As we exited the area of turbulence; automation was reengaged successfully. About 45 minutes prior to our turbulence encounter; we received SIGMET CHARLIE 1 for frequent thunderstorms. Most of the area plotted was west of our route; however; we tried to maintain situational awareness with the weather radar. Prior to our encounter; we saw no radar returns; or other visual cues of cells ahead such as lightning. Therefore the encounter came as somewhat of a shock. Upon realizing we were entering turbulence I looked up to engage the continuous ignition. In hindsight; as pilot flying I should have requested the continuous ignition and stayed on the instruments. The level of turbulence made it difficult to even read the instruments. Temporarily looking away only compounded the challenge.
Air Carrier pilot reported encountering unexpected severe turbulence which caused an uncontrolled altitude deviation.