Military Transport
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Workload
Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Provided Assistance; Flight Crew Regained Aircraft Control; Flight Crew Requested ATC Assistance / Clarification
Weather
This flight was scheduled as four-man crew.  We decided to divide the rest up in half for this flight. I opted to take the first rest period so I could be up for the second period as I might be encountering weather enroute in the last two or three hours of the flight.  The flight was uneventful until the last two hours. The conditions were dark - no moon and no lightning associated with the cumulous clouds. I had my radar on the 40nm scale and the First Officer on the 80nm scale. We were looking at the stars and looking for any potential weather. We were cruising at FL400. We flew through the top of a cumulonimbus cloud that was not painting on the radar. We encountered moderate turbulence for about a minute and then we were back in the clear.  After flying through the top of that cloud; we saw three more off in the distance on the radar. The First Officer and I decided to climb higher and deviate around the cells.   The max altitude on the VNAV cruise page was FL434.  I opted to ask for FL420 and requested it.  The controller then came back and approved a block altitude of FL410 to FL430.  I put the mask on and climbed to FL424 hoping to clear the tops.  Next; I asked for a 20nm right deviation around the weather. The First Officer then deviated to the right of the weather splitting the two green cells by a few miles. We avoided flying through the cells; but were in the tops of the clouds as we encountered severe turbulence. The position of North 16 degrees; East 154 degrees.  We lost 25 knots airspeed with an 'airspeed low' caution message; the aircraft could not maintain altitude. We lost 400 feet and the yoke stick shaker was rattling.  I kept the autopilot engaged. The turbulence felt like it went on forever but only lasted a minute or two. The aircraft recovered with no injuries and no altitude violation.  I asked ATC to descend to a lower altitude of FL400. I also reported the severe turbulence to ATC and the company.  No emergency was declared. We continued onto ZZZ1 with no further incident.
747 Captain reported encountering severe turbulence at cruise altitude and loss of control of the aircraft.