Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 180; Flight Crew Total 14000; Flight Crew Type 3000
Workload; Distraction
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence
Flight Crew Regained Aircraft Control
Weather
Aircraft was being flown by the First Officer (FO) with the International Reserve Officer (IRO) in left seat. I; the Captain; was sleeping in the bunk. I woke up when I felt some bumps than went back to sleep. Upon return to the Flight Deck the crew informed me of the Moderate Turbulence and 10 seconds of Severe Turbulence. They considered the Turbulence Sever due to the abrupt change in altitude. A drop of 100 feet than a climb of 400 feet which resulted in a Caution and Altitude Alert due to the plane climbing to 38;300 feet. The autopilot remained engaged and the aircraft returned to 38;000 within seconds. The crew informed Fukuoka Control which begin advising other traffic. Dispatch was notified and an entry was made in the logbook. I followed up with maintenance on the ground in HKG.
[Report narrative contained no additional information.]
Air carrier flight crew reported experiencing severe turbulence which caused momentary excursions from their assigned altitude.