Date: 201204
Local time of day: 1201-1800
Location: MUHA.ARTCC, FO
Flight conditions: Marginal
Weather elements / visibility: Thunderstorm; Turbulence
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Cruise
ATC / advisory: Center MUHA
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; Captain
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Regained Aircraft Control
Contributing factors: Human Factors; Weather
Primary problem: Weather

Narrative: Level at filed cruise altitude of FL400 in constant light to intermittent moderate turbulence. There was a large storm system over South Florida; the Keys; and the eastern portions of the Gulf of Mexico. Miami Center requested we climb to FL410 for a smoother ride; we agreed as we were right in the tops and the CRZ page in the FMC indicated a max altitude of FL410. We accepted a climb to FL410. Now with Havana Center; we had a build up along our route just to the north that we estimated topped FL430-FL450. Westerly winds exceeding 100 KTS at altitude. We requested a southwesterly heading from Havana Center to keep a safe distance from this cell. Approximately 20-30 miles east of this cell (and directly downwind) we experienced very abrupt moderate chop. Immediately the speed trend vector decreased very rapidly into the min safe range/lower fishhook. As the stick shaker activated the Captain stated 'I have controls'. The airplane banked right approximately 30 degrees and back to the left approximately 20-30 degrees. I observed altitude deviation of up to +300 and -200 FT. As the now pilot not flying; I requested immediate descent to FL400 from Havana Center. Descent clearance was approved. We descended to FL400 with no further incident. The event occurred because we did not maintain an adequate buffet margin at altitude with the present weather conditions. When the Captain took control he added thrust and hand flew the airplane attempting to maintain assigned alt and heading. When operating in the weather conditions that we were; a greater margin of safely operable speed range is required. I.e. [we should have been at] a lower altitude. We also had the option to go the long way around the storm cell to the north which would have been on the upwind side yet required a deviation of a great distance. This option would have been prudent.


Second narrative: We accepted a climb to FL410 anticipating being above all of the buildups and a smooth ride. We were direct to UCL talking to Havana Control. There was a TCU 30 NM +\- 43;000 FT on our route. We requested deviation to the left (east) of the TCU with a heading of 210 degrees. Havana Control approved the deviation and stated; 'Direct to UCL when able'. The winds aloft were 270/110; remaining 20-25 NM from the TCU. As we approached abeam; downwind of the TCU; we encountered turbulence which activated the stick shaker; them aircraft rolled to the right 30-35 degrees then to the left 25-30 degrees. I took the controls. To be honest; I do not recall if the autopilot had disengaged on its own or not. I did however; disengage the auto-throttles and began the stall recovery maneuver. Adding thrust climbing 41;300 FT. The onset of the stall recovery maneuver; the First Officer requested a lower altitude of FL400. This was immediately received from Havana Control; stating 'maintain FL400; direct to NOSAT when able'. At that time I had the aircraft stable at 40;800 FT descending to the new assigned FL400. We maintained FL400 and proceeded direct to NOSAT. Flight continued with no further incident. The flight crew of 7 sustained no injuries. However one flight attendant reported hitting her chin on an armrest. Upon landing; I went back to check on her. She said; 'I'm fine'. The First Officer and I discussed how this incident could have been avoided. We concluded a lower altitude is the obvious solution; providing more of a buffet margin.


Synopsis: B737-800 flight crew at FL410 reported encountering turbulence downwind of a thunderstorm causing the aircraft to roll sharply right and left and activating the stick shaker. The Captain assumed control; adding thrust; and the First Officer requested descent to FL400.