B757-200
Flight Crew Air Transport Pilot (ATP)
Fatigue
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control; Flight Crew Took Evasive Action
Human Factors; Weather
It was a dark and stormy night.  I was getting ready to begin my break after an hour and a half of flying in continuous light; occasional moderate chop and deviating around thunderstorms. At FL350; the ride smoothed out and I set the cruise Mach in the FMS to .80.  Waiting for the relief pilot to return from break; we began to pick up light to moderate chop and I selected M.78 in the auto throttle window.  The airspeed decrease was minimal as I fought the auto throttles to retard; so I clicked them off until M.78 was achieved; put .78 in the FMS cruise page; and selected VNAV on the Mode Control Panel. I then got out of my seat to let the relief pilot into the cockpit and exchanged seats. I passed a meal tray to him then exited. I walked to the aft galley to check on the Flight Attendants and passengers.  Most were asleep or on break.  I entered the aft lav to relieve myself; when the toilet flapper valve began to vibrate.  Thirty seconds later; moderate buffeting began.  A call to the cockpit from the aft galley went unanswered; so I walked to the cockpit and called for entrance.  The Relief Pilot (FB) opened the door and I observed we were in a climb at .74 Mach passing 30;500 feet. I returned to the seat for the remainder of the flight. We advised ATC of the altitude loss due to turbulence; and returned to FL350. Discussion of the event over the next day occurred among the three of us as to the likely cause.  A. The auto throttles were not engaged.  When I slowed down for the turbulence just before the FB was to relieve me; I clicked the auto throttles off; but did not verify they re-engaged in Speed or VNAV mode. B. Distraction from the seat swap; food tray; ATC radio chatter (loud squeal).C. Late night 'fatigue' compounded by 90 minutes of flight in Lt to Mod chop and Thunderstorm deviations.  I was tired and the relief FB had no rest during his break with the turbulence. When the vibrations started; I thought it was due to another aircraft's wake vortices.  The FO thought it was a Mach over speed buffet or engine vibration.  The moderate vibrations during the few minutes of buffeting; made reading the panel instruments very difficult.  The noise from the autopilot disconnect warning was adding to the stress.  Stick shaker was noticed by one pilot; but not the other. Aircraft recovery was smooth.  During my walk from the back of the aircraft to the cockpit; I saw no wing rock and no sensation of descent.  The vibrations did wake some passengers; and trays of glasses were broken in the forward galley. The mantra; Maintain aircraft control; analyze the situation; and take appropriate action is paramount. Mach recovery is paramount at high altitude recovery. When exchanging seats or aircraft control; verify you have the automation doing what you think you have it doing.  The Flight Mode Annunciators make that a good check.  Auto throttles are not needed on many aircraft; so avoid distractions in keeping a timely scan on speed and altitude.
I returned to the cockpit after taking the first break. The Captain met me at the door; and I assumed the position in the left seat as the flying pilot. I was briefed on current flight conditions; altitude; and the fact we were maintaining FL350 due to company traffic ahead and 2000 feet above. (I made visual contact). The Captain handed me my crew meal and left the cockpit to take the second break. Approximately 5 minutes later; the First Officer commented about an abnormal vibration. The vibration went from light; increasing to moderate; to the point that the autopilot disconnected. The First Officer assumed control of airplane. We scanned the flight instruments to ascertain the problem; but because of continuous buffeting it made it impossible. We quickly realized our speed was slow. We maintained wings level; lowered the nose; and made a smooth recovery. Our altitude went from FL350 to approximately FL290. After full recovery; we began a climb back to FL350; contacted Center; and notified them of our situation. I then made contact with the Captain on the interphone; he re-entered the cockpit; and remained there the rest of the flight. Late night flying in continuous turbulence can be very fatiguing; so adequate rest facilities for quiet; uninterrupted rest is ideal.
B757 flight crew describes the circumstances surrounding a stick shaker incident at FL350 and loss of 6;000 feet before recovery. The autothrottles had been turned off just prior to a crew change and not reengaged.