B737-800
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Total 24000
Situational Awareness; Workload; Human-Machine Interface
Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Took Evasive Action; Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control
Aircraft; Human Factors; Weather
We were at FL340; approaching an area where there was convective activity and moderate turbulence had been reported. I advised the flight attendants and passengers of the possibility of turbulence; the seat belt sign would be on; and to remain seated. We elected to climb to FL360 to try to top all of the clouds. The winds aloft were out of the west at 120 kts; we were downwind; but we were more than 20 miles from any precipitation. Maneuver margin was still adequate. I was in the green.As we crossed the area; we were still in the cloud tops and we encountered the moderate turbulence. I immediately realized my first mistake. We were still too close to the weather with the winds aloft; and I told the PF (Pilot Flying) to turn right while I coordinated with ATC. He did so and the autopilot went from LNAV to CWS (Control Wheel Steering) roll. I was now in the yellow.We were still in the bumps; and the airplane started a roll to the left. I waited a second for the PF to correct; and when it didn't happen; I told the PF to roll wings level. Rather than roll wings level; we rolled more left. Bank angle alerted; we were rolling towards 45 degrees; the red/black low speed zipper came up; and the stick shaker went off. I was now way beyond red.I ordered 'Roll Right; Roll Right; Power; Power!' He rolled to the right and we regained wings level; reduced the power so we wouldn't overspeed; engaged heading select; and shortly thereafter exited the bumps. There was no altitude deviation. I checked with the flight attendants and they and the passengers were OK. I apologized profusely. We advised ATC and Dispatch of the moderate turbulence.I then set out to find out why we hadn't immediately corrected the roll. The PF was under the false impression that when the autopilot reverted to CWS it would correct back to wings level. Small bank yes; but not this. He was waiting for the autopilot to correct. I cannot answer as to why we initially rolled the wrong way during the correction. I made sure he understood how we had screwed this up and how close we came to stalling this airplane! This was not a minor event. I cannot begin to express the disappointment I have in myself. I would like to think I am a professional. I was not. There are steps I should have taken to prevent what happened. First; I should have went further away from the convective activity. Being downwind with a 120 kt wind aloft; near the cloud tops was not the place to be. Had I done that; none of this would have happened. I should have reviewed the moderate turbulence procedures with the PF. The autopilot can be on; but you still have to fly the airplane. You cannot wait for the autopilot to make the corrections. At the first instance I realized the PF was having difficulty; I should have taken the airplane. This was not the time to coach a recovery.I contemplated whether this was moderate or severe turbulence. The TAPS (Turbulence Auto-PIREP System) event listed as moderate. I reviewed the intensity levels; and decided that positive control of the aircraft was maintainable; but we failed to do so.
We were in cruise flight at FL360 in IMC and a speed of Mach .77 at the time of the event and noticed a very small area of green precipitation displayed ahead of us on the onboard weather radar. As we approached the small area of precipitation we experienced the onset of continuous light turbulence; and decided to deviate to the right. Since we were IMC we only had the radar to rely on for weather avoidance; but we assume that we flew through the cloud tops of that area of weather resulting in approximately 30 seconds of moderate turbulence. This caused airspeed fluctuations; initially an increase; then a decrease in indicated airspeed. It also caused the aircraft to roll to the right. The stall shaker activated for a split second during the bank to the right. The autopilot reverted to Control Wheel Steering Roll (CWS R) and I rolled the aircraft manually back to the left. I manually controlled the thrust levers to maintain airspeed during the turbulence event. I manually controlled aircraft roll during the event to try to maintain wings level; then re-engaged the HDG SEL mode of the autopilot once the event was over. I do not believe any engine limitations were exceeded nor did we overspeed the aircraft during the event. The area of embedded weather could not be identified visually while flying in IMC; and the radar only showed a very small area of green intensity precipitation due to the auto-tilt of the radar. Our heading change and cruise altitude were not enough to avoid the embedded and hidden cloud tops of the weather. The quick onset of the moderate turbulence was unexpected; as were the airspeed and bank angle changes. The bank angle at high cruise altitude combined with the fluctuating airspeed resulted in the momentary activation of the stall shaker that I would characterize as a 'very quick buzz' lasting less than a second.
B737-800 Flight Crew reported a momentary stick shaker event at FL340 while crossing through an area of convection.