B737 flight crew reported temporary loss of control when they encountered unexpected severe turbulence.
Synopsis
B737 flight crew reported temporary loss of control when they encountered unexpected severe turbulence.
Narrative
Encountered severe turbulence in the vicinity of ZZZ about 40nm. Aircraft was traveling at flight level 370 and Mach .78. FO (First Officer) was pilot flying. Aircraft pitched up and rolled right abruptly. Airspeed immediately jumped to exceed Vmo/mmo; into red/black band and aural clacker sounded. Estimating .83. Aircrew used speed brake to reduce the airspeed and immediately requested lower. Aircraft returned to wings level and lower to FL 310 was assigned and 310 was set and verified in MCP (mode Control Panel. VNAV was slow to pitch over and start decent; FO disconnected Autopilot and manually flew to get the nose over and exit the continuing moderate to severe turbulence. Aircraft leveled at FL310 and automation was restored. Flight crew checked with FA's (flight attendants) and they reported ok; had immediately gotten seated or alter on ground and were secured by passengers. Time in severe turbulence was roughly 30-40 seconds from onset to escape.ATC reported another B737 had reported a good ride and no mod or severe turbulence experienced ahead of us along the same route of flight at 370. We requested 370 in an attempt to find a better ride than FL310 which was constant light to moderate at the time. Ride at 370 was smooth for 50-100 miles prior to onset of event.Cause: Crew could have requested a further south route to avoid forecasted area of turbulence.
Second reporter narrative
FO (First Officer) was PF; I was PM. We were sent a message from Dispatch about turbulence in the ZZZ area in which we took the time to draw the area of concern on our ND (Navigation Display) using the fix page. We were at FL 290 and got PIREPs and ATC updates about the ride ahead. We could see an area of weather at our 10-11 o'clock that coincided with our Dispatch notice of turbulence; and determined we were well south of the problem area still in clear air. There was a flight miles or so ahead of us that reported smooth at FL 350. Requested Fl350 with flight plan having us climb to FL 370 after ZZZ. Requested ride report at 370 and told good ride; so we continued to Fl370. Cruised at FL370 for 5 mins or so and then encountered severe turbulence. Seat belt sign was off because the ride up to that point had been good. Flight attendants were in the aisle with cart out serving. At encounter; immediately requested descent to FL 310. FO used automation to begin descent. Airspeed jumped into the high speed red band and according to safety software data; it reached .83 M. One of us got the speed brake out upon the overspeed as the FO was trying to use mode control panel to descend us. With speed brake we began slow descent and after a short time we got slow ( due to not using level change to descend quickly) and speed crept just into the slow yellow band. As soon as I saw it approaching yellow band I instructed FO to push it over and throttle up." The Autopilot was disengaged and FO used v/s to get the aircraft descending quicker. According to the data we got 2 knots into yellow slow band. We were out of the turbulence by FL360 in the descent and automation was re-engaged approximately by FL350. Speed brake was still back as we were in shock from the event and I eventually stowed it.The number one flight attendant was near his seat and as turbulence first hit. He made a quick PA to the passengers seated in the isle near the standing flight attendants; to hold down the flight attendants since they immediately got on the floor for protection. The passengers also held the carts down so they wouldn't hurt anyone. As the turbulence hit; I made a PA for Flt attendants "take your seats."As we descended and cleared the turbulence; the flight attendants got to their seats and stowed carts. I made a PA to the passengers about the no notice turbulence and called to assess the cabin crew condition. No injuries reported.Cause: When there are turbulence reports at a defined area; and we are flying anywhere near these areas; at a minimum the seat belt sign should be on and flight attendants notified of the possibility of turbulence. Routes should be filed to avoid these areas especially when planned for high altitude long flights.Automation is great but we needed to disconnect it sooner to get out of the turbulence. There is not a large margin for error at FL 370 and speed can degrade quickly if not descending quick enough. The biggest lesson learned for me was the shock of severe turbulence in clear air and how quickly it hit and stunned us. We were honestly in shock and it took us a few minutes to regroup. The FO has 900+ hours in the 737 and he has been with the company less than one year. I was complacent thinking he would quickly handle the descent; when I could have intervened stating my aircraft and descending quicker.Very grateful for the quick acting number 1 Flight Attendant to make his pa and have passengers help flight attendants leading to no injuries!"
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.