B737 Undifferentiated or Other Model
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 176; Flight Crew Total 4916; Flight Crew Type 4916
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Took Evasive Action
Weather
At cruise we were informed of moderate turbulence and light to moderate mountain wave at various altitudes about 10 minutes ahead. Severe turbulence was reported at FL410. At our altitude the turbulence was reported as light to moderate. We stayed at our altitude due to the fact that all altitudes above FL310 were reporting some turbulence and I didn't want to take the fuel hit to descend to FL310 or below. We expected nothing worse than light to moderate turbulence and some mountain wave. We had the flight attendants seated with seat belt signs on and made 2 separate announcements warning the passengers of the possible impending turbulence. We were at MACH .78. We experienced moderate to severe mountain wave coupled with severe turbulence. We lost roughly 700 feet and gained a plus 400 feet during the wave. We experienced stick shaker as the autopilot attempted to recover the altitude before I clicked it off and unloaded to approximately 0 to -.5 gs. Speed got down to approximately 240 kts. Even with the unload during the climb back up; we went as high as FL385 before descending back to FL380. During this the Co-pilot informed ATC of our altitude deviations and asked for a decent to FL340. After the turbulence and wave; we initially leveled off at FL380 then descended to FL340 after confirming our new clearance to FL340. This all lasted about 2 minutes after which the flight was unremarkable.
We were in cruise at FL380 in VNAV PTH and FMC SPEED with the speed intervention window open and MACH 0.78 selected in smooth air with the seatbelt sign off and had just been switched off to a new Salt Lake Center frequency. On check in; we were advised of a report of moderate turbulence and moderate mountain wave with +/- 10-15 kts and +/- 300 ft approximately 30 miles west of GTF. At that time; the location of the turbulence was about 15-20 minutes ahead of us. As we continued along; I heard several more air carrier aircraft in that area report moderate turbulence and mountain wave. All of those aircraft descended to FL320 for better rides where the reports were light chop and no mountain wave. I opened WSI and added the PIREP layer and confirmed those reports through the displayed PIREPS. Additionally; there was a PIREP from a Falcon 900 in the same vicinity as the air carriers reports at FL410 that showed severe turbulence. I suggested to the Captain maybe we should descend to a lower altitude for better rides. He asked what the rides were ahead and I then asked ATC for expected rides ahead. They gave us about 4-6 PIREPs of moderate turbulence and mountain wave and indicated everyone had gone down to FL320 and were experiencing only light chop down there. I again asked if he wanted to go down. He said it would take a lot of fuel; but I pointed out that we were only about 1:20 from landing and it would not make a big difference. I then plugged in a step to FL320 in the FMC and it estimated a 500 pound penalty to go down. The Captain said he was 'Up for an adventure.'  Based on that; I did not feel we were going to descend. I suggested we proactively seat the flight attendants and the passengers. The Captain turned on the seat belt sign and made a PA indicating the impending turbulence and that we would be seating the flight attendants as well. About 5 minutes later; I heard the first class lavatory door close; so I told the Captain I was just going to make a quick PA to reinforce the seatbelt sign. After that I called the flight attendants on the interphone to make sure they were seated.Approximately 2 minutes after that PA we encountered moderate turbulence quickly followed by a 25 kt loss of airspeed (or more) and severe turbulence. The airspeed had decayed into the amber low speed band and I said 'We have to descend'. The Captain increased thrust; but it was insufficient. The airspeed decayed and the stick shaker activated. At that point; the autopilot disconnected. At that point I remember looking at the airspeed tape which was entirely black and red; the high and low speed barber poles had met. We were at 38;000 feet and 10 degrees nose up; climbing. I said 'Get the nose down' and I began to bring my hands up to push forward on the control column. At that time the Captain had begun to bring the nose down. We reached approximately 38;600 ft. at the highest point. I told ATC 'We are descending and need a lower altitude'. They cleared us to descend to FL350. We reported the severe turbulence and ATC asked if we had any injuries. During the descent and recovery from the upset; still in manual flight; the Captain called the flight attendants to inquire about injuries to report to ATC. I told him to let me call them; but he said it's fine. I then said to give me the airplane; but he continued to hand fly while talking to the flight attendants. The aircraft was recovered from the upset at about 37;000 ft and we leveled off at FL350 where we continued uneventfully to Seattle. There were no injuries to any flight attendants or passengers.There were several areas where this could have been avoided. First; I feel I could have and should have been more assertive in the need to descend. I was reluctant to continue pushing after having suggested it twice with no success; but I should have continued to press. Although I do not recall whether the exact PA commands were used to seat the flight attendants; the communication wasexcellent and they were all safely seated. I am very thankful there were no injuries. There was an FPG for light to moderate turbulence in the area; but that was from FL320-FL360. We were above that; but based on PIREPS; we should have planned better to avoid the area we flew through. There were issues in the recovery; the autopilot should have been disconnected as soon as we got into the amber speed band; but it happened so quickly; I'm not sure it would have made a significant difference. Also as discussed above the workload during the recovery and immediately after could have been better divided between us. One item that was an unexpected threat was the lack of a latched hot mic position for the crew interphone. Both the Captain and I use a headset and communicate on the interphone using a rubber band. After the autopilot disconnected and the recovery was begun; I did not want to use the control wheel mic to talk to ATC for fear of interfering with the recovery. As a result; I used the switch on the audio control panel to transmit. I think the rubber band slipped off the switch when I transmitted to ATC; so when I returned to talk to the Captain; I had to look for the switch and secure it with a rubber band. If that switch latched in the interphone position; I could have just slapped it into that spot after ATC communications. Also with the noise of the stick shaker; the interphone was very helpful in our ability to communicate over the noise. If there was ever a justification for installing that new panel with the latched position; this is it.
B737 flight crew reported severe turbulence during cruise.