Date: 201702
Local time of day: 1201-1800
Location: ZDV.ARTCC, CO
Flight conditions: VMC
Weather elements / visibility: Windshear; Turbulence
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZDV
ATC / advisory: Center ZDV
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Landed As Precaution; Flight Crew Regained Aircraft Control; Flight Crew Took Evasive Action; Flight Crew Diverted; General Maintenance Action
Contributing factors: ATC Equipment / Nav Facility / Buildings; Weather
Primary problem: Weather

Narrative: Approximately 40 miles southwest of PUB at FL370 we encountered moderate mountain wave activity followed very shortly by severe turbulence. This encounter lasted for approximately 10 minutes with the severe turbulence lasting about 3 to 5 minutes. In my over 20;000 hours and 40 years of flying experience I have never encountered turbulence of this severity lasting for such an extended period of time. In cruise flight I attempted to use WSI but was unable to achieve a WIFI connection. Therefore I relied on my preflight data and our flight plan; which showed the worst ride conditions to be approximately the Grand Canyon area. During preflight a conversation with the inbound crew indicated this was the area they encountered the worst rides. In the vicinity of Durango; CO; we began to hear aircraft reporting mountain wave activity and some moderate turbulence over the Rockies. I turned on the seat belt sign and instructed the F/A's via intercom to clean up the cabin and take their jump seats. We also started to pick up light mountain wave and light to moderate chop as we proceeded east. During this time we were watching a 125-knot tail wind slowly bleed off to a 54 knot quartering tailwind. The turbulence and chop seemed to increase but the mountain wave seemed to settle down a bit. The wind bleed off reminded me of an experience I had in the same vicinity when I had less than 100 hours on the 737. That time wind shift had led to an aircraft over speed. So this time we used the opportunity to talk about 737 throttle response and spool up time. To back up my earlier instructions and make an impression on the passengers to stay in their seats; I made a PA instruction the F/A's to take their jump seats. I followed this up with an intercom conversation with the F/A's checking to make sure they were seated; the cabin was secure; and telling them how long I thought these conditions might last. At some point during this time we inquired from ATC about the rides lower as the mountain wave continued. We were told that the rides were about the same at all altitudes. Having hit a fairly smooth area; we decided to stay at FL370. The mountain wave activity started to pick up in intensity. I attempted to contact ATC for clearance to a lower altitude; but the frequency was blocked with other aircraft enquiring about or reporting ride conditions. Finally with the airspeed bleeding off in mountain wave; without clearance; I instructed the F/O to initiate a turn 30 degrees right to avoid lower TCAS traffic ahead of us and to begin a descent to FL350. The turn was never completed as we flew into the area of severe turbulence. Auto throttles and autopilot both disengaged as the aircraft became uncontrollable. I took control of the throttles as the F/O struggled to maintain a wings level attitude and some sort of reasonable descent. I was finally able to [advise ATC]; state we were leaving altitude; for severe turbulence. ATC cleared us to descend to FL290. At times during the event the aircraft was virtually uncontrollable with uncommanded bank angles reaching 30 degrees or greater; which the F/O stated he used full control deflection at times to return the wings to level attitude. Airspeed fluctuations of plus and minus 20 knots or greater were on going and at one time we experienced a brief stick shaker. The severe turbulence encounters continued frequently; becoming less frequent as we descended. The moderate to severe turbulence finally ended at about FL295 and was a much more comfortable moderate chop at FL290. ATC cleared us further down to FL270 where the ride was reasonably comfortable but still choppy. We spent 2 to 3 minutes reengaging the automation and getting settled back on course. We then assessed the aircraft condition from the flight deck finding it to appear normal. A call was made to the F/A's checking if they were okay; enquiring about the condition of the aircraft and the passengers. I instructed the F/A's to walk throughthe cabin and check on the aircraft and our passengers. They reported back that the aircraft was a mess; but appeared structurally sound. None of the passengers were reported as injured; however several had become sick and vomited and concern was expressed for two pregnant passengers.We were over southeast Colorado and 800+ miles from our destination. I told the F/O I wanted to divert to ZZZ. He expressed valid concern about flying back into the vicinity of the turbulence. I felt we were below the worst of it and other aircraft were departing and landing at ZZZ. We informed ATC we wanted to divert to ZZZ. They cleared us direct. I called the F/A's on the interphone; once again and checked the condition of the passengers and aircraft. I informed them of our diversion; time to landing and my intention to have CFR meet the aircraft. I explained that I anticipated a normal landing; that we would stop the aircraft and have the rescue vehicles check it's condition prior to proceeding to the gate and that I would use the 'remain seated' PA. I followed this briefing up with a similar briefing to the passengers. The QRH was consulted for severe turbulence encounters finding that the only checklist was applicable only during the actual event. The Electronic Engine Controls (EEC) had also disengaged and the QRH procedures only applied on the ground.A normal landing was made. We cleared on the high speed taxiway and stopped the aircraft. The 'remain seated' PA was made. The CFR checked the aircraft visually finding no abnormalities. We taxied to the gate escorted by the CFR. During taxi ATC passed us a phone number to call after arrival. EMT's; fire personnel and many company staff met us at the gate. During post flight we completed our checklists and pulled the Digital Flight Data Recorder and Cockpit Voice Recorder circuit breakers. I than went to the main cabin to check on the condition of the aircraft and passengers and meet the emergency personnel as they entered the aircraft. Log entries were made for a severe turbulence encounter; EEC trip off and pulling the breakers.This event occurred in clear air with little real warning. Frequency congestion contributed to our inability to request a timely descent; but we probably would have encountered severe turbulence even if we had descended to FL350 earlier. A working WSI system available in the aircraft would have been beneficial but would not necessarily have prevented the encounter. Provide a working WSI platform with an own ship position indicator such as on the Jeppesen FD-Pro airport page. An enhanced system to send via dispatch or automatically updates to aircraft enroute.

Callback: After the event the reporter thought that the turbulence may have been beyond severe; but Maintenance found no discrepancies and released the aircraft for flight with a different crew.

Second narrative: We were filed at 370 and that is the altitude we flew the flight. The flight was mostly smooth until approximately 170 miles southwest of ZZZ. We encountered a couple pockets of light turbulence and the Captain asked ATC for ride reports ahead at lower altitudes. ATC advised us that there were reports at all altitudes of light; with some pockets of moderate chop; and some reports of light mountain wave. At that point the Captain asked the flight attendants to take their seats. At the onset of turbulence I immediately slowed the jet to .76 mach. The Captain and I agreed that the ride at 370 was just as good as a lower altitude. ATC informed us that east of ZZZ the rides improved to light chop. As we continued east the Captain and I were commenting about the wind changes. We had 100 knots on the tail; then 120 knots; it was up and down. There was nothing abnormal at this point in the flight. Just prior to the severe turbulence encounter; the windspeed began to decrease to 85 knots and was trending down. I also noticed that the direction of the wind was changing rapidly. The wind was shifting from westerly to southerly at approximately 60 knots when we encountered the severe turbulence. First; the autothrottles disconnected; followed by the autopilot disconnecting. I then took manual control of the jet. It became immediately apparent that the jet was becoming increasingly uncontrollable. At this point; I made a 30 degree turn to the right and began to descend. It was nearly impossible to do both keep the wings level and descend; so I was just trying to keep the jet from an upset position. The jet was violently rolling from left to right and I was using both aileron and rudder to keep the jet flying. I instructed the Captain to operate the thrust levers; as I needed both hands to try and control the jet. We continued to descend and from what I remember; gained full control of the jet around 30;000 feet. I estimate the event lasted approximately 5 minutes. Once out of the severe event; the Captain called the back to see if there were injuries; and at that point we knew the cabin crew was not injured. I slowed the jet down in the event we had any structural failure and we decided to divert to ZZZ to get the jet inspected and have the injured passengers receive medical attention. The Captain coordinated with the back and I flew the jet. We completed the severe turbulence checklist and we requested ARFF upon landing. We also informed the passengers that emergency vehicles would be present upon landing. I landed the jet; we taxied clear of runway and had the jet inspected for obvious damage. We asked to be followed to the gate as a precaution.


Synopsis: B737 flight crew reported diverting after encountering a severe mountain wave over the Southern Rockies.