B777 flight crew reported an over speed situation; due to severe turbulence encounter; resulted in destination diversion.
Synopsis
B777 flight crew reported an over speed situation; due to severe turbulence encounter; resulted in destination diversion.
Narrative
Due to encountering moderate to severe turbulence enroute to ZZZZ with a resulting overspeed event requiring an inspection; the operational decision was made to divert to RKSI. While keeping the safety of flight as the primary intent; it became evident that the diversion to RKSI was in the best overall interest considering: the amount of time the aircraft would need to be grounded for a maintenance inspection; crew duty limitations; fuel and weather factors; and our customer's needs. The Chinese ATC was very helpful and accommodating with our diversion request.While operating as the relief crew in cruise flight at FL351 (10;700 meters) over China we encountered moderate to severe clear air turbulence. Airspeed rapidly increased through the Maximum Speed indication to at least 340 kts. indicated; and the overspeed warning sounded. Crew coordination and control inputs were applied to stabilize the encounter. The airspeed remained above the Maximum Speed indication for a number of seconds before dropping down. The airspeed then dropped rapidly past the airspeed bug and the stick shaker was activated for a brief second requiring continued control inputs by the PF (Pilot Flying) to stabilize the situation. Once we cleared the turbulence (or what appeared to be a strong rogue wave of air) and stabilized the airspeed; we reported the encounter to ATC. Another aircraft approximately 5 minutes ahead of us at 11;300 meters reported a windshear encounter to ATC just before our report. We performed the Overspeed non-normal checklist. We then woke up the PIC (Pilot in Command) and FO (First Officer) of the flight who were in rest and briefed them of what had occurred. We then contacted Company Dispatch and informed them of the occurrence. The operational decision was made to divert to RKSI due to the reasons mentioned above. We concurred with the decision and diverted to RKSI. The remainder of the flight was uneventful. After we blocked in we made an appropriate entry into the aircraft log for an inspection.1. Be more cognitive of and place more emphasis on areas of forecasted turbulence while route planning our flights. 2. Better turbulence forecasting technology.
Second reporter narrative
In cruise flight at approximately XA00z north of OMBON waypoint in CHINA the relief crew encountered moderate clear air turbulence which rapidly increased in intensity to severe causing a violent increase in airspeed above MMO activating a red master warning overspeed message. The PF (Pilot Flying) ( relief CA (Captain)) immediately responded with decrease of thrust and use of speed brakes to reduce speed to normal regime however the clear air turbulence was severe enough to increase speed roughly 40 - 50 kts. above MMO after a few seconds the speed was reduced below MMO however the wind shear was strong enough to decrease speed to the min speed regime and a momentary stick shaker activation was felt for a brief second as the airspeed was restore to normal. During this event the PM (Pilot Monitoring) (relief FO (First Officer)) supported the PF monitoring flight condition; communicating to ATC to coordinate a change of altitude and reported large speed fluctuations and requested pilot reports if available. ATC assigned a lower altitude from 35;100 ft. to 33;100 ft. converted in meters. Several minutes later another aircraft reported wind-shear conditions with violent airspeed fluctuations in the same region roughly 2;000 ft. above our altitude. After recovery to normal flight condition the crew followed with checklists and company manuals to assure flight was safe to continue; since it was determined that a maintenance write-up had to be done the relief crew immediately informed the PIC (Pilot in Command) and FO of the situation to consult Dispatch; Maintenance and on-call duty pilot. Maintenance and Crew duty limitations being an issue; operations therefore decided that it was in the companies best interest to divert the flight to RKSI instead of continuing to our original destination of ZZZ1. At which time ATC was informed by the PM (Pilot Monitoring) to change destination to RKSI. While dispatch worked in the re-routing of flight ATC provided vectors in direction to our new destination. Flight was re-released and completed uneventfully to RKSI where a maintenance crew promptly received the aircraft. North of OMBON waypoint in CHINA the was a pocket of severe clear air turbulence with violent horizontal wind-shear producing large speed fluctuations. These conditions were experienced by other aircraft in the region from 35;100 ft. to 37;100 ft. I feel our crew did an amazing CRM job; everybody put their best attitude and effort with safety in mind and prioritized to keep the aircraft safe as well as to cooperate to the fullest with the company. However this scenario makes me aware and wanting to emphasize overspeed recovery techniques. Clear air turbulence is as hazardous as any other weather phenomena and awareness of its significance is a very valuable lesson that the pilot group can benefit from.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.