B777 Undifferentiated or Other Model
Flight Crew Last 90 Days 240; Flight Crew Total 12000; Flight Crew Type 4000
Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Exited Penetrated Airspace; Flight Crew Took Evasive Action; General Maintenance Action
Weather
During cruise at the noted location we encountered moderate plus mountain wave turbulence. We seated the passengers. We were initially in the clouds and we made a PIREP of loss of 15 KTS airspeed. A few minutes later we exited the clouds and were on top of all clouds with clear day VMC conditions. We noted some rotor cloud activity ahead at our 10 o'clock position. It was initially smooth with winds around 30 KTS. We then encountered a series of strong mountain wave; winds aloft went up to 80 KTS and our airspeed gained 20 KTS (target + 20); then suddenly lost 50 KTS (target -30). We were within a couple of KTS of the yellow approach to stall speed and the altitude went up and down 300 FT. I believed we were very close to a potential stall and were ready to declare an emergency and [descend to a] lower altitude. After a couple of minutes it smoothed out. We [had come] within a few KTS of both overspeed and approach to stall speeds; however; no limits were reached. We reported this info to ATC; Dispatch; and Maintenance. Maintenance misinterpreted our call and wrote up an overspeed condition (VMO+20 KTS) which was not the case. We were only target speed (.82 MACH + 20 KTS).
15 to 20 NM in front of the aircraft what looked to be a very large horizontal rotating core of wind and clouds; more than 40 NM in length; with shear separation occurring both above and below the core. Aircraft settled to just above the stall buffet; full max thrust being applied by myself; for 45 to 60 seconds; followed again by several more speed excursions. [The] cabin [was] checked for injuries and damage. None noted. Worst wave I have seen in 30 plus years.
A B777 flight crew reported significant airspeed and altitude variation as a result of weather and windshear.