A B777 flight crew reported significant airspeed and altitude variation as a result of weather and w...

Attributes
ACN	1165657
Time
Date	201404
Local Time Of Day	0601-1200
Place
Locale Reference	ZLC.ARTCC
State Reference	UT
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B777 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Person 1
Function	First Officer Pilot Not Flying
Experience	Flight Crew Last 90 Days 240 Flight Crew Total 12000 Flight Crew Type 4000
Person 2
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Speed All Types Inflight Event / Encounter Weather / Turbulence

Narrative:

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).

Original NASA ASRS Text

Title: A B777 flight crew reported significant airspeed and altitude variation as a result of weather and windshear.

Narrative: 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).

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.