2025-01 · NASA ASRS report 2201748
EMB-170 flight crew reported encountering turbulence and mountain wave in cruise flight that resulted in large airspeed changes; and required a descent to avoid a stall.
About 2 hours into the flight we began to experience continuous light chop at FL360. We had asked ATC for a descent to FL340; ATC said it would be a few minutes because of crossing traffic. Soon after; we experienced what seemed to be a patch of moderate turbulence followed by a drastic airspeed increase and decrease in indicated airspeed of approximately +/- 20 kts. My initial thought was mountain wave or wake turbulence. I instantly decreased power as I saw the airspeed climb close to the high end of the speed tape. Soon after the airspeed began to decrease drastically. I then added power to the aircraft and the airspeed seemed to continuously decrease. At that point I disconnected Autopilot in order to pitch down to gain airspeed before it got too slow. Doing so we vacated our assigned altitude in order to regain airspeed and stop the aircraft from getting too slow. Simultaneously I told the First Officer (FO) to let ATC know we were descending. ATC had noticed as well and had issued us a traffic alert and wrong altitude warning.Cause: The cause of this event was unexpected turbulence/possible mountain wave while at high altitude. The startle and surprise moment had caused me to react quickly to the changing airspeed indications. As the airspeed of the aircraft climbed I had reduced power. Then as the airspeed decreased I added power. When adding power wasn't changing the trajectory of the decreasing airspeed I had to disconnect the Autopilot and lower the nose/descend in order to prevent the speed from getting too low. Doing so caused us to leave our assigned altitude.Suggestions: In order to prevent this from happening I should have done more planning on the possibility of encountering turbulence/mountain wave. Whether that be looking at forecasts or asking for more pilot reports. Having a better knowledge of that could have allowed me to try to descend to a better altitude prior.
While at FL360 we started experiencing turbulence. The difference between overspeed and green dot decreased as well as a gain in around 15-20 kts. of indicated airspeed was noticed. Pilot flying corrected by reducing thrust; afterwards a reduction of 15-20 kts. indicated was experienced. Pilot flying (PF) then increased thrust; the engines took too long to spool back up; and upon realizing that; PF determined that a loss of altitude was acceptable to avoid putting the aircraft into a worse situation.Cause: Mountain wave turbulence; possibly poor decision making when reducing power by too much to decrease airspeed.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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