B787 flight crew encountered an unreported and sudden severe mountain wave during cruise flight; resulting in an over speed condition and uncontrolled loss of altitude. The crew recovered from the speed exceedance and loss of altitude; reported the incident to ATC and maintenance; and continued the flight to destination.
Synopsis
B787 flight crew encountered an unreported and sudden severe mountain wave during cruise flight; resulting in an over speed condition and uncontrolled loss of altitude. The crew recovered from the speed exceedance and loss of altitude; reported the incident to ATC and maintenance; and continued the flight to destination.
Narrative
While in cruise at FL380 at a speed of Mach 0.86; we encountered an unreported and sudden severe mountain wave. The aircraft was on A/P and the speed increased rapidly. After retarding the throttles and extending the speed brake the plane continued to accelerate past MMO to mach 0.91 (5 to 10 kts into the overspeed band). This was followed by a rapid deceleration; I retracted the speed brakes and increased the power immediately but the plane started to loose altitude. At 37;700 feet the plane regained speed and altitude. We leveled off at 38;000 feet and reduced speed to mach 0.83 to maintain a stable flight. The mountain wave activity continued for about 10 min but never as severe as the first encounter. We reported the incident to ATC; sent a PIREP and reported the overspeed in the maintenance log. The flight continued uneventfully to ZZZ. Upon arrival; the Captain (CA) contacted maintenance to ascertain that they had received the overspeed [notification] and gave maintenance the details of the incident.
Second reporter narrative
[Report narrative contained no additional information.]
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.