Flight crew reported encountering severe turbulence and momentary loss of control.
Synopsis
Flight crew reported encountering severe turbulence and momentary loss of control.
Narrative
In cruise flight at 40;000 ft and 0.88 Mach talking to JAX Center approxamitely area of CHRGE intersection over the Gulf of Mexico. Center had been fielding requests from many aircraft about diverting for storms and turbulence; predominantly below us. We were a few thousand feet above a smoothish undercast layer of clouds; with a higher cloud area of blow off some 30ish miles to the south of our course. We were experiencing continuous light turbulence with occasional moderate for about 10 minutes. At one point we began to see wisps of moisture rising from the mostly smooth layer in sort of a line perpendicular to our route. We requested 43;000 ft to see if we could get up a little higher. ATC advised us that Aircraft Y; 12 miles in front of us; had just reported severe turbulence at 43;000 ft while all reports from 39;000 ft to 41;000 ft indicated only moderate turbulence. We chose to stay at 40;000 ft.As we crossed some line; the turbulence had increased significantly; and the autopilot was struggling to keep the wings level. Turbulence increased even more with the autopilot somewhat falling behind on maintaining control. I noticed at one point our altitude had dipped to as low as 39;200ft while the airspeed was dropping quickly; trending slower than 0.81 Mach. I added power; to stem the airspeed loss; while the turbulence increased. At some point the autopilot disconnected and I manually tried to just keep the plane in a wings and nose level attitude. Seconds later Pilot Monitoring (PM) noticed the airplane was climbing rapidly while the airspeed was also accelerating well into the red overspeed zone; probably as high as 0.95 or 0.96 Mach. There were several aural warning sounds going off; (autopilot; overspeed; CAS messages; etc) When the turbulence subsided; it became very smooth quickly and we were at an altitude of 40;800 ft. At some point we had notified ATC of our inability to maintain altitude and they had cleared us for a block; but I couldn't hear the specific altitudes through all the aural warnings. We descended to 40;000 ft and continued the flight. The entire episode of Severe turbulence probably lasted no more than 20 to 25 seconds. We originally reported the incident to ATC as moderate turbulence. In the next few minutes we re-reported the turbulence to ATC as severe after verifying the delineations between the categories of turbulence. Severe up and downdrafts associated with severe turbulence.
Second reporter narrative
We encountered unexpected severe turbulence at 40;000 ft over the eastern Gulf of Mexico while flying approximately 6;000 feet above the tops of thunderstorms. We were given only a very short heads up by ATC that Aircraft Y ahead of us had just reported severe turbulence at 43;000 ft; but we were also told that everybody below 43;000 ft was encountering light with occasional moderate. I used the cabin speaker to brief the passengers just before the turbulence started and they were seated and buckled up at the time of encounter. Very shortly after that; we encountered a pocket of severe turbulence with +/- 750 ft altitude deviations; greater than 30 degrees of abrupt roll left and right; and significant airspeed deviations. The airspeed indicator exceeded red line for a few seconds as well. The severe turbulence was short lived and we were able to recover the aircraft from the turbulence upset. We of course reported all this to ATC. No injuries or perceptible structural damage to the aircraft occurred. Company and Maintenance was also immediately notified after it happened. A write-up was entered in the log after landing and Maintenance grounded the airplane after our arrival so a mechanic could perform a severe turbulence inspection. Unexpected severe clear air turbulence above a line of thunderstorms.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.