An SF340 Flight Crew encountered severe turbulence during descent to their destination; resulting in temporary loss of control.
Synopsis
An SF340 Flight Crew encountered severe turbulence during descent to their destination; resulting in temporary loss of control.
Narrative
We spent the entire flight deviating around two very large convective cells and medium buildups. We were paralleling an elongated convective cell (approx. 20-25 NM away) while we initiated our descent. During the descent we encountered occasional light and moderate turbulence which had been pretty normal for the afternoon. The turbulence in one particular buildup started as normal and progressed into very uncomfortable continuous moderate turbulence. I asked the Captain to ask ATC for an exit heading and to query what we were flying into as our weather radar was not depicting anything. Approximately three seconds after making my request the aircraft started to violently pitch down/to the left and then pitch up/to the right I directed the Captain to tell ATC we were taking an exit heading of 210 degrees to remove ourselves from the turbulence. The autopilot made seemingly erratic and insufficient corrections for the aircraft's attitude that the EADI (Electronic Attitude Director Indicator) displayed. Just prior to entering this situation our torque was set at 55%; 210 KIAS; 8;400ft descending to 7;000ft and approximately a 250 degree heading. I disengaged the autopilot in an effort to regain control of the aircraft. As I disengaged the autopilot the overspeed horn sounded and while reducing power to flight idle I noted that we were indicating 270 KIAS. I also noted that the VSI was pegged at the maximum indicated of 6000ft/min down and that our Left Engine Oil Pressure CWP (caution and warning panel) light had illuminated along with its corresponding triple chime. I attempted to raise the nose of the aircraft to bring our speed under control and to arrest the descent but the control yoke was; at first; while using both hands; unresponsive. By this time the Captain had secured a lower altitude with ATC and informed them of our situation. In my estimation we lost 2;000 ft in less than 30 seconds. Upon exiting these conditions all engine parameters were normal; the CWP light had extinguished; and we were able to control the aircraft without any further issues. We turned back towards destination; climbed 250 ft to maintain 6;000 ft and made appropriate power adjustments to maintain speed. We landed without incident and called Maintenance on arrival.
Second reporter narrative
The severe portion of the turbulence lasted about thirty seconds; during which we experienced nose attitudes from about fifteen degrees down to ten degrees up; descent rates of 1000 to 6000 fpm; bank angles up to twenty five degrees; and changes in airspeed of plus or minus thirty knots. We leveled off at about 5700 feet and corrected up to 6000 after exiting the severe turbulence. A PIREP was issued to ATC after which we continued to destination without incident. After landing; company was notified and maintenance inspected the plane. The flight attendant said that no passengers complained of injuries.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.