Gulfstream IV / G350 / G450
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 45; Flight Crew Total 7000; Flight Crew Type 1200
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Regained Aircraft Control
Weather
While descending through FL280 for FL260 the aircraft encountered severe turbulence. We were unable to maintain altitude and during an uncontrolled descent we were instructed by Center to maintain FL240. The aircraft was finally recovered from the descent at FL200. We notified ATC of our altitude and were instructed to maintain FL220. We were able to climb and maintain FL220. At this point the deviation was complete. I don't believe at any point did we have a conflict with other traffic.
While descending for Nashville; TN thru FL280 FT near the Louisville VOR (IIU) we were notified of moderate to severe turbulence by an aircraft in our area between FL280 and FL220. We immediately instructed the flight attendant and two passengers to be seated with their seatbelts firmly fastened. We also slowed the aircraft to turbulence penetration speed of 270 KTS or .75 Mach. We began to experience severe turbulence and immediately asked to a lower flight level to try and descend out of it. We were told to stand by and shortly after began to experience severe down drafts followed by an immediate increase in airspeed well over 50 KTS indicated. Throttles were pulled back to the stops but the airspeed increased as well as the descent was uncontrollable. Speed brakes were extended and it was almost 20;400 FT before the aircraft was brought under control once again. We informed Indianapolis Center of what we had just experienced and asked for a level off at FL220 which we were granted. The rest of the trip continued on without incident. I would classify the clear air turbulence we experienced far greater than anything I have ever experienced in 26 years of flying. We reported this to Indy Center and suggested they divert aircraft away from the area we went through.
A G-IV flight crew reported a severe turbulence encounter descending through FL280 that resulted in loss of vertical path control and an involuntary descent to FL200.