Date: 201312
Local time of day: 0001-0600
Location: ZLC.ARTCC, UT
Flight conditions: VMC
Weather elements / visibility: Turbulence; 100
Light: Night
Aircraft: Gulfstream V / G500 / G550
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZLC
ATC / advisory: Center ZLC
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Second reporter function: Captain; Pilot Flying
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; Flight Crew Regained Aircraft Control
Contributing factors: Weather
Primary problem: Weather

Narrative: At FL470 we encountered severe turbulence and could not maintain altitude. We were in light chop with occasional moderate turbulence and possible Mountain waves. We were on frequency with Salt Lake Center. The Captain had slowed the aircraft to Mach .80 (best turbulence penetration speed). The speed increased to Mach .81 then .83 and then dropped to .73 in a matter of seconds. I informed the Captain 'Speed; Speed' he acknowledged and asked for a lower altitude; I asked Salt Lake Center for lower and that we were in severe turbulence and could not maintain altitude. Salt Lake Center did not answer my request. The aircraft then pitched up; down; rolled 20-30 degrees right; then 20-30 degrees to the left ;the speed went up to .86-.87 and we lost approximately 300-400 FT; the Captain said tell them we are leaving FL470. As we started our descent Salt Lake Center said they had missed the last 'call' and to please 'say again'. I informed them that we were in severe turbulence and that the aircraft could not maintain FL470 and that we were descending and needed a lower altitude. I requested FL430; they immediately gave me FL400 and said that if we found smooth air we could stop our descent anywhere. During the descent I kept informing the Captain 'Speed; Speed' as the speed kept increasing .85-.87; he deployed the speed brakes and was working to keep the wings level as we were descending. He was in a shallow descent and it was not normal to see that much speed with the angle of descent we were in. The PLI indicator illuminated 3 times during this incident.  Around 43;700 the air started to smooth out. I asked the Captain if he wanted to stay at FL430 and he said 'yes'. I informed Salt Lake Center and they approved FL430. At FL470 the OAT was -53 C and once level at FL430 the OAT was -64 C! Before; during and after this event the wind was shifting from the front of the aircraft; to the side and the tail. The wind speed also changed; increasing and decreasing more rapidly than normal. About 5-6 minutes after this event Salt Lake Center asked if there was any damage to the aircraft; I replied 'no.' I also described the event to them and our loss of speed; altitude and that the aircraft would not maintain that altitude. After landing; I did a walk around and could not find any damage.


Second narrative: [Report narrative contained no additional information.]


Synopsis: G-V flight crew reports encountering severe turbulence at FL470 and inability to maintain altitude with large roll and airspeed excursions. A descent to FL430 is requested and granted. Temperature decreased from -53C to -64C and wind direction and velocity changed dramatically during the descent.