Date: 202102
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Weather elements / visibility: 10
Aircraft: Gulfstream IV / G350 / G450
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: FMS/FMC
Component problem: Malfunctioning
Reporter organization: Corporate
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overrode Automation; Flight Crew Regained Aircraft Control; Flight Crew Overcame Equipment Problem; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We were flying from ZZZ to ZZZ1.We were at FL410 and given a descent to FL350. I was the PF and after the PM acknowledged the altitude and set it in the ALT selector I initiated a 1000fpm descent in vertical speed mode. At approximately 40;500 feet the aircraft pitch over 60-70 degrees and accelerated from Mach .80 to Mach .90+ in a matter of a few seconds. I turned off the auto throttles and pulled the throttles to idle while simultaneously canceling the auto pilot then extended the speed brakes. We were descending at such a high rate I had difficulty pulling back on the yoke; but was able to pull it back stopping out descent around 36;000 feet and after pulling back the aircraft climbed to almost 43;000 feet before I able to arrest the ascent and bleed our airspeed off. This entire event took place in the space of about 10-15 seconds. After bleeding the speed off and regain control of the aircraft I started our decent once again by hand and flew it down to a lower requested altitude. We were sure that our flight guidance computer was to blame and had it replaced.


Second narrative: We were flying from ZZZ to ZZZ1.We were at FL410 and given a descent to FL350. I was the PM and after the PFrequested the altitude change and set it in the ALT selector I initiated a descent of 1000 fpm in vertical speed mode. At approximately 40;500 feet the aircraft pitch was over 55-70 degrees and accelerated from Mach .80 to Mach .90+ in few seconds. the PF turned off the auto throttles and he's pulled the throttles to idle and simultaneously canceling the autopilot then extended the speed brakes. We were descending at such a high rate the PF had difficulty pulling back on the yoke; but he was able to pull it back stopping the descent at around 36;000 feet and after pulling back the aircraft climbed to almost 43;000 feet before the PF was able to arrest the ascent and bleed our airspeed off. This entire event took place in the space. After bleeding the speed off and regain control of the aircraft the PF started the decent by hand and flew it down to a lower requested altitude. We were sure that our flight guidance computer was to blame and had it replaced.


Synopsis: Gulfstream flight crew reported that during descent the aircraft pitched over excessively and quickly accelerated. The flight crew disengaged aircraft automation and regained control. Crew reported that the flight guidance computer had malfunctioned and was subsequently replaced.