Gulfstream G200 (IAI 1126 Galaxy)
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Engineer; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 110; Flight Crew Total 26000; Flight Crew Type 155
Time Pressure; Troubleshooting
Aircraft Equipment Problem Critical; Deviation - Altitude Crossing Restriction Not Met; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Flight Crew Took Evasive Action; Flight Crew Regained Aircraft Control; Flight Crew Executed Go Around / Missed Approach
Aircraft
I was PM; Pilot Monitoring; on RNAV GPS Runway XX at ZZZ. We were radar vectored to intercept final just outside of [the] fix at 4;200 feet. Autopilot was connected and indicated ALT hold and NAV. As autopilot intercepted final approach course; PF (Pilot Flying) called for 'Gear Down; Landing Checklist.' As I was reading and referring to checklist; Approach Control said 'I told you to cross [the fix] at 4;200; Climb Immediately.' I immediately checked the altimeter; and we were rapidly descending through 3200 feet. Concurrent with this; the GPWS warning activated 'TERRAIN; TERRAIN; PULL UP!' I commanded the PF to execute terrain avoidance. He disconnected the autopilot to initiate the escape.As he pulled the yoke back; he stated it was taking full back pressure and he was unable to trim the aircraft. His effort would only bring the nose to about 10 degree above the horizon. I reached down to the center console to use the standby trim system. I noticed the red main trim warning light was already indicating primary trim [and] had disconnected. Used standby trim to execute escape maneuver and get control of aircraft. I advised Approach we had runaway trim; and he vectored us for another RNAV GPS Runway XX. We landed without further incident. I wrote up the aircraft on landing for nose down runaway trim and maintenance was performed. As to the cause; I can only guess that moderate turbulence; heavy snow; moderate icing; correcting back to course for 50 knot cross wind at altitude; and gear coming down; overloaded the autopilot and/or trim computer.
G200 Captain reported a runaway trim at low altitude resulted in GPWS terrain alert and marginal controllability.