Narrative:

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 altitude hold and navigation. 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.

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Original NASA ASRS Text

Title: G200 Captain reported a runaway trim at low altitude resulted in GPWS terrain alert and marginal controllability.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.