A g-iv crew reported an excessive nose up trim during climb and cruise. During descent; the aircraft...

Narrative:

On the climb out; no issues presented itself. It wasn't till we passed through FL240; that we noticed the aircraft had an excessive amount of nose up trim. The autopilot was engaged at the time. The aircraft; aside from the trim; acted normally. We chose to level off at FL410; in case we lost the mach trim computer. At cruise; the autopilot held the aircraft level; but trim was still full nose up.we tried a numerous amount of trouble shooting and nothing worked. On descent; the autopilot descended the aircraft; but the trim then went full nose down! I pulled back on the yoke so the aircraft wouldn't nose over. This is when the flight guidance computers (FGC) failed due to pilot input overriding autopilot. We went to manual and I had difficulty flying the aircraft with full aft trim. Weather at our destination was around BKN030 and getting worse.landing was a bit challenging. Fully configured the trim was full nose down. I had to put an excessive amount of back pressure to prevent the aircraft from nosing over. Landing was accomplished safely and the problem was reported to maintenance.

Original NASA ASRS Text

Title: A G-IV crew reported an excessive nose up trim during climb and cruise. During descent; the aircraft trimmed full nose down causing flight computer failure with the pilot's excessive yoke pull. Flying manually in the landing configuration; preventing a nose over was challenging.

Narrative: On the climb out; no issues presented itself. It wasn't till we passed through FL240; that we noticed the aircraft had an excessive amount of nose up trim. The autopilot was engaged at the time. The aircraft; aside from the trim; acted normally. We chose to level off at FL410; in case we lost the Mach Trim computer. At cruise; the autopilot held the aircraft level; but trim was still full nose up.We tried a numerous amount of trouble shooting and nothing worked. On descent; the autopilot descended the aircraft; but the trim then went full nose down! I pulled back on the yoke so the aircraft wouldn't nose over. This is when the Flight Guidance Computers (FGC) failed due to pilot input overriding autopilot. We went to manual and I had difficulty flying the aircraft with full aft trim. Weather at our destination was around BKN030 and getting worse.Landing was a bit challenging. Fully configured the trim was full nose down. I had to put an excessive amount of back pressure to prevent the aircraft from nosing over. Landing was accomplished safely and the problem was reported to maintenance.

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.

Attributes
ACN	1459572
Time
Date	201706
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Gulfstream IV / G350 / G450
Operating Under FAR Part	Part 135
Flight Phase	Landing Descent Climb Cruise
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Elevator Trim System
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Instrument Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe

A g-iv crew reported an excessive nose up trim during climb and cruise. During descent; the aircraft trimmed full nose down causing flight computer failure with the pilot's excessive yoke pull. Flying manually in the landing configuration; preventing a nose over was challenging.