Figure: G450 elevator trim control rigging, from G450 Maintenance Manual, §27-33-00, figure 501, sheet 1.
The airplane is certainly flyable without pitch trim, so long as you still have control of the stabilizer. But if you lose pitch trim, there are steps you can take to get it back. It helps to understand how the elevator pitch trim system works and how each of several switches impacts operation of electric and manual pitch trim. More about this: G450 Systems / Elevator / Trim. This checklist assumes the elevator pitch trim tabs are frozen, that is, iced over.
If you follow the procedure and still end up with frozen pitch trim, don't fret. The airplane is perfectly flyable using the stabilizer to trim pitch forces. I worry about trying to repeatedly free a stuck control surface, even a trim tab. Why? See: Mishaps / Alaska Airlines 261.
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
[G450 Airplane Flight Manual §3-13-70] Inoperative pitch trim may be due to mechanical failure or binding of one or more system components from ice accumulation in the trim mechanism. The first indication of frozen pitch trim is an AP 1-2 Fail caution message on CAS caused by the inability of the autopilots to trim elevator control forces.
Photo: Pitch trim servo, from Eddie's aircraft.
Your normal pitch trim mode is through an electric trim servo that reacts to your inputs via the electric trim switches on each yoke, or from the autopilot. If the electric servo is unable to move the trim tab, you will probably get an AP 1-2 Fail CAS message. The problem could be one of these servos, in which case checking their circuit breakers would be in order. This procedure assumes you've already done this.
Each elevator trim tab is heated whenever the right main AC bus has power. You may need to cycle the power to these heaters or you may need to adjust altitude to find warmer air.
There is a lot of cable and are a lot of connection points between the manual trim wheel in the cockpit and the trim tabs on the elevator. (See the drawing at the top of this page.) A lot can conceivably go wrong, though this airplane does not have a track record of that. Fortunately, the GV series has an answer to the unlikely problem of frozen pitch trim. If all else fails, you can take control of the stabilizer away from the flap / stabilizer electronic control unit and trim pitch forces directly by moving the stabilizer by activating the EMER STAB switch.
[G450 Airplane Flight Manual §3-13-70]
If either or both APs have hard failed because of frozen trim, check MAU #1A and MAU #2B circuit breakers listed below:
A binding electric trim servo will stop the cables from moving and could be the source of trouble. Pressing the A/P DISC switch will remove electrical power to the servos as long as the switch is depressed.
The electric pitch trim switch removes power from the servos completely, however engaging the autopilot will attempt to reengage the servos.
It might be a good idea to pull up the AC synoptic and watch the load on the right main AC bus. The problem could very well be a short in one of those heaters and you may not want to reset it. Remember, the airplane is flyable with frozen pitch trim.
CAUTION DO NOT USE EXCESSIVE FORCE IN MOVING THE MANUAL PITCH TRIM WHEEL. THE PITCH TRIM SYSTEM HAS A SHEAR PIN INCORPORATED TO PROTECT THE SYSTEM FROM EXCESSIVE FORCES. IF THE PIN IS SHEARED, FUTURE USE OF PITCH TRIM WILL NOT BE POSSIBLE.
If climbs or descents are necessary, disconnect the autopilot and manually fly the airplane to new altitude. Determine trimmed airspeed for new altitude and adjust power as required. Engage autopilot if desired.
Keep in mind that a warmer altitude may actually be above you.
Attempt to free pitch trim by cycling electric pitch trim switches. If unable, select pitch trim OFF and re-attempt to free pitch trim using firm pressure on manual pitch trim wheel.
Certainly give this a try, but if it doesn't work remember the next step gives you a perfectly flyable airplane.
CAUTION CARE SHOULD BE TAKEN WHEN CHANGING CONFIGURATION TO AVOID RAPID CONTROL FORCE BUILDUP.
Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013