what is control surface trim system?

Answer 1

Kevin P has some good points. It allows the pilots to correct for trim problems and ease their workload.

When we do our 737 trim and alignment checks, the airplane is flown at all stages inflight, flaps 0-40. The course bug is then lined up with the heading bug on the navigation display to see how far the airplane is out of trim. Giving the airplane either more aileron trim or rudder trim will correct how straight the airplanes flies. It is an interesting procedure that we do to ensure the plane flies straight during all phases of flight, cruise through flaps 40.

In this production stage, if there happens to be a trim problem that exceeds our trim limits, we squawk this problem and the engineers have a formula in which they can figure out how to fix the trim to allow the airplane to fly straighter.

During a flight, as the airplane burns fuel, the nose tends to pitch down. Mach trim automatically trims the elevator system to keep the nose up. This is one of the systems that the pilot doesn't have to do anything to, it does for him.

Answer 2

The trim system on an aircraft is used to keep the aircraft at the desired attitude and heading without the pilot having to make constant adjustments on the main elevators and ailerons.
Most aircraft have what are called trim tabs attached to the elevators, ailerons, and rudder. These are essentially smaller control surfaces that move independently of the primary controls. They can be adjusted to provide a constant pitch, forcing the aircraft to pitch up or down (or side to side) without input to the main control column.These are controlled in the cockpit with trim wheels (literally wheels that are found either next to the control yoke, or on the control pedestal). When these are rotated, the trim tabs move, and the aircraft can be fine-tuned to a level pitch, or a certain rate of climb.
For example, if a pilot wished to maintain a constant nose-up attitude of 5 degrees while he climbed to cruise, instead of pulling back on the yoke for minutes on end to achieve the climb, he would trim the aircraft to 5 degrees up, and be able to more comfortably fly the plane.