Originally Posted by
fweinrebe
The purpose of the program is to read some values from FS like the MAC%. Entering the MAC% on the FCU / FMS then gives a trim value. Setting this trim value results in a better rotation on take-off. Setting the trim too far forward will result in a heavy to lift nose, while too aft will result in an aircraft that just wants to get airborne by itself.
In flight the autopilot uses the trim to set the correct pitch for climb, cruise etc. On a B737 for example one can actually see the trim wheels move. And hear them cluck cluck cluck if it is bigger movements.