EASA learning objective '022.06.02.01.11' states: 'Describe the purpose of the synchronisation function when engaging the autopilot and explain why the autopilot should be engaged when the aircraft is in trim.'

Engaging the autopilot is similar to handing over control to a human pilot, if you want a smooth takeover the aircraft needs to be in trim! Therefore autopilots use synchronising circuits to prevent engagement unless the aircraft is properly trimmed to prevent an uncomfortable snatching of the controls on engagement. It's not just trim that needs to be checked to ensure a smooth takeover, the synchronising function also checks, amongst other things, the position of the flying control servos to ensure that autopilot keeps the control surfaces in their current position when taking control.

The autopilot will not engage unless the synchronisation system interlocks all receive a valid response from their respective systems, if the synchronisation system fails it will prevent autopilot engagement.

Note: When the autopilot is engaged the automatic pitch trim system ensures a smooth transition to manual flight on autopilot disengagement, this has nothing to do with the synchronisation function.