In the context of aviation and aerospace, an "Autopilot control unit" (ACU) is a component of an autopilot system that controls and stabilizes an aircraft's flight trajectory. The autopilot control unit is essentially the "brain" of the autopilot system, responsible for receiving and processing data from various sensors and systems to make adjustments to the aircraft's flight control surfaces.

The autopilot control unit typically performs the following functions:

1. Receiving sensor data: The ACU receives data from sensors such as accelerometers, gyroscopes, and air data computers, which provide information about the aircraft's attitude, altitude, airspeed, and heading.
2. Processing data: The ACU processes the received data using algorithms and software to determine the aircraft's current state and make corrections to maintain a stable flight trajectory.
3. Sending control signals: The ACU sends control signals to the aircraft's flight control surfaces, such as ailerons, elevators, and rudder, to make adjustments to the aircraft's attitude, altitude, and heading.
4. Stabilizing flight: The ACU continuously monitors the aircraft's flight trajectory and makes adjustments as necessary to maintain a stable and level flight path.

The autopilot control unit can be used in various modes, including:

1. Altitude hold: Maintaining a constant altitude.
2. Heading hold: Maintaining a constant heading.
3. Vertical navigation: Following a pre-programmed vertical profile.
4. Lateral navigation: Following a pre-programmed lateral route.

The autopilot control unit is typically connected to other components of the autopilot system, such as:

1. Flight control computers: Which provide additional processing power and control functions.
2. Sensor suites: Which provide the necessary data for the ACU to make control decisions.
3. Actuators: Which convert the control signals from the ACU into physical movements of the flight control surfaces.

In summary, the autopilot control unit is a critical component of an aircraft's autopilot system, responsible for processing data and sending control signals to stabilize and control the aircraft's flight trajectory.