How Does Auto-Pilot Works?

In this Post, we will explore the reasons behind the introduction of autopilots. We will also examine the typical closed-loop pilot control system and compare it with a typical autopilot closed or inner loop system. Additionally, we will look at the different types of autopilots, typical autopilot redundancy on modern airliners, and the European Operations (EU Ops) requirements.

Why Autopilots Were Introduced:

Autopilots were introduced to relieve the pilot of the physical and mental effort required to fly the airplane manually over extended periods. The autopilot takes care of the basic functions of flying the aircraft, allowing the pilot to focus on other tasks such as navigation and managing the aircraft systems. Moreover, autopilots react more quickly to disturbances, making the airplane more comfortable to fly and ensuring more accurate maintenance of heading and altitude.

Understanding Control Loops:

To better understand how autopilots control the airplane, it's essential to analyze how human pilots maintain various parameters, like keeping the wings level. This requires an external reference, which can be the horizon outside or the artificial horizon (attitude indicator) inside the cockpit. When a disturbance occurs, the pilot assesses it mentally and applies the necessary corrections to bring the aircraft back to the desired attitude. This process can be referred to as a control loop and is essentially a closed-loop control system with aerodynamic feedback.

Inner Loop Control System:

We can replace the human pilot with components that follow a similar process. An attitude sensor (rate gyro) detects disturbances, and a transducer converts this into an electrical signal. The flight control computer compares this signal with the attitude reference signal and calculates an error signal for corrective action. A servo motor converts the electrical signal into mechanical movement of the flight controls, which repositions the aircraft to the desired attitude. This control loop is known as an inner loop, enhancing the aircraft's stability.

Feedback and Monitoring:

To ensure the autopilot's proper operation, feedback is crucial. The feedback from the servo motor, referred to as rate and position feedback or control position feedback, informs the computer of the mechanical movement. This feedback helps the computer compare the servo motor's movement with the command signal, ensuring that the autopilot responds correctly. If discrepancies are detected, the autopilot can be disengaged for safety.

Single, Two, and Three-Axis Autopilots:

Autopilots come in different configurations. A single-channel autopilot controls the roll axis, providing lateral stability. A two-channel autopilot controls the roll and pitch axes using ailerons and elevators. The three-channel autopilot, standard on modern airliners, controls all three axes: roll, pitch, and yaw. The yaw axis is controlled by the rudder, and it often interacts with the roll channel to facilitate coordinated turns.

EU Ops Requirements:

The EU Ops requirements specify that an operator should not conduct single-pilot operations in Instrument Flight Rules (IFR) conditions unless the airplane is equipped with an autopilot with at least altitude hold and heading mode. This means the aircraft must have at least a two-axis or two-channel autopilot.

Conclusion:

In this lesson, you learned how autopilots were introduced to assist pilots and saw the components involved in replicating a pilot's actions in a closed-loop system. You also learned about the different types of autopilots, from single to three-axis systems, and the EU Ops requirements for autopilot systems.