Autopilot — Page 363, Lesson 427

Let me walk you through the autopilot chapter, starting with the regulatory requirements that govern its use. First, we have the EU-OPS requirements for single-pilot operation under IFR or at night. If an operator wants to conduct single-pilot IFR operations, the aeroplane must be equipped with an autopilot that has at least two functions: ALTITUDE HOLD and HEADING MODE. Altitude hold means the autopilot maintains the selected altitude. Heading mode means it maintains the selected heading. And here's the key point — this requirement means the aircraft must have at least a two-axis autopilot. One axis controls pitch, the other controls roll. That's why altitude hold and heading mode together demand two axes. Now let's look at the installation requirements for an automatic pilot system. Each automatic pilot system must be approved, and it must be designed so the autopilot can be quickly and positively disengaged to prevent it from interfering with your control of the aeroplane. That's a safety-critical design feature — you must always be able to take over. Next, unless there is automatic synchronizing, each system must have a means to readily indicate to the pilot the alignment of the actuating device in relation to the control system it operates. In plain terms, if the autopilot isn't automatically syncing itself, you need a way to see whether the actuator is lined up with the control system it's driving. Each manually operated control for the system must be readily accessible to the pilots. And here's a specific one: quick release, or emergency, controls must be on both control wheels, on the side of each wheel opposite the throttles. So both pilots have immediate access to disengage the autopilot. Attitude controls must operate in the plane and sense of motion specified for cockpit controls. The direction of motion must be plainly indicated on, or adjacent to, each control. So if you move a control, the aircraft must respond in the expected direction, and that direction must be clearly marked. The system must be designed and adjusted so that it cannot produce hazardous loads on the aeroplane, or create hazardous deviations in the flight path — either during normal operation or in the event of a malfunction. That's a fundamental safety requirement. If the autopilot integrates signals from auxiliary controls, or furnishes signals for operation of other equipment, there must be positive interlocks and sequencing of engagement to prevent improper operation. Protection against adverse interaction of integrated components is also required. So if the autopilot talks to other systems, there must be safeguards so they can't interfere with each other incorrectly. Finally, means must be provided to indicate to the pilots the current mode of operation and any modes armed by the pilot. You need to know what the autopilot is doing right now, and what it's set up to do next. Now let's move to the types of actuator. Actuators produce the physical movement of the control surfaces. They can be of different types depending on their principle of operation: electromechanical, electrohydraulic, or pneumatic. Electromechanical uses electric motors, electrohydraulic uses hydraulic pressure controlled electrically, and pneumatic uses air pressure. There are two types of configuration in which actuators are connected to the flying controls: parallel and series. In a parallel configuration, the actuator produces the movement of the control surface as well as providing feedback to the control stick. That means the stick will move when the autopilot is controlling the control surfaces. You can see this in Figure 26.5, which shows the A/P actuator in parallel. In a series configuration, the actuator produces movement of the control surface but not the control stick. So the surfaces move, but the stick stays still. That's Figure 26.6, the A/P actuator in series. It's also possible to have a combined series/parallel configuration, which gives you characteristics of both. Finally, let's talk about the torque limiter. In flight, particularly where high rates of control are to be produced, the movement of the flight control surfaces can result in loads which may impose excessive stresses on the aircraft structure. So under automatically-controlled flight conditions, it's necessary to safeguard against such stresses. The torque limiter also safeguards against a servomotor 'runaway' condition — that's when the servomotor, the motor driving the actuator, runs away and would cause control surfaces to be displaced to their maximum hard-over positions. The torque limiter prevents that dangerous condition. So to tie it together: the autopilot needs a two-axis capability for single-pilot IFR, it must be safely disengageable and clearly indicate its state, and its actuators — whether electromechanical, electrohydraulic, or pneumatic — connect to the controls in parallel, series, or a combination, with a torque limiter protecting the structure from excessive loads and runaway.