Autopilot — Page 368, Lesson 444

Let's start with the Mode Annunciator, because it's your window into what the autopilot is actually doing. The Mode Annunciator will indicate the current auto-flight system status. It can be a separate indicator, or it can be an integrated part of the EFIS primary flight display — that's the Electronic Flight Instrument System's main screen in front of you. The electronic display indicates armed and engaged modes of the auto-flight system in different colours. So "armed" means a mode is standing by, ready to capture when conditions are met, and "engaged" means it's actively controlling right now. The different colours tell you which is which at a glance. It can also indicate autopilot, autothrottle, autoland, and flight director status — so one display can tell you the state of all four of those systems. Beyond that annunciator, there will also be illuminated switches or simply warning lights that indicate whether an autopilot is engaged, warning of disengagement, warning of autothrottle disengagement, failure to achieve target speed, and of auto-trim failure. So you have a dedicated light for each of those five conditions: autopilot engaged, autopilot disengaged, autothrottle disengaged, target speed not achieved, and auto-trim failure. Now let's look at where the autopilot gets its information. The auto-flight computer needs data about the aircraft's state, and that comes from several sensor groups. The first is Manometric, or Air, Data. Raw data inputs under this heading are those associated with altitude, airspeed or Mach number, and vertical speed. Each of these provides current aircraft status for outer loop control in the pitch channel of the autopilot. So the pitch channel — the autopilot's control of the aircraft's nose up and down — uses these air data values as its outer loop, meaning the slower, corrective layer of control that keeps the aircraft on the commanded flight path. Sensing may be carried out either by independent sensor units, or by a Central Air Data Computer, the CADC. The sensors operate on the same fundamental principles as the basic pitot-static instruments, the measuring elements being coupled to appropriate types of electrical transducers instead of instruments. So instead of driving a needle on a dial, the pressure sensing elements convert their measurement into an electrical signal that the computer can read. The next group is Attitude Reference. Attitude reference data — roll, pitch, yaw — is fed into the auto-flight computer from the primary attitude sensors. Those could be a Vertical Gyro and Directional Gyro combination, an Inertial Navigation System, or an Inertial Reference System, depending on the age of the aircraft. These sensors may also transmit data to the ADI and HSI — the Attitude Director Indicator and the Horizontal Situation Indicator, your primary flight and navigation displays. Finally, there's the Magnetic Heading Reference. A Magnetic Heading Reference System, or MHRS, also called a gyro-magnetic compass, combines inertial heading with magnetic compass heading, providing magnetic heading signals to the HSI and reference data to the auto-flight computer. So it blends the stability of the gyro with the absolute truth of the magnetic compass to give you a reliable heading reference. That figure shows trimming by variable incidence tailplane — the output is applied to the trim tab actuator or the horizontal stabilizer, which is how the autopilot manages trim. And this next one shows the auto-trim failure light. So to tie it together: the Mode Annunciator tells you what the autopilot is doing, the warning lights tell you when something's wrong, and the sensor inputs — air data, attitude reference, and magnetic heading — give the computer the raw information it needs to fly.