Autopilot — Page 368, Lesson 444

I want to walk you through the autopilot chapter now, and we're starting with the cockpit displays and the sensor inputs that feed the whole system. First, let's look at the Mode Annunciator. This is the display that tells you the current status of the auto-flight system. It can be a separate indicator, or it can be integrated right into the EFIS primary flight display — that's the Electronic Flight Instrument System's main screen. On that electronic display, it shows you which modes are armed and which are engaged, and it does this in different colours. So armed modes might be one colour, engaged modes another. It can also show you the status of the autopilot, the autothrottle, autoland, and the flight director. Beyond that annunciator, there are also illuminated switches or warning lights. These tell you specific things: whether an autopilot is engaged, if there's a warning of disengagement, a warning of autothrottle disengagement, a failure to achieve target speed, and a warning of auto-trim failure. So you have a whole suite of annunciations to keep you informed. Now, let's get into the sensor inputs that feed the auto-flight computer. These are the raw data sources. The first category is Manometric, or Air, Data. These are the inputs associated with altitude, airspeed or Mach number, and vertical speed. Each of these gives the current aircraft status for what's called outer loop control in the pitch channel of the autopilot. So the pitch channel uses these to manage the aircraft's vertical profile. The sensing here can be done either by independent sensor units, or by a Central Air Data Computer — that's the CADC. The key point is that these sensors work on the same fundamental principles as the basic pitot-static instruments. The difference is that instead of driving mechanical instruments, the measuring elements are coupled to electrical transducers. So you're converting the pressure measurements into electrical signals the computer can use. Next, we have Attitude Reference. This is the roll, pitch, and yaw data fed into the auto-flight computer from the primary attitude sensors. Depending on the age of the aircraft, this could be a Vertical Gyro and Directional Gyro combination, an Inertial Navigation System, or an Inertial Reference System. These sensors can also transmit data to the ADI and HSI — that's the Attitude Director Indicator and the Horizontal Situation Indicator. Finally, there's the Magnetic Heading Reference. This is a Magnetic Heading Reference System, or MHRS, also called a gyro-magnetic compass. It combines inertial heading with magnetic compass heading. So you're blending the gyro's stable heading with the magnetic compass's absolute reference. This system provides magnetic heading signals to the HSI and reference data to the auto-flight computer. So to tie it together: the auto-flight computer needs air data for pitch control, attitude data for all three axes, and heading data for navigation. Each of these comes from dedicated sensors, and they all feed into the computer that drives the autopilot, autothrottle, and flight director.