Autopilot — Page 368, Lesson 442

Let's pick this up right where the outer loop feeds the autopilot. We've got the raw data coming in, and now I want to walk you through how the pilot actually commands the machine. That interface is the Mode Control Panel, or MCP. It sits on the glare-shield, right in front of the pilots, and it's the pilot's primary interface with the entire auto-flight system. On a modern twin jet, it's a row of selector switches and knobs. Each switch has a light in it, so you can see at a glance which autopilot is engaged and which command modes are selected. Now, here's a hard rule you need to remember: only one command mode may be engaged in a single channel at any one time. One in roll, one in pitch. That's it. So it is physically impossible for the autopilot to hold speed and maintain altitude at the same time by pitching the aeroplane, because both of those would be pitch commands. If you want both, you need an autothrottle — then the autopilot holds altitude while the autothrottle maintains speed. That's the division of labour. Let me give you the precise language for what's happening. When we feed raw data relevant to a particular flight path into the system, we call that 'coupling', or we call it a 'mode of operation'. Two other terms you'll hear constantly are 'hold' and 'capture'. If an aircraft is flying automatically at a selected altitude, we say it's in 'altitude hold' mode. The term 'capture' relates principally to modes associated with the selection and interception of beams from ground-based radio navigation aids — so you'll hear 'glide slope capture'. Here's where it gets clever. In some cases, mode switching is automatic. To switch from intercepting a beam or a heading to tracking the beam once you reach it, we install a Beam Sensor. This device senses beam deviation and switches modes automatically when the aircraft flies into the beam. So glide slope capture can happen automatically: the pitch control channel switches from 'altitude hold' mode to 'glide slope track' the moment the aircraft flies into the glide slope beam. No pilot input needed. Now, where does the raw data come from? It's supplied from the aircraft sensors — attitude, air data, heading, radio, and so on — to the relevant auto-flight computer. That computer compares the actual data with the selected values on the MCP and computes the control inputs needed to achieve those selected values. That's the whole loop: sense, compare, command. Let me run you through the actual modes you'd find on a modern transport aircraft with automatic landing capability. In the Roll Channel, you've got: Heading hold, Heading select, VOR intercept and track, LOC intercept and track — that's localizer — and Inertial Nav or L NAV. In the Pitch Channel: Altitude hold, Speed hold, Mach hold, Vertical Speed, and V NAV. That's your complete outer loop menu. One roll mode, one pitch mode, at any given time.