Autopilot — Page 358, Lesson 424

Let’s start with the big picture, because this whole chapter hangs on one idea: an aircraft can be disturbed about three control axes, and an autopilot has to counter those disturbances about the same three axes. Those axes are longitudinal, which is roll; lateral, which is pitch; and vertical or normal, which is yaw. Notice the naming here — it’s easy to trip over. Longitudinal axis gives you roll, lateral axis gives you pitch, and vertical or normal axis gives you yaw. So the autopilot breaks down into three basic control channels: a roll channel that controls the ailerons, a pitch channel that controls the elevators, and a yaw channel that controls the rudder. From that, we get the classification of autopilots as single axis, twin axis, or triple axis. And here’s a key structural point: there will be a separate inner loop for each axis of autopilot control. If an aircraft has more than one autopilot, then there is one inner loop for each axis of control for each autopilot. So two autopilots, three axes each — that’s six inner loops total. Keep that in mind, because it comes back when we talk about autoland. Now let’s walk through the three types. A single axis system is normally limited to the roll axis only — that’s one autopilot channel controlling the ailerons. At its most basic, it only gives lateral stability or levels the wings. The roll axis here is called the primary axis. This system is sometimes simply called a Wing Leveller. So if you hear “wing leveller,” that’s the single axis roll-only autopilot. A two axis system controls attitude in the roll and pitch axes. The pitch axis is now called the secondary axis. There are two autopilot channels — one for the ailerons, one for the elevators. A three axis system gives attitude control in all three axes — roll, pitch, and yaw. The yaw axis is the third or tertiary axis. Now here’s an important distinction: the roll and pitch channels are used as the primary control channels. It is these two channels to which outer loop signals are fed to control the various modes. So the roll and pitch channels carry the outer loop — that’s where the mode commands come in. The rudder channel, by contrast, is basically a stability channel. It’s not the primary control path; it’s there to stabilise. And there’s a common interaction between the roll channel and the rudder channel. That interaction assists in co-ordinated turns and gives a faster stability response. So when you bank, the rudder works with the ailerons to keep the turn co-ordinated and to settle the aircraft more quickly. Finally, the punchline: three axis control is required for autoland. That’s why the full triple axis system matters — you can’t do autoland with just roll and pitch; you need the yaw channel as well. So the hierarchy is: single axis is roll only, primary axis, the wing leveller. Two axis adds pitch, the secondary axis. Three axis adds yaw, the tertiary axis, and that’s what autoland demands.