Autopilot — Page 358, Lesson 424

Let's start with the very foundation of the autopilot. An aircraft in flight is constantly being pushed around by disturbances — turbulence, wind shear, asymmetric thrust — and these disturbances act about the three control axes. We have the longitudinal axis, which gives us roll; the lateral axis, which gives us pitch; and the vertical or normal axis, which gives us yaw. So to stabilize the aircraft, we have to control it about those same three axes. That's why autopilot systems are broken down into three basic control channels. The roll channel controls the ailerons. The pitch channel controls the elevators. And the yaw channel controls the rudder. Each channel is a separate control loop for that axis. Now, depending on how many of these channels an aircraft has, we classify the autopilot as single axis, twin axis, or triple axis. And here's an important structural point: there will be a separate inner loop for each axis of autopilot control. So if an aircraft has more than one autopilot, there will be one inner loop for each axis of control for each autopilot. That's how redundancy is built in. Let's look at the single axis system first. A single axis attitude control system would normally be limited to the roll axis only — that is, a single autopilot channel controlling the ailerons. At its most basic, this system will only give lateral stability, or level the wings. The roll axis is known as the primary axis. This system is sometimes simply called a Wing Leveller. Next, the two axis system. This controls the aircraft attitude in the roll and pitch axes. The pitch axis is known as the secondary axis. So here we have two autopilot channels, one controlling the ailerons and one controlling the elevators. Then the three axis system. This gives attitude control in all three axes — roll, pitch, and yaw. The yaw axis is the third or tertiary axis. Now here's a key design point. The roll and pitch channels are used as the primary control channels. It's these two channels to which the outer loop signals are fed to control the various modes — things like altitude hold, heading select, and so on. The rudder channel, by contrast, is basically a stability channel. It's not a primary mode carrier; it's there to keep the aircraft stable. It's common to have interaction between the roll channel and the rudder channel. This 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 balanced. And finally, the reason this matters at the professional level: three axis control is required for autoland. That's a hard requirement — you cannot do an automatic landing with anything less than full three-axis control. So to tie it all together: roll is primary, pitch is secondary, yaw is tertiary. The roll and pitch channels carry the outer loop modes, the rudder channel is purely for stability, and the interaction between roll and rudder gives you co-ordinated turns and faster response. And autoland demands all three axes.