Autopilot — Page 358, Lesson 422

I want to walk you through the inner loop control system — the heart of how an autopilot actually flies the aircraft. This is the closed loop that gives the aircraft auto-stability, and everything else the autopilot does builds on top of it. Let me start with the components, because each one has a specific job in the chain. The first is the Attitude Sensor. This is a rate gyro, and its job is to sense disturbance of the aircraft in one axis only. So it's not measuring everything at once — it's watching a single axis, like pitch or roll, and detecting when the aircraft moves away from its stabilized condition. Next is the Transducer. This converts the mechanical movement of the gyro into an electrical signal. So the gyro physically moves, and the transducer turns that physical movement into something the electronics can work with. Then we have the Signal Processor, which is the autopilot computer. This is the error detector. It compares the signals coming from the transducer with the input signals — the signals that represent what you want the aircraft to do. It determines the required corrective action, which is the error, and transmits a signal to the servomotor. But it doesn't just send a one-way command. It also receives and compares position and rate of movement feedback signals from the servomotor. So it's constantly checking what the servomotor is actually doing against what it commanded. The Servomotor is the actuator. It converts the processed signal into movement of the aircraft flight controls, and that movement is proportional to the rate and direction of the signal. So if the signal says "move quickly in this direction," the control surface moves quickly in that direction. The servomotor uses hydraulic, electrical, or pneumatic power to do this. Now here's the key part — the Aerodynamic Feedback. The attitude reached by the aircraft is sensed by the rate gyro, which gives a measure of the output. So the gyro isn't just sensing the initial disturbance; it's also sensing the result of the correction. That's what closes the loop. Let me trace the whole sequence for you. A disturbance to the selected flight path produces an error signal. The autopilot operates to move the aircraft back towards its stabilized condition. This causes the error signal from the transducer to be progressively reduced, and therefore removes the control surface deflection after the disturbance has been corrected. So the system corrects, then relaxes — it doesn't keep pushing once the aircraft is back where it should be. Now let me distinguish between the two levels of loops, because this is a critical concept. Inner loop systems provide auto-stability only. This is the innermost control loop — the one we just described, with the gyro, transducer, signal processor, and servomotor all working together to keep the aircraft stable. Outer loop systems are the extra facilities offered — things like Altitude Hold, Heading Hold, LNAV, and VNAV. LNAV is lateral navigation, and VNAV is vertical navigation. These are still essentially loops, but they act from an external position on the inner loop. They make the inner loop manoeuvre the aircraft into the position required by the outer loop control. So the outer loop says "I want you at this altitude," and the inner loop does the actual flying to get there. Let me show you how this all fits together with the diagram. Look at Figure 26.3 — this is the closed loop or inner loop. You can see the chain: the attitude sensor feeds into the transducer, which produces an electrical signal that goes to the signal processing, which is the autopilot computer. From there it goes to the servomotor, which is the actuator, and that moves the control surface. Then you have the aerodynamic feedback path — the attitude reached by the aircraft is sensed by the rate gyro, closing the loop. And you also have the rate and position feedback from the servomotor going back to the signal processor. Plus there's the autopilot engage and manual functions, and the mechanical coupling between the components. Then Figure 26.4 shows the relationship of an outer loop function to the inner loop. You can see the inner loop contains the error detector, the autopilot computer, the servomotor, and the control surface. The outer loop sits outside it, and it includes the gyro-magnetic compass and the MCP — that's the Mode Control Panel — with the heading hold function. So the outer loop takes information from things like the compass, and it commands the inner loop to achieve what it wants. The key takeaway is this: the inner loop is the stabilizer, the outer loop is the navigator. The inner loop keeps the aircraft stable and responds to disturbances. The outer loop tells the inner loop where to go. And together, they give you the full autopilot capability.