Autopilot — Page 358, Lesson 422

I want to walk you through the inner loop control system, because this is the heart of how an autopilot actually flies the aircraft. Let's start with the components, and I'll define each one precisely as we go. First, the Attitude Sensor. This is a rate gyro, and its job is to sense a disturbance of the aircraft in one axis only. So it's not measuring everything at once — it's dedicated to a single axis, like pitch or roll. When the aircraft gets bumped off its path, this gyro detects that angular movement. Next, 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 motion into something the electronics can read. Then we have the Signal Processor, which is the error detector. This is the brain of the loop. It compares the signals coming from the transducer with the input signals — that's the desired state you've commanded. It determines the required corrective action, which is the error, and then transmits a signal to the servomotor. It also receives and compares position and rate of movement feedback signals from the servomotor, so it's constantly checking what the servo is actually doing against what it should be doing. The Servomotor converts that processed signal into movement of the aircraft flight controls. And here's the key relationship: the movement is proportional to the rate and direction of the signal. So a bigger error signal means more control surface deflection, and the direction of the signal dictates which way the surface moves. The servomotor uses hydraulic, electrical, or pneumatic power to do this work. Finally, we have Aerodynamic Feedback. This closes the loop. 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 makes this a closed loop. Let me walk you through the sequence of events, because this is the cause-and-effect chain that makes the whole thing work. 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, the error shrinks, and the control surface returns to neutral once you're back on path. That progressive reduction is what stops the aircraft from overshooting. Now, let's distinguish between the two loop levels, because this is a critical concept. Inner loop systems provide auto-stability only. It is the innermost control loop. That's the closed loop we just described — the gyro, transducer, processor, servomotor, and aerodynamic feedback working together to keep the aircraft stable. Outer loop systems are those extra facilities offered — for example, Altitude Hold, Heading Hold, LNAV, and VNAV. LNAV is lateral navigation, 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 commands the target, and the inner loop actually flies the aircraft to achieve it. Let me tie this together with the diagram. In Figure 26.3, you can see the closed loop or inner loop. The attitude sensor feeds the transducer, which sends an electrical signal to the signal processing — that's the autopilot computer. The computer drives the servomotor, which is the actuator, and that moves the control surface through mechanical coupling. Then you have rate and position feedback coming back from the servomotor to the computer, and aerodynamic feedback coming back from the aircraft to the attitude sensor. There's also the autopilot engage and manual functions, which is how you turn the system on and take control. Now look at Figure 26.4, which shows the relationship of an outer loop function to the inner loop. You have the inner loop at the core — the error detector, autopilot computer, servomotor, and control surface. Then the outer loop wraps around it. The gyro-magnetic compass feeds into the MCP, which is the Mode Control Panel — that's where you select your modes. The MCP commands Heading Hold, and that outer loop acts on the inner loop to achieve the heading you've selected. So the mental model is this: the inner loop is the muscle that keeps the aircraft stable, and the outer loop is the brain that tells the muscle where to go. The outer loop commands a heading or an altitude, and the inner loop executes the manoeuvre to get there. That's the complete picture of how the autopilot controls the aircraft.