Autothrottle — Page 393, Lesson 484

I want to walk you through the autothrottle system now. This is a big one, because it's the system that actually moves the thrust levers for you, and it ties together the autopilot, the flight management system, and the engine itself. Let's start with the definition. An autothrottle system is a computer-controlled, electromechanical system. That means it's driven by a computer, and it uses electrical and mechanical parts to do its work. Its job is to control the thrust of the aircraft's engines within specific design parameters. So it's not just slamming the levers forward — it's holding thrust to a precise, commanded value. Now, the throttles control the thrust, and in some aircraft the preferred name for the throttles is the thrust levers. That's worth remembering, because you'll hear both terms. Thrust itself is the force generated by the engines. And here's an important point: although there are thrust computation systems, there is, as yet, no direct indicator of thrust value in use. We don't have a gauge that reads "thrust" directly. Instead, we use two engine parameters as a measure of thrust — N1, which is fan speed, and EPR, which is engine pressure ratio. So the autothrottle controls the throttle position of each engine to maintain a specific value of thrust, and it can do that in terms of three things. First, fan speed, N1. Second, engine pressure ratio, EPR. Third, target airspeed, which is set by SPD on the mode control panel. So you can command the autothrottle to hold a fan speed, hold an engine pressure ratio, or hold a target airspeed. Using these modes, the autothrottle can control either engine thrust or aircraft speed from the beginning of the take-off roll all the way until the system is disconnected after an automatic landing. So it's active across the whole flight envelope — from the moment you start rolling on take-off, through climb, cruise, descent, approach, and right up to after the automatic landing, when it's disconnected. Now, the autothrottle can also be called the Thrust Management System, or TMS. And it works in conjunction with the autopilot and the FMS — the Flight Management System. So it's not a standalone box; it's part of an integrated flight control and guidance setup. Let's look at the system components and the inputs. The autothrottle takes in a lot of information from various aircraft systems and sensors. I want to walk you through each input, because each one feeds a specific function. First, mode selection and the A/T Arm switch on the MCP — that's the Mode Control Panel. The A/T Arm switch is what arms the autothrottle, and the mode selection tells it which mode to operate in. Next, from the ADC — the Air Data Computer — it gets TAS, which is true airspeed, Mach number, and TAT, which is total air temperature. These are needed for speed control and for thrust calculations at altitude. From the IRS — the Inertial Reference System — it gets attitude and acceleration. That tells the system the aircraft's orientation and how it's accelerating. From engine sensors, it gets N1 speed and/or EPR — the same parameters it uses to measure thrust. From the AoA sensor, it gets angle of attack. That's important for protecting against stalls and for managing thrust in certain flight phases. From the radio altimeter, it gets radio altitude — that's the height above the ground, which matters especially during approach and landing. From the landing gear switch, it gets air/ground logic. That tells the system whether the aircraft is on the ground or in the air, which changes how the autothrottle behaves. From the engine accessory unit, it gets a reverse thrust requirement. That's for when you need reverse thrust after landing. Then there are a few more inputs. A thrust command from the FMS, or thrust mode selection from the thrust mode select panel. An A/T Disconnect switch on the throttles — that's your manual way to disconnect the autothrottle. PLA, which is power lever angle, position from transducers — that tells the system the actual physical position of the power levers. And flap position. So you can see, the autothrottle is constantly blending air data, inertial data, engine data, and aircraft configuration data to decide exactly where those thrust levers need to be. That's the core of the system — it's a computer that takes all these inputs and commands the electromechanical actuators to move the throttles to hold the thrust or speed you've commanded. That's the foundation of the autothrottle. Next we'd look at how those inputs are processed and how the system actually commands the throttle movement, but for now, make sure you've got the inputs and the three control modes clear — N1, EPR, and target airspeed.