Autothrottle — Page 393, Lesson 484

We're starting a new topic now: the autothrottle system. Let me walk you through what this is and how it fits into the aircraft's overall control. An autothrottle system is a computer-controlled, electromechanical system. That means it uses electronics and mechanical parts working together, all managed by a computer. Its job is to control the thrust of the aircraft's engines within specific design parameters. So it doesn't just slam the throttles anywhere—it keeps them within carefully defined limits. Now, the throttle position of each engine is controlled to maintain a specific value of thrust. And here's the key: thrust can be expressed in three different ways, depending on the mode you're using. First, Fan Speed, which we call N1. Second, Engine Pressure Ratio, or EPR. Third, Target Airspeed, which is set by the SPD function on the mode control panel. Let me define these clearly. Thrust is the force generated by the engines. The throttles control that thrust, and in some aircraft the preferred name for the throttles is thrust levers. Now, an important point: although there are thrust computation systems, there is, as yet, no direct indicator of thrust value in use. Instead, we use N1 and EPR as indirect measures of engine thrust. So when you see N1 or EPR on your instruments, you're looking at a proxy for thrust, not thrust itself. Using the modes I just mentioned, the autothrottle can control either engine thrust or aircraft speed. And it does this from the beginning of the take-off roll all the way until the system is disconnected after an automatic landing. So it's active through the entire flight envelope, from the moment you start rolling down the runway to the moment you've touched down and the system is switched off. Now let's look at the system itself. The autothrottle can also be called the Thrust Management System, or TMS. It works in conjunction with the autopilot and the FMS—the Flight Management System. So it's not a standalone unit; it's part of a larger integrated control network. Let me walk you through the inputs, because this is where you see how many different systems feed into it. The inputs include: - Mode selection and the A/T Arm switch on the MCP—that's the Mode Control Panel. - TAS, Mach Number, and TAT from the ADC. TAS is True Airspeed, TAT is Total Air Temperature, and ADC is the Air Data Computer. - Attitude and acceleration from the IRS—the Inertial Reference System. - N1 speed and/or EPR from engine sensors. - Angle of attack from the AoA sensor. - Radio altitude from the radio altimeter. - Air/ground logic from the landing gear switch—this tells the system whether the aircraft is on the ground or in the air. - Reverse thrust requirement from the engine accessory unit. Plus, there are additional inputs: thrust command from the FMS or thrust mode selection from the thrust mode select panel. There's also the A/T Disconnect switch on the throttles—that's your manual override. And we have PLA, which is power lever angle, from transducers. Finally, flap position. So you can see, the autothrottle is fed by a wide array of sensors and systems. Each one provides a piece of information the computer needs to decide how to position the thrust levers. The mode you select on the MCP tells it what to control—N1, EPR, or airspeed—and the sensors tell it the current state of the aircraft and engines. That's the foundation of the autothrottle system. We've covered what it is, what it controls, and what feeds into it. Next, we'll look at how these inputs are processed and what the system actually does with them.