Autothrottle — Page 405, Lesson 499

Let's start with the autothrottle, and I want to begin with a very specific lever position, because it sets up everything else. When the thrust lever is placed in the FLX-MCT detent, the system has to decide which thrust rating to actually use. FLX stands for flexible takeoff, and MCT stands for maximum continuous thrust. The detent is a physical notch in the lever's travel that the pilot can feel. Here's the decision logic. On the ground, with the engine running, the system selects the flexible takeoff thrust rating, but only on one condition: a flex takeoff temperature greater than the current TAT must have been selected through the MCDU. TAT is total air temperature, the temperature the aircraft actually feels in flight. The MCDU is the multipurpose control and display unit, the main computer interface in the cockpit. So if the pilot has entered a flex temperature that is higher than the current total air temperature, the system uses the flexible takeoff rating. If that condition is not met, then the MCT thrust, maximum continuous thrust, is selected instead. Now, after takeoff, the situation changes. To change from FLX to MCT, the pilot sets the thrust lever to the TO/GA position, which is takeoff/go-around, or to the CL position, which is climb. Then the lever is moved back to MCT. Once that change is made, the FLX rating setting is no longer possible. So the flexible takeoff rating is a one-way door after takeoff. There is one more important point about the lever. Maximum takeoff power, MAX TO, is always available simply by pushing the thrust lever fully forwards. No matter what mode the system is in, the pilot can always get maximum takeoff power by moving the lever all the way forward. Now let's move to the automatic mode. When the auto-thrust function, abbreviated A/THR, is active, the thrust is computed by the FMGC. That's the flight management and guidance computer. The computed thrust is limited to the value corresponding to the thrust lever position. In other words, the computer calculates what it wants, but it cannot exceed what the lever position allows. There is one exception to this limit: if the alpha floor mode is activated. Alpha floor is a protection function that commands maximum thrust to prevent the aircraft from stalling, and when it activates, the lever position limit is overridden. Let me now walk you through the inputs and outputs of this system, because that's where the engineering detail lives. The inputs are the data the system needs to compute thrust. First, air data parameters come from the air data inertial reference system. That's the combined system providing airspeed, altitude, and inertial data. Second, operational commands come from the engine interface unit, specifically a target N1. N1 is the low-pressure compressor speed, expressed as a percentage, and it's the primary thrust parameter on modern engines. Third, we have the thrust lever angle, abbreviated TLA, which is the physical position of the lever. Fourth, engine pressure sensors provide N1 and N2. N2 is the high-pressure compressor speed. Fifth, we have temperature, specifically EGT, which is exhaust gas temperature. And finally, fuel flow is an input. Now the outputs. The system sends data outputs necessary for the FMGS, the flight management and guidance system. It sends thrust parameters and TLA to the FMGS. It sends control signals to the fuel metering unit, which is the device that actually regulates fuel delivery to the engine. And it sends command N1 to the ECAM display. ECAM is the electronic centralized aircraft monitor, the screen that shows engine and system status to the crew. So the whole picture is this: the system gathers air data, engine speeds, lever position, temperature, and fuel flow as inputs. It computes thrust in the FMGC, respects the lever position limit unless alpha floor overrides it, and then outputs control signals to the fuel metering unit and display data to the ECAM and FMGS. That's the complete autothrottle loop.