Autothrottle — Page 405, Lesson 499

We're starting the autothrottle chapter now. Let's begin with the thrust lever detents, because that's where the system's logic starts. When the thrust lever is set in the FLX-MCT detent, the behavior depends on the phase of flight. FLX stands for flexible takeoff thrust, and MCT stands for maximum continuous thrust. On the ground, with the engine running, the system selects the flex takeoff thrust rating — but only if a flex takeoff temperature greater than the current TAT has been selected through the MCDU. TAT is total air temperature, and the MCDU is the multipurpose control and display unit, the pilots' interface for entering performance data. If that flex temperature hasn't been set, or isn't higher than the current TAT, then the system selects MCT thrust instead. After takeoff, the change from FLX to MCT is achieved by moving the thrust lever to the TO/GA or CL position, then back to MCT. TO/GA is takeoff/go-around, and CL is the climb detent. Once you've done that, the FLX rating setting is no longer possible — you can't go back to flex. Now, maximum takeoff power is always available by pushing the thrust lever fully forwards. That's the MAX TO detent, and it overrides everything. Let's move to the automatic mode. When the auto-thrust function, abbreviated A/THR, is active, the thrust is computed by the FMGC — the flight management and guidance computer. That computed thrust is limited to the value corresponding to the thrust lever position. In other words, the lever sets a ceiling on what the computer can command. There's one exception: if the alpha floor mode is activated, that limit is bypassed. Alpha floor is a protection that commands maximum thrust regardless of lever position. Now let's look at the inputs and outputs of this system. The inputs are: air data parameters from the air data inertial reference system; operational commands from the engine interface unit, specifically a target N1; the thrust lever angle, abbreviated TLA; engine pressure sensors giving N1 and N2; temperature, which is EGT, exhaust gas temperature; and fuel flow. The outputs are: data outputs necessary for the FMGS, the flight management and guidance system; thrust parameters and TLA to the FMGS; control signals to the fuel metering unit; and command N1 to the ECAM display, the electronic centralized aircraft monitor. So the picture is: the FMGC computes thrust from air data and engine inputs, respects the lever limit, and sends commands down to the fuel metering unit and up to the displays. That's the autothrottle system in its essentials.