Autothrottle — Page 393, Lesson 486

I want to walk you through the autothrottle system now. This is the system that actually moves the thrust levers for you, so let's start with what it produces. The main outputs from the system are signals sent to several places. First, the A/T servo actuator, which is the motor that physically moves the throttles. Second, the A/T disengage circuit, which is what cuts the system out. Third, the BITE circuits — that's built-in test equipment — located in the FCC and the FMC. The FCC is the flight control computer, and the FMC is the flight management computer. Fourth, mode annunciation to the EFIS symbol generator, which is how the flight mode annunciations get displayed on your electronic flight instrument system. Fifth, thrust limits and the EICAS or ECAM display — EICAS on Boeing, ECAM on Airbus. And sixth, failure warning annunciations, which can be a lamp, an aural warning, or an electronic display. Now, the feedback side. The autothrottle system compares the actual values with the reference values, and passes control signals to the servomotors of the thrust levers. So it's a closed-loop system — it looks at where the thrust levers actually are versus where they should be. To control the speed at which the thrust levers are moved, there's suitable feedback from the servo actuators back to the TMC. The TMC is the thrust management computer. That feedback is what governs how fast the levers travel. Let me give you the Boeing 737-400 general picture, because that's the aircraft this section is built around. The A/T system provides automatic thrust control from the start of the take-off, all the way through climb, cruise, descent, approach, and go-around or landing. So it's with you for the entire flight envelope. In normal operation, the FMC provides the A/T system with N1 limit values. N1 is the fan speed of the engine, expressed as a percentage — that's your primary thrust reference. So the FMC tells the autothrottle what the N1 limit should be, and the A/T works to that. The A/T moves the thrust levers with a separate servomotor on each thrust lever. So each lever has its own motor. Now here's an important operational point. Manually positioning the thrust levers does not cause A/T disengagement — unless 10 degrees of thrust lever separation is exceeded during a dual channel approach, after FLARE armed is annunciated. So if you grab the levers and move them by hand, the system stays engaged. The exception is that specific condition: a dual channel approach, FLARE armed annunciated, and you exceed 10 degrees of separation between the two levers. That's the trigger for disengagement. And one more key behaviour. Following manual positioning, the A/T may reposition the thrust levers to comply with computed thrust requirements — except while in the HOLD and ARM modes. So if you move the levers manually, the autothrottle can take them back to where the computed thrust says they should be. But not in HOLD mode, and not in ARM mode. In those two modes, it leaves them where you put them. So the whole picture is: the FMC computes the N1 limit, the autothrottle compares actual to reference, drives the servomotors with feedback to the TMC, and annunciates its modes and failures — with that one specific manual-override disengagement condition on the 737-400.