Autothrottle — Page 393, Lesson 486

Let’s start with the big picture, because the autothrottle is one of those systems that sounds simple but has a lot of moving parts. I want to walk you through what the system actually does, what it sends out, and how it keeps itself honest. First, the name. Autothrottle, often abbreviated A/T. Its job is automatic thrust control. On the Boeing 737-400, that control is available from the start of the take-off, all the way through climb, cruise, descent, approach, and even go-around or landing. So it’s not just a cruise aid — it’s with you for the whole flight envelope. Now, how does it actually move the throttles? On the 737-400, the A/T moves the thrust levers with a separate servomotor on each thrust lever. So each lever has its own motor. That’s an important detail, because it means the system can move each throttle independently if it needs to. But before we get too deep into the hardware, let’s look at the outputs — the signals the system produces. I want you to think of these as the things the autothrottle "tells" the rest of the aircraft. There are six main outputs. First, a signal to the A/T servo actuator to move the throttles. That’s the direct command that physically drives the levers. Second, a signal to the A/T disengage circuit. That’s the logic that decides when the system should be switched off. Third, signals to the BITE circuits — that’s built-in test equipment — inside the FCC and the FMC. The FCC is the Flight Control Computer, and the FMC is the Flight Management Computer. BITE is the self-test circuitry that monitors the health of the system. Fourth, mode annunciation to the EFIS symbol generator. EFIS is the Electronic Flight Instrument System. So the autothrottle tells the display system what mode it’s in, so you can see it on the screen. Fifth, thrust limits, and those go to the EICAS or ECAM display. EICAS is Engine Indication and Crew Alerting System, used on Boeing. ECAM is Electronic Centralised Aircraft Monitor, used on Airbus. So the autothrottle sends the computed thrust limits to the engine display. And sixth, failure warnings — annunciations. These can be a lamp, an aural warning, or an electronic display message. So if something goes wrong, you get told, visually and/or by sound. Now, feedback. This is the part that makes the system accurate. The autothrottle compares actual values with reference values. In other words, it looks at where the throttles actually are, and where they should be, and it passes control signals to the servomotors of the thrust levers to correct any difference. But here’s the key detail: to control the speed at which the thrust levers are moved, there is suitable feedback from the servo actuators to the TMC. TMC is the Thrust Management Computer. So the servo actuators report back to the TMC, and that feedback loop lets the system move the levers at a controlled rate, not just slam them to a position. Now let’s talk about normal operation on the 737-400. In normal operation, the FMC — the Flight Management Computer — provides the A/T system with N1 limit values. N1 is the fan speed of the engine, expressed as a percentage. So the FMC tells the autothrottle what the maximum fan speed should be, and the autothrottle works to those limits. One more critical point, and this is a real operational detail. Manually positioning the thrust levers does not cause A/T disengagement — normally. There’s an exception. If you exceed 10 degrees of thrust lever separation during a dual channel approach, after FLARE armed is annunciated, then the A/T will disengage. So during a dual channel approach, with FLARE armed showing, if the two thrust levers get separated by more than 10 degrees, the system drops out. That’s a specific limit you need to remember. And finally, after you manually position the thrust levers, the A/T may reposition them to comply with computed thrust requirements. But there’s a catch: it will not do this while in the HOLD and ARM modes. So in HOLD and ARM, the system leaves the levers where you put them. Outside those modes, it can take over and move them back to match the computed thrust. So let me tie it together. The autothrottle is a closed-loop system. It gets reference values, compares them to actual values, sends control signals to servo actuators, gets feedback from those actuators to the TMC to control the speed of movement, and it announces its mode and failures to the displays. On the 737-400, it’s active through the entire flight, driven by N1 limits from the FMC, and it respects your manual inputs except for that 10-degree separation limit during a dual channel approach with FLARE armed. That’s the core of the autothrottle system.