Unlike many other autothrottle systems you'll study, there's no motor… — Page 401, Lesson 498

I want to walk you through the thrust control system on this engine, and I'll start with something that might surprise you: the thrust levers themselves are only ever moved manually. Unlike many other autothrottle systems you'll study, there's no motor driving these levers around. The pilot's hand is the only thing that moves them. Let's look at the lever's physical layout. The thrust lever moves over a sector that's divided into 4 segments, and those segments are defined by 5 detents, or stops. A detent is a physical notch or gate in the lever's travel that gives you a positive mechanical feel — you can feel the lever click into place. So you have 5 of those stops creating 4 distinct segments between them. Now, here's the key relationship: the position of the thrust lever is transmitted to the FADEC. FADEC stands for Full Authority Digital Engine Control — it's the computer that has complete authority over the engine's operation. The FADEC takes that lever position and computes two things: the thrust rating limit, and the N1 TLA. N1 is the fan speed — the rotational speed of the low-pressure compressor and fan, expressed as a percentage. TLA stands for Thrust Lever Angle — the angular position of the thrust lever. So the FADEC is continuously computing what the thrust rating limit should be and what the corresponding N1 should be for that lever angle. There's an important detail about the reverse thrust side. Note that there is no reverse idle detent. In other words, there's no separate notch that holds the lever at a reverse idle position. Instead, when the idle stop is cleared by pulling up the reverse thrust levers, reverse idle is selected. So the reverse levers have to be physically lifted up to get past the idle stop, and that action itself selects reverse idle — there's no dedicated detent for it. Now let's talk about the thrust rating limit. This is computed according to the thrust lever position. If the thrust lever is set in a detent — meaning it's clicked into one of those 5 stops — the FADEC will select the rating limit corresponding to that detent. So each detent has a specific rating limit associated with it, like IDLE, CLB, or TO/GA. But here's the interesting part: if the lever is set between 2 detents — in one of those 4 segments — the FADEC will select the rating limit corresponding to the higher mode. So the FADEC always errs on the side of the higher thrust rating when you're between detents. Let me now cover the thrust control modes, starting with manual mode. The engines are in the manual mode provided that the ATS function is either: not armed, or armed and not active. ATS stands for Autothrust System — that's the automatic thrust control. "Armed" means the system is ready and waiting, but "not active" means it hasn't actually taken control. The ATS is not active when the thrust lever is not in the ATS operating range, and/or there's no alpha floor condition. Alpha floor is a protection feature — alpha refers to angle of attack, and the alpha floor condition triggers when the aircraft's angle of attack gets dangerously high, commanding maximum thrust automatically. So in manual mode, each engine is controlled by the position of the corresponding thrust lever. Each engine has its own lever, and that lever directly commands that engine. Thrust modulation — that is, changing the thrust — is performed by the pilot moving the thrust lever from IDLE to TO/GA position. TO/GA stands for Takeoff/Go-Around — that's the maximum thrust position. Each position of the thrust lever within these limits corresponds to an N1 value. So as you move the lever smoothly from IDLE up to TO/GA, the N1 increases correspondingly. And here's the final piece: when the thrust lever is positioned in a detent, the corresponding N1 is equal to the N1 rating limit computed by the associated FADEC. So when you're clicked into a detent, the engine runs at exactly the rating limit that the FADEC has computed for that detent — no more, no less. The FADEC holds it precisely at that N1 rating limit. Let me tie this all together. The thrust lever is a purely manual input device. Its position tells the FADEC what the pilot wants. The FADEC then computes the thrust rating limit — choosing the higher mode if you're between detents — and the corresponding N1. In manual mode, with the ATS not armed or armed but not active, the pilot has full authority over thrust by moving the lever between IDLE and TO/GA. And when the lever is in a detent, the engine settles exactly at the FADEC-computed N1 rating limit.