Let's pick up with the thrust levers and how they talk to the FADEC — Page 401, Lesson 498

Let's pick up with the thrust levers and how they talk to the FADEC. I want to walk you through the physical layout first, because everything else hangs off it. The thrust levers on this engine are only moved manually. That's a deliberate design point — unlike many other autothrottle systems, there's no motor driving them. The pilot's hand is the only thing that moves them. They travel over a sector, and that sector is divided into 4 segments, defined by 5 detents or stops. A detent is a physical notch or stop in the lever's travel that gives you a positive, click-in position. So you have 5 stops creating 4 segments between them. Now, the lever position itself is transmitted to the FADEC — that's the Full Authority Digital Engine Control, the engine's computer. The FADEC takes that lever position and computes two things it then displays: the thrust rating limit, and the N1 TLA. N1 is the fan speed, the low-pressure compressor speed, expressed as a percentage. TLA stands for Thrust Lever Angle — the angular position of the lever. So the FADEC is telling you, based on where the lever sits, what the fan speed limit is and what the lever angle actually is. One important detail: there is no reverse idle detent. That means there's no separate notch for reverse idle. When you want reverse, you pull up the reverse thrust levers — that clears the idle stop — and once the idle stop is cleared, reverse idle is selected. So reverse idle isn't a position you find on the sector; it's a state you enter by lifting the levers past the idle stop. Now let's talk about how the thrust rating limit is actually computed. It's computed according to the thrust lever position. If the lever is sitting exactly in a detent, the FADEC selects the rating limit that corresponds to that detent. But here's the key behavior: if the lever is set between two detents — in other words, in one of those 4 segments — the FADEC selects the rating limit corresponding to the higher mode. So it always errs toward the higher of the two bounding detents. That's a safety-driven choice, and you should remember it: between detents, you get the higher rating. Finally, let's look at thrust control and the manual mode. The engines are in manual mode provided that the ATS function — that's the Autothrottle System — is either not armed, or it's armed but not active. "Armed and not active" means the thrust lever is not in the ATS operating range, and/or there's no alpha floor condition. Alpha floor is a protection feature that commands maximum thrust in certain low-speed, high-angle-of-attack situations. So if ATS isn't armed, or it's armed but not actually commanding, you're in manual. In manual mode, each engine is controlled by the position of its corresponding thrust lever. Thrust modulation — changing the thrust — is performed by the pilot moving the lever from IDLE to TO/GA. TO/GA is Takeoff/Go-Around, the maximum thrust position. Every position of the lever within those limits corresponds to an N1 value. And when the lever is positioned in a detent, the corresponding N1 is equal to the N1 rating limit computed by the associated FADEC. So in a detent, you get exactly the rated fan speed; between detents, you get a continuous range of N1 values. So the whole picture is: manual lever position feeds the FADEC, the FADEC computes the rating limit and N1, and the pilot's lever movement from IDLE to TO/GA is the direct thrust control in manual mode.