Autothrottle — Page 393, Lesson 487

All right, let's get into the autothrottle system. This is the system that actually moves your thrust levers for you, so it's a big one. We're going to start with the heart of the thrust control itself, which is the Power Management Control, or PMC. Think of the engine's thrust control as a two-part system mounted on each engine. The first part is a hydromechanical unit called the Main Engine Control, or MEC. The MEC is the mechanical brain that schedules fuel to provide the thrust called for by the Thrust Lever setting on the flight deck. So, when you move the thrust lever, the MEC responds by giving the engine fuel to produce that thrust. Now, the second part is the PMC, the Power Management Control. This is an electronic unit that refines that fuel flow. Here's the key: the PMC adjusts the fuel flow without moving the thrust levers. So the levers stay where you put them, but the PMC fine-tunes the fuel electronically to get a more precise result. To do this, the PMC takes in a few inputs: the MEC power lever angle, the N1 speed, the inlet temperature, and the inlet pressure. N1, remember, is the fan speed of the engine, usually expressed as a percentage. The PMC uses these inputs to adjust, or trim, the MEC to obtain the desired N1 speed. In other words, it adjusts fuel flow as a function of the thrust lever angle. Here's a really useful feature: the PMC provides a constant thrust climb feature. Once you set the thrust lever at the beginning of climb, the PMC automatically maintains that thrust throughout the entire climb profile. You don't have to touch the thrust levers again. If you do reposition the thrust lever, the PMC simply maintains the setting that corresponds to that new thrust lever angle. Now, what happens if the PMC fails? The system has failure detection and annunciation modules. These detect PMC failures and provide a signal to the crew. For detectable failure conditions, the PMC schedules a slow N1 drift over approximately 30 seconds. So the engine speed slowly changes, giving you a warning. Then, it illuminates three things: the PMC INOP light, the ENG system annunciator light, and the MASTER CAUTION lights. For a PMC failure, you can select the PMC OFF using a switch on the aft overhead panel. When you do that, the engine speed is then controlled by the hydromechanical MEC only. One important detail: the PMC INOP light is suppressed below starter cut-out engine speed. So during engine start, you won't see that light even if there's a problem. Now let's talk about how the autothrottle, the A/T, works with the PMCs. The A/T system operates properly with the PMCs either ON or OFF. In either case, the A/T computer controls the FMC N1 limits. FMC is the Flight Management Computer, and it sets the N1 limits that the autothrottle will respect. Here's a recommendation: during A/T operation, it's recommended that both PMCs be ON or both OFF. Why? Because this produces minimum thrust lever separation. If one engine's PMC is on and the other's is off, the thrust levers could drift apart. And note this: A/T take-offs may be performed with both PMCs OFF. So it's allowed, just keep them consistent. Now, how do you engage and disengage the A/T? To arm it, you move the A/T Arm switch to ARM. This arms the A/T for engagement in one of three modes: the N1 mode, the MCP SPD mode, or the FMC SPD mode. MCP is the Mode Control Panel, and SPD is just speed. The A/T Arm switch is magnetically held at ARM, and it releases to OFF when the A/T becomes disengaged. So what disengages the A/T? There are several conditions. Moving the A/T Arm switch to OFF. Pressing either A/T Disengage switch. An A/T system fault being detected. Two seconds having elapsed since landing touchdown. And finally, thrust levers becoming separated by more than 10 degrees during a dual channel approach after FLARE is annunciated. FLARE is the mode that raises the nose just before touchdown. When the A/T disengages, two things happen: the A/T Arm switch releases to OFF, and the A/T Disengage light flashes red. That's your visual cue that the autothrottle is no longer in command. So to tie it all together: the MEC is the mechanical fuel scheduler, the PMC is the electronic trimmer that refines it, and the A/T is the autopilot for your thrust levers, working on top of both. Got it?