Autothrottle — Page 393, Lesson 487

I want to walk you through the autothrottle system, and I'm going to start with the Power Management Control, or PMC, because it's the heart of how the engines actually deliver the thrust the autothrottle asks for. Every engine on this aircraft has two control units mounted on it. The first is a hydromechanical Main Engine Control, which we call the MEC. The second is the PMC. The MEC is the primary fuel scheduler—it takes the Thrust Lever setting from the flight deck and schedules fuel to produce the thrust that lever position calls for. But here's the key refinement: the PMC takes that fuel flow and electronically fine-tunes it, all without moving the thrust levers. So the pilots set the levers, and the PMC does the precise adjusting behind the scenes. Now, what does the PMC actually use to do its job? It monitors four inputs: the MEC power lever angle, the N1 speed, the inlet temperature, and the inlet pressure. Using those, it adjusts, or trims, the MEC to achieve the desired N1 speed. And notice the relationship—the PMC adjusts fuel flow as a function of thrust lever angle. So the fuel flow is continuously matched to where the thrust lever sits. This gives us a really important feature: constant thrust climb. 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 touch the levers again. If you do reposition the thrust lever, the PMC simply maintains the new setting corresponding to that new lever angle. The PMC also has built-in failure detection and annunciation modules. These detect PMC failures and send a signal to the crew. Here's the sequence for a detectable failure: the PMC schedules a slow N1 drift over approximately 30 seconds, and then it illuminates three things—the PMC INOP light, the ENG system annunciator light, and the MASTER CAUTION lights. If the PMC fails, you can select it OFF using a switch on the aft overhead panel. Once it's off, the engine speed is controlled by the hydromechanical MEC only. And one more detail: the PMC INOP light is suppressed below starter cut-out engine speed, meaning it won't illuminate until the engine is spooled up past that point. Now let's move to how the autothrottle, the A/T, works with the PMC. The A/T system operates properly whether the PMCs are ON or OFF. In either case, the A/T computer controls the FMC N1 limits. During A/T operation, the recommendation is that both PMCs be ON or both OFF, because that produces minimum thrust lever separation. And importantly, A/T take-offs may be performed with both PMCs OFF. Finally, let's look at A/T engagement and disengagement. Moving the A/T Arm switch to ARM arms the autothrottle for engagement in one of three modes: N1, MCP SPD, or FMC SPD. The Arm switch is magnetically held at ARM, and it releases to OFF when the A/T becomes disengaged. There are several conditions that disengage the A/T, and I want you to know each one. Moving the A/T Arm switch to OFF. Pressing either A/T Disengage switch. An A/T system fault being detected. Two seconds elapsing since landing touchdown. And finally, thrust levers becoming separated by more than 10 degrees during a dual channel approach after FLARE is annunciated. When disengagement happens, two things follow: the A/T Arm switch releases to OFF, and the A/T Disengage light flashes red. That's your visual cue that the autothrottle has dropped out.