Autothrottle — Page 401, Lesson 492

I want to walk you through the autothrottle system now, and we're going to start with how the FMC commands speed. The FMC SPD mode is an autothrottle mode that's commanded by the FMC during V NAV operation. So when you're flying a vertical navigation profile, the FMC is in charge of the speed target. When this mode engages, the MCP IAS/Mach display goes blank, and the airspeed cursors position themselves at the FMC commanded airspeed. The autothrottle then maintains that commanded speed. But here's the key limitation: the autothrottle is limited to the N1 value shown on the thrust mode annunciators. So even though the FMC wants a certain speed, the thrust can't exceed that N1 limit. Now let's talk about N1 equalization. The autothrottle attempts to equalize N1 between the two engines through the dual servo individual thrust lever control. That means each thrust lever has its own servo, and the system works to balance the N1 readings. But the equalization control is limited to 8 degrees of thrust lever separation. So if the levers drift more than 8 degrees apart, the system won't try to force them back together beyond that limit. Next is ARM mode. The autothrottle annunciates ARM when the A/T Arm switch is at ARM and no autothrottle mode is engaged. In this state, you can move the thrust levers manually without any interference from the autothrottle system. The autothrottle automatically transfers to ARM from the SPEED or N1 mode when you deselect that mode by pressing the respective mode selector switch while the switch light is illuminated. So if you're in SPEED mode and you press that selector, the system drops back to ARM. Now, the descent retard mode. The autothrottle engages and annunciates RETARD during LVL CHG and V NAV descents. RETARD changes to ARM when the thrust levers reach the aft stop, or when they're manually prevented from reaching the aft stop. So the system is trying to pull the thrust back, and once the levers hit the stop, it reverts to ARM. Then we have the landing flare retard mode. During landing, RETARD mode engages, reduces thrust, and annunciates RETARD 2½ seconds after FLARE mode engagement, or at 27 feet radio altitude, whichever comes first. During a non-precision or visual approach with flaps extended to 15 or greater, and the AFDS not in ALT ACQ or ALT HOLD, the autothrottle RETARD mode engages at 27 feet radio altitude. The autothrottle automatically disengages approximately 2 seconds after landing touchdown. Let me make sure that's clear. Radio altitude is the height above the ground measured by the radio altimeter. So during a normal ILS approach, the flare mode triggers the retard, and it happens 2½ seconds after flare engagement or at 27 feet, whichever is earlier. On a non-precision approach, there's no flare mode, so the retard engages purely at 27 feet radio altitude, provided the flaps are at 15 or greater and the autopilot isn't capturing or holding an altitude. Now the go-around mode. With the A/T Arm switch at ARM, the go-around mode is armed when you descend below 2000 feet radio altitude, with or without the AFDS engaged. Once armed, the go-around mode can be engaged until 2 seconds have elapsed after landing touchdown. Pressing either TO/GA switch engages the go-around mode. GA is annunciated for the autothrottle, and the thrust levers advance to the reduced go-around thrust setting. That setting produces a 1000 to 2000 feet per minute rate of climb. After reaching reduced go-around thrust, pressing either TO/GA switch a second time signals the autothrottle to advance thrust to the full go-around N1 limit. After reaching reduced or full go-around thrust, the GA mode can be terminated by selecting another AFDS pitch mode, or when ALT ACQ annunciates engaged. During a single engine FD go-around, the autothrottle will increase thrust to the full N1 limit. Finally, the autothrottle disengage switches. Pressing an Autothrottle Disengage switch disengages the autothrottle. The A/T Disengage light flashes, and the A/T ARM switch on the MCP trips off. Pressing the Autothrottle Disengage switch a second time extinguishes the A/T warning. After an automatic autothrottle disengagement, pressing the Autothrottle Disengage switch will extinguish the A/T warning. So the key distinction there is that the disengage switch serves two purposes. First press disengages the system and gives you the warning. Second press clears the warning. And even if the system disengages automatically for some reason, you can still press that switch to clear the warning light.