From AMM 22-31-00
FMC
The FMC calculates thrust N1 limits and N1 targets for each flight phase. The data goes to the DEUs. The DEUs show the N1 limits on the engine display. The DEUs send the N1 targets to the EECs which calculate equivalent TRA targets to send to the A/T to set thrust. The FMC also sends N1 targets directly to the A/T. During takeoff and max thrust go-around, the A/T uses EEC TRA targets and FMC N1 targets to set thrust. During takeoff, climb, and max thrust go-around, the FMC N1 targets are the same as the N1 limits. During reduced thrust climb and cruise operations, the FMC N1 targets are less than the N1 limits. The FMC calculates gross weight and sends it to the A/T to use in calculation of thrust and T/L rate commands. The FMC has an interface for A/T for BITE.
EEC
The DEUs send FMC N1 targets to the EECs. The EECs use the data to calculate equivalent TRA targets. The A/T uses the EEC TRA targets to set thrust during takeoff, climb, and max thrust go-around. For takeoff and max thrust go-around, the A/T initially uses EEC TRA targets to advance the T/Ls. As the T/Ls get to within 4 to 6 degrees of the FMC N1 limit, the A/T then uses FMC N1 targets to make final T/L adjustments to the FMC N1 limit.
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And here's the lots-o-words version
A/T SYSTEM - FUNCTIONAL DESCRIPTION - MODE SELECTION
General
The A/T computer receives digital and analog data from different systems and sensors to determine operational modes. This data comes into the A/T computer through the input devices.
The A/T has two primary modes of operation, speed control and N1 control. These are the only two A/T modes that are selectable on the MCP. Depending on the flight phase and mode selected, the A/T can enter these additional modes:
Arm
Retard
Throttle hold
Go-around
Test (on the ground).
After the A/T enters a mode, it puts mode selection data on the general ARINC output bus.
These systems use the mode selection data:
DEUs to show A/T modes
FDAU for data download and storage to the FDR
FCCs for A/T mode status.
The A/T controls power to the ASMs.
The A/T removes power from the ASMs during throttle hold mode operation during takeoff.
This section discusses each A/T mode of operation.
N1 Mode
The N1 mode is used during these flight phases:
Takeoff
Climb
Maximum thrust go-around.
In N1 mode, the A/T controls thrust to the EEC TRA target, calculated by the EEC for the equivalent FMC N1 target.
The N1 mode can be selected in these four ways:
Pilot manually selects N1 mode from the MCP
DFCS requests N1 mode when DFCS is engaged
TO/GA switch is pushed for takeoff (on ground)
TO/GA switch is pushed a second time during reduced thrust go-around (in the air).
When DFCS is engaged in VNAV climb or LVL CHG climb, the FCCs command the A/T to N1 mode.
When the DFCS is not engaged, the pilot can push the N1 selector switch on the MCP to manually select the N1 mode. Throttle Hold Mode
The throttle hold mode is automatic and the A/T goes into this mode during the takeoff ground roll. In this mode, the A/T removes power to the ASMs to prevent the A/T from moving the T/Ls during the takeoff roll and initial climbout.
The A/T uses two separate functions to remove power from the ASMs. One is a software function and the other is a hardware function. When both throttle hold functions agree and remove power to the servos, the A/T mode shows THR HLD on the FMA.
Arm Mode
ARM mode means that no active A/T mode has been selected. In the ARM mode, the A/T is enabled and ready to receive commands. The ASMs are powered but the A/T control logic prevents the servo motors from moving the throttles.
The A/T goes to ARM mode during these conditions:
On the ground when A/T is armed during preflight
After THR HOLD mode above 800 feet barometric alt
During Descent RETARD when T/Ls reach aft stops.
Speed Mode
In the speed mode, the A/T controls engine thrust to control airplane speed. It does this by comparing the actual computed airspeed (CAS) from the ADIRU, to the target speed from the MCP.
These are the two A/T Speed modes:
FMC SPD from the FMC target speed
MCP SPD from the speed selected on the MCP.
In FMC SPD mode, the A/T controls thrust to control airplane speed to the FMC flight plan target speed. When the A/T is in MCP SPD mode, it controls thrust to control airplane speed to the target speed selected on the MCP.
The A/T Speed mode can be selected automatically or manually. If the DFCS is engaged, the DFCS selects the A/T speed mode automatically, either FMC SPD or MCP SPD, consistent with the active DFCS pitch mode. A/T MCP SPD mode may also be selected manually by a push of the A/T speed mode selector switch on the MCP.
Retard Mode
In the RETARD mode, the A/T moves the T/Ls to the aft stops.
These are the two retard modes.
Descent RETARD
Flare RETARD.
Descent RETARD occurs during descent from altitude.
Flare RETARD occurs during flare to landing.
Both modes show as RETARD on the FMA on CDS.
Descent RETARD occurs during DFCS VNAV SPD decent, or when LVL CHG descent is selected on the MCP. During DFCS VNAV SPD descent, the A/T usually starts to retard the T/Ls to idle at the FMC top of descent (TOD) point. When the T/Ls get to the aft stops, the A/T mode changes from RETARD to ARM.
The A/T stays in the ARM mode until a new mode is selected.
Flare RETARD occurs during landing flare. During flare retard, the A/T retards the T/Ls to idle and RETARD shows on the FMA. The T/Ls move back to the aft stops as the airplane flares for landing and touchdown. The A/T disengages 2 seconds after touchdown.
Go-Around Mode
During approach when you push a TO/GA switch once, the A/T commands a reduced thrust go-around. The A/T mode on the FMA shows GA.
During go-around, if you push a TO/GA switch a second time, the A/T commands maximum thrust go-around to the FMC go-around N1 limit. The A/T mode on the FMA changes from GA to N1. The go-around mode resets if the N1 or the speed mode is selected on the MCP.
Man I gotta learn to type faster...