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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
22-31-00-015
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 goaround
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.
A/T SYSTEM - FUNCTIONAL DESCRIPTION - MODE SELECTION
EFFECTIVITY
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D633A101-SHZ1 Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
22-31-00-015
A/T SYSTEM - FUNCTIONAL DESCRIPTION - MODE SELECTION
A/T SYSTEM - FUNCTIONAL DESCRIPTION - MODE SELECTION
EFFECTIVITY
SHZ ALL 22-31-00
Page 47
D633A101-SHZ1 Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
22-31-00-015
General
The autothrottle (A/T) receives digital data from many airplane
systems to calculate thrust lever commands. The A/T calculates
rate commands for the servo motors to set thrust for each mode
of operation. The two primary A/T operational modes are the N1
mode and speed mode. Other A/T modes are ARM, RETARD,
GA and THR HLD.
Arm
When the A/T is engaged, it is in ARM mode unless one of these
modes are selected or are active:
* N1
* Speed
* Throttle Hold
* Retard.
The A/T goes into the ARM mode during takeoff after THR HOLD
mode and during descent retard after RETARD mode when the
T/Ls get to the aft stops.
Throttle Hold
The A/T uses these inputs during takeoff while in throttle hold
mode:
* Computed airspeed
* Barometric corrected altitude
* Air/ground input.
The A/T goes into the throttle hold mode during the takeoff
ground roll when computed airspeed is 80 knots. THR HLD
mode annunciates on the FMA at 84 knots. In the THR HLD
mode, the T/Ls remain at the takeoff N1 setting and the pilot
may move the T/Ls if desired. The A/T stays in throttle hold
mode until both of these conditions occur:
* Barometric altitude is 800 feet AGL
* 10 seconds after lift-off.
After takeoff is complete, the A/T changes from THR HLD to
ARM mode. The A/T stays in the ARM mode until the N1 or
speed mode is selected.
N1
The N1 mode is used during takeoff, climb and maximum thrust
go-around. In the N1 mode, the A/T uses these inputs:
* Target N1 (FMC)
* N1 command (EEC)
* TRA target (EEC)
* TRA actual (EEC)
* TRA for N1 max (EEC).
A/T SYSTEM - FUNCTIONAL DESCRIPTION - COMMAND CALCULATION
EFFECTIVITY
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Page 48
D633A101-SHZ1 Jun 10/2003
BOEING PROPRIETARY - Copyright#- Unpublished Work - See title page for details.
737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
22-31-00-016
In the N1 mode, to set thrust, the A/T uses EEC TRA targets. The
EECs receive N1 targets from the FMC and calculate an
equivalent TRA target and send these to the A/T. Using these
EEC TRA targets, the A/T sends rate commands to the ASMs
and TR packs to set thrust. Actual TRA data is transmitted from
the TR packs to the EECs. The EECs compare this with the
actual thrust set on the engines to provide feedback to the A/T.
The EECs calculate a maximum TRA target to make sure the
engine limits are not exceeded.
For takeoff and maximum thrust go-around only, the A/T uses
FMC N1 targets with EEC TRA targets for more precise thrust
control to the FMC N1 limit.
In takeoff or go-around, the A/T moves the T/Ls at a rate of 13.5
deg/sec to a predicted position. For final adjustments, the A/T
moves the T/Ls at a maximum rate of 5 deg/sec. In modes other
than TO/GA, the A/T limits thrust lever rates to 3 deg/sec.
When the A/T is not engaged and the T/Ls are moved manually,
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