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Thrust limits
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Thrust limit modes
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Fault messages.
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TMCS -TMCF FUNCTIONAL DESCRIPTION -INTRODUCTION
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TMCS -TMCF FUNCTIONAL DESCRIPTION -AUTOTHROTTLE CONTROL LAWS
General
These are the four autothrottle control laws:
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Speed (SPD)
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Vertical speed (V/S)
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Thrust
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Trim equalization.
For mode logic, there are these five selectable autothrottle modes:
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A/T SPD
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VNAV (SPD/THR)
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FLCH
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Takeoff (TO)
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Go-around (GA).
Each control law uses these:
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Thrust limit protection
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Pilot override logic
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A/T servo motor control.
Mode logic determines which control law is active for servo control.
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Speed
Two modes are available for the speed control law. They are VNAV SPD and MCP (A/T) SPD. The VNAV SPD mode uses a command true airspeed (TAS) from the FMCF. The MCP SPD mode uses the speed in the IAS/MACH window on the MCP as command speed. The TMCF compares the command speed with the air data inertial reference unit (ADIRU) speed and the difference is an error. The error goes to the A/T servo motor (ASM) as a rate command.
The control law uses throttle position and adjusts the rate command that drives the throttles. When the airplane is at the command speed from the FMCF (VNAV active in cruise) or the IAS/MACH window (A/T switch active) the error goes to zero.
The thrust limit protection supplies thrust commands to prevent engine thrust limit exceedance for that thrust limit mode.
Both of these modes operate to hold mach or airspeed.
Automatic Speed Mode Engagement
If the autothrottle is not engaged, it will automatically engage in the speed (SPD) mode when all these conditions are true:
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At least one autothrottle ARM switch is in the ARM position
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Radio altitude is more than 100 feet, or more than 400 feet after take off
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Engine thrust is below the engine thrust limit
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TMCS -TMCF FUNCTIONAL DESCRIPTION -AUTOTHROTTLE CONTROL LAWS
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The autopilot or the flight director are not in a speed-through-elevator mode
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The airspeed is just above stall speed.
Vertical Speed (V/S)
The V/S control law calculates a vertical speed for the flight level change (FLCH) mode and the GA mode. The control law calculates a fixed thrust setting. When the actual V/S is less than the initial calculation, the thrust control law increases thrust. If the actual V/S is more than the initial calculation, the control law does not decrease its thrust calculation. The thrust limit protection prevents engine overboost.
For the FLCH mode, it supplies a vertical speed command to get to the MCP altitude in 125 seconds. THR shows for the A/T mode on the PFD.
During the FLCH mode, if a pilot moves the throttles, the A/T mode changes to throttle HOLD. HOLD shows as the A/T mode on the PFD. This occurs when a difference of eight degrees exists between the throttle position and the throttle command. This occurs for a pilot override or when the throttles move to the aft mechanical stop. When the throttles get to the aft mechanical stop, the ASM continues to drive electrically until there is a difference of eight degrees between the throttles and the servomotor command.
The go-around (GA) mode controls V/S for the first TO/GA lever push. The control law calculates a thrust setting and compares actual V/S with 2,000 FPM. From this, the TMCF control law calculates a rate command to move the throttles to maintain the set vertical speed. THR shows for the A/T mode on the PFD. Push the TO/GA lever again for a full thrust calculation. The A/T mode changes to TO/GA on the PFD.
Thrust
The thrust control law operates to maintain engine thrust in takeoff (TO), GA (full GA thrust), or VNAV climb/descent thrust limit modes. The thrust limit protection process does not let the engines over boost, exceed the thrust for the thrust limit mode, or go over the maximum speed/maximum mach (Vmo/Mmo) limit. There is no thrust limit protection in the TO mode below 400 feet above ground level (AGL).
Flare retard is a thrust mode. Flare retard engages when the SPD mode is on and the autopilot flight director system (AFDS) sends a flare retard command. Flare retard occurs for both autoland and manual land in the final approach at 25 feet AGL. The AFDS commands the throttles to move aft at a 5 degree rate. IDLE shows as the A/T mode on the PFD.
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