(b)
Depressing the SPD pushbutton sends the mode change request to the TMS and AFDS. If the autothrottle is armed and there are no faults affecting operation in the speed mode, the autothrottle engages and controls airspeed (or Mach) to the reference value selected in the IAS/MACH display on the MCP.
(c)
The MCP IAS/MACH display is controlled by the SEL button and the unlabeled speed select knob located below the display window. The SEL button permits IAS or MACH reference speed to be displayed. Selection of the IAS or MACH reference speed is made by turning the unlabeled speed select knob. Pushing the center of the speed select knob transfers control of the speed reference from the MCP to the FMC. When a TMS control mode of SPD, FLCH or EPR (TO or GA) is engaged, pushing the speed select knob will not cause any speed reference change. When in VNAV mode, the IAS/MACH display will be blank when the FMC is in control of the reference and driving the airspeed bugs on the MACH/airspeed indicators. When the speed window is blank, pushing the speed select knob causes the current airspeed to be displayed in the window. If the airspeed reference is invalid, 200 knots is displayed.
(d)
The ranges of the speed select knob are:
1) 0.4 to 0.95 for Mach
2) 100 to 399 knots for airspeed
(e)
The SEL pushbutton allows selection of Mach or airspeed for display, provided it is within the allowable range.
(20)
Thrust Management System - Speed Mode Signal
(a) Speed Mode Functions
1) The A/T COMMAND (SPEED CONTROL THR/RATE CMD) signal commands a throttle rate to capture and maintain the selected speed on the MCP. This command is automatically limited between minimum and maximum speed limits. The TMC fast/slow control law drives the fast/slow indicator on the EADI to indicate speed error of | 10 knots full scale. When the minimum speed limit is reached, the fast/slow display is automatically referenced to and controlled by the minspeed limit function. When the EPR mode is engaged, the thrust levers advance toward the EPR REF but are limited to the flap limit speed or maximum speed (VMO). Normally the EPR REF limit overrides before the maximum speed is obtained.
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MAINTENANCE MANUAL
. A/T ARM
ATTITUDE DIRECTOR INDICATION
. SPEED SELECTED
. MACH MODE DISPLAYED
. NO SPEED ERROR
UPPER EICAS DISPLAY . CRZ SELECTED . REF EPR DISPLAYED
AIRPLANES WITH STANDARD FORMAT FAST/SLOW DISPLAY AIRPLANES WITH SPEED TAPE DISPLAY AIRPLANES WITH EURO FORMAT FAST/SLOW DISPLAY
Autothrottle - Speed Mode
Figure 19
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M21505
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MAINTENANCE MANUAL
(b) A/T COMMAND
1) The A/T COMMAND controls the thrust levers to maintain a target speed. The TMS uses true airspeed (TAS) in controlling to the CAS reference target speed. TAS is computed from the CAS output from the ADC. The commanded true airspeed is limited by the maximum speed limits to prevent the speed from violating MMO, VMO, and flap placard limits.
(c) Minimum Speed 1) The minimum speed limit automatically becomes active when the A/T command permits the speed to go below the critical minimum speed (30% above stall speed). A reference AOA is computed from Mach and airplane configuration data. A load factor is added to compensate for the change in AOA due to body loads.
2) The TMC causes an ALPHA speed message when the airplanes speed is at or below Vref. The TMC calculates Vref as a function of AOA and then compares the calculated Vref with actual speed. This eliminates nuisance ALPHA speed messages when the airplane AOA is high.
(d) Fast/Slow Display Command 1) The fast/slow control law drives the fast/slow speed indicator on the EADI. If the A/T is engaged or the FCC pitch channel is controlling speed, the true airspeed is compared to the limited selected speed to determine raw error. The turbulence cancelation factor is summed with this error to form the command that drives the display. 2) If no automatic mode is engaged, the fast/slow command is the greater of the proportional signals from the speed control or minimum speed control functions. The error between filtered true airspeed and process selected speed, along with the turbulence cancelation factor, combine to form the command signal when the speed control law is in control (proportional path).
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