(b)
The monitoring of the following failures will be done continuously. 1) Failure to maintain at least 16 volts dc on the servo card
for use by the power amplifier.
2) Failure to maintain at least 80 volts ac on the servo card for use by the cards demodulator function.
3) Failure of the servo card to apply excitation voltage when requested by software.
4) Failure of the servo card to remove excitation voltage when requested by software.
5) Failure of the servo card to properly detect and debounce a disconnect switch request.
6) Failure of the servo card to properly detect and debounce a go-around switch request.
7) Failure of the A/T ARM circuitry on the servo card such that it does not match a digital bus version of the MCP A/T ARM switch. This is meant to detect servo card circuit failure, not an actual switch failure.
8) Unexpected autothrottle disconnect. If the autopilot is disconnected for any reason other than pilot or autopilot initiation, the BITE will be notified.
(c) If any of the above failures are detected, the autothrottle mode will be inhibited.
(5) Control Laws
(a)
Three major processes exist in this function. They are speed control, FLCH (flight level change) control and cruise control. Other processes that exist are throttle rate control, control A/T servo, control engine trim and airframe/engine dependent gains. The three major control law outputs are routed through an engine normalization and limiting function. The switching and limiting of these control laws determine the speed and thrust modes for the autothrottle. The throttle rate commands are then sent to the servo motor for the physical maneuvering of the thrust levers.
(b)
Two general modes exist in the speed control process. These modes are VNAV speed mode and MCP speed mode. The VNAV speed mode uses a True Airspeed (TAS) target supplied by VNAV. The MCP speed mode uses the Calculated Airspeed (CAS) and MACH speeds. These speeds are converted to true airspeed and used as targets. These targets will be limited to a minimum and maximum airspeed. The performance computations will supply the minimum and maximum limits.
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ALL ú ú 23 Page 80D ú Jun 18/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
(c)
(d)
(e)
(f)
(g)
(h)
(i)
A
747-400
MAINTENANCE MANUAL
During automatic takeoff these control laws will also apply. When the airplane is 400 feet AGL and there has been no commanded speed by the autopilot, the maximum speed will be supplied by the performance function. This is also true in the A/T EPR G/A mode.During the autothrottles FLCH mode, it will be the autopilots responsibility to control the airspeed through the altitude change. It is the autothrottle's responsibility to control the thrust through the altitude level change. These control laws apply to both descending or climbing and is also used in VNAV IDLE and A/T go-around modes.During cruise EPR control, the control laws will minimize the throttle movement at the expense of speed fluctuations. The control laws use the average actual EPR and airspeed errors to calculate throttle position.The throttle rate control will select either the speed or level change throttle rate command and limit this selection to produce a final throttle servo rate command.The control A/T servo is responsible for throttle rate servo command, manual override detection and line voltage compensation. 1) Manual override occurs when the throttle rate does not
track the commanded servo rate. This may be caused by the pilot manually opposing the A/T lever motion or the levers reaching their physical position. Whatever the cause, the control A/T servo will detect it and cause the throttles to become dormant.
2) The voltage compensation refers to gain adjustments in software or hardware to counteract the effect of voltage variations. These variations are due to the tachometers excitation winding. A change in the excitation windings will produce a change in the gain values which in turn will produce a change in the throttle movement.
The control engine trim function provides an EPR equalization signal. This trim signal is sent to the Electronic Engine Controllers (EECs) which in turn adjusts the EPR values for their respective engine.The base EPR value for engine trim will be that value corresponding to the engine with the second highest thrust resolver angle position. Engine equalization will be enabled if the following is true: 1) The A/T arm switch on the MCP is armed. 2) The on-side FMC is master. 3) The EEC signals are valid. 4) No more than one engine failed. 5) The Thrust Limit Computation (TLC) reference signals are
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