(11)
The monitor servo loops function is to detect servo loop failures which in turn will remove excitation to the servo motor.
(12)
Three failure detectors exist in the A/T servo monitor. The first detector uses the throttle rate command and the tachometer feedback difference to produce an error voltage. If the value of this error is greater than a predetermined value and the error is greater than 25 percent of the throttle rate command then, the detector No. 1 is set.
(13)
If the value of the throttle rate command is less than 0.2 deg/sec and the servo amplifier current is greater than a threshold value, then detector No. 2 is set.
(14)
The demodulator pulse that is used to demodulate the tachometer feedback and servo excitation signal is passed through a pulse detector. If a pulse is not detected than detector No. 3 is set.
(15)
The three detector outputs are ORed together and this result is ANDed with software generated autothrottle energize. The I/O switch inhibits triggering an ATS latched fail condition if the servo line is open during the throttle stopper test or during takeoff thrust lockout. The output will be the latched ATS monitor fail flag.
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E. Thrust Limit Computations
(1)
The Thrust Limit Computations (TLC) function provides the autothrottle control function with the maximum and reference thrust limits. These thrust limits (EPR) are displayed on the CDU and EICAS.
(2)
The TLC consists of two internal processes. The foreground
processor TLC and the task processor TLC.
(3)
The foreground processor provides the basis for the thrust limit computations. It provides the thrust limit mode selection for both processes as well as the thrust limits to be used by the autothrottle controller. It also controls the transmission of the thrust limit from the FMC over the general busses.
(4)
The task processor computes the reference thrust limit using the raw air data from the servo card and compares this reference value to the value from the foreground processor for increased reliability. This raw air data from the servo card provides an alternate path for thrust limit computations.
(5)
The foreground processor consists of selecting the thrust limit mode, determining the bleed configurations, computing thrust limits, and monitoring foreground rating computations. These processes are as follows:
(a)
The thrust limit mode is selected through the CDU's THRUST LIM page by entering an assumed temperature or by a request from VNAV. This process insures that the mode requested can be engaged therefore insuring the correct thrust limits for the engaged mode.
(b)
The first process is to detect a mode request from either the VNAV or CDU. The detection of a valid mode request will initiate the mode selection logic.
(c)
The mode selection logic process will determine if the requested mode is a new mode. It will also select the takeoff and climb submodes (derate 1 and derate 2 and bump for takeoff). This process will also provide the selected mode from the TLC and transmit the selected mode to the CDU for display on the THRUST LIM page.
(d)
The determine bleed process compares the bleed discretes (switch position) with the bleed status validity discretes. This is to ensure protection against over compensating for the engine bleeds in the thrust limit computations. If the validity discrete is invalid, this indicates no bleed regardless of the switch positions. This process also provides the data formatting for the A/T bleed status which is transmitted over the general busses, and the configuration used in the task processor active limit calculations.
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(e) The compute thrust limit process computes either maximum or reference thrust limits. The maximum thrust limit is the engine specified maximum operational thrust rating. The reference limit is the maximum thrust limit commanded by the autothrottle. 1) This process utilizes the Thrust Management Model Data Base
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