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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400 MAINTENANCE MANUAL
2) Generator Power Load Management a) The BCU load management function receives overload signals from the GCUs thorugh the ARINC 429 data link and two discrete signals. The GCUs determine when an overload condition is occurring on their respective bus and transmits this information to the BCU. The GCU transmits the amount of load sensed by its CT to the BCU.
3) Automatic Load Restorations a) Generator power automatic load restoration occurs when loads have been shed as a result of an overload condition. Loads are restored in reverse order of load shedding. If an overload condition occurs during load restoration. The load restoration process is stopped and load shedding takes place. Following a load shedding operation, load restoration does not occur until an additional generator or generators supply power. Therefore, at least two generators must be operating before load shed will start to restore. If one of the BCUs has failsafed, only four of the loads can be restored. b) Ground power automatic load restoration occurs when loads have been shed as a result of an overload condition during auxiliary power source or external power source operation. Loads are restored in reverse order of load shedding when an opposite auxiliary power source or an opposite external power source is added. When an IDG is added, loads may be momentarily restored but will be load shed to IDG thresholds at the completion of the power transfer process. Restoration may then be initiated by reapplying an opposite ground source (SSB opens) or an additional IDG.
The BCU receives information from each GCU on the status of its
channels transformer rectifier unit (TRU). If three of the
four TRUs have failed, the BCU provides a ground signal to the
single TRU operation relay which reduces the dc bus load
(Ref 24-30-00).
The BCU provides autoland isolation, which is the process of
dividing the airplane electrical system into three separate
power sources. This is done to provide the three flight
control computers (FCC) with three separate power sources when
they are engaged during the approach mode.
1) When three FCCs are engaged during approach (triple channel autoland), an isolation request is sent to both BCUs. The decision on how to divide the electrical system is done by BCU 1. This decision is based on the condition of the electrical system, monitoring the position of the BTBs, GCBs, and SSB, and the operation of the generator and transformer rectifier units (TRUs).
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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6) 7) 8)
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747-400 MAINTENANCE MANUAL
After BCU 1 receives an autoland isolation request from the FCCs, it determines the number of the generators and TRUs that are operating. Based upon this information the BCU directs the GCUs to operate the BTBs and dc isolation relays (DCIRs) to configure the electrical system in one of five possible configurations labled A, B, C, D, and E.The configurations selected are determined by which generator or TRU is inoperative if any. The configurations for generator failures are: a) A - No generators inoperative b) B - No. 1 generator inoperative c) C - No. 2 generator inoperative d) D - No. 3 generator inoperative e) E - No. 4 generator inoperativeThe configuration for the failure of any TRU is identical to a generator failure except that the GCB of the affected channel is not tripped.Each generator and TRU is considered an individual power generator (PG) by the autoland logic, except when they are on the same channel. A generator and TRU on the same channel are considered a single PG. If more than one PG is inoperative, BCU 1 ignores the autoland request.When there are four generators and four TRUs operating, BCU 1 directs the GCUs to configuration A. Configuration A is the normal mode for triple channel autoland.Once the system has been isolated, BCU 2 sends a confirmation signal back to the FCCs that isolation has been completed.If a PG fails while in configuration A, BCU 1 reconfigures the system to B, C, D, or E depending upon which PG failed. The BCU configures the BTBs, SSBs, and DCIRs as necessary to re-power the lost ac and dc buses. However, if the SSB or any BTB opened prior to the autoland isolation request the SSB or BTBs will not be closed during the autoland mode.
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