AC DISTRIBUTION -FUNCTIONAL DESCRIPTION -AUTOLAND BUS ISOLATION INTERFACES
General
Autoland is an automatic flight director system (AFDS) mode. In this mode, the AFDS automatically lands the airplane.
During autoland, the electrical system divides into three different channels (bus isolation). This is to ensure that a single electrical power problem does not endanger the automatic landing.
Bus Isolation Process
The automatic flight director computers (AFDC) get an autoland request from pilot input. They send a bus isolation request signal on the systems ARINC 629 buses to these electrical power system components:
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ELMS
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BU generator converter
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GCUs.
Under normal conditions, the ELMS and the B/U generator converter divide the electrical system. The ELMS operates these components:
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Main battery charger
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Main battery relay
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Battery-captain isolation relay
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AC standby power relay
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DC bus tie relay.
The B/U generator converter operates the CCBs and TBBs.
When the bus isolation is complete, the B/U converter and the ELMS send autoland confirm signals to the AFDCs.
If the electrical power system is in a non-normal configuration before an autoland, other power configurations support the bus isolation. The GCUs then control part of the bus isolation.
If the electrical system cannot divide to support an autoland, the electrical control components send cannot-support-autoland signals to the AFDCs. The primary display system then shows a NO LAND 3 advisory message.
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AC DISTRIBUTION -FUNCTIONAL DESCRIPTION -AUTOLAND BUS ISOLATION INTERFACES
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AC DISTRIBUTION -FUNCTIONAL DESCRIPTION -NORMAL AUTOLAND BUS ISOLATION
General
When the electrical power system receives a bus isolation request signal from the AFDS, it divides into three different channels. This is to supply power to critical autoland components from different power sources. For normal autoland operations, these are the power sources for the three electrical power channels:
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Left IDG
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Right B/U generator
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Right IDG.
B/U Generator Converter Control
The B/U generator converter closes the right CCB to perform a no-break power transfer between the right B/U generator and the right transfer bus. The converter then opens the right TBB. The converter also prevents any operation of the left TBB and left CCB during autoland.
ELMS Control
The ELMS does these operations:
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Sends a signal to put the main battery charger into a TRU mode
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Closes the main battery relay
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Opens the battery-captain isolation relay
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De-energizes the ac standby power relay to connect the standby ac bus to the static inverter
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Prevents operation of the dc bus tie relay.
Autoland Components
There are three sets of critical autoland components; left, center, and right. These are the components in each set and their power requirements:
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Instrument landing system (ILS) (115v ac)
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Radio altimeter (115v ac)
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AFDC (28v dc)
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ILS antenna switch (28v dc).
During autoland bus isolation, the left set gets power from the left transfer bus and the left dc bus. The center gets power from the standby ac bus and the battery bus. The right set gets power from the right transfer bus and the right dc bus.
For more details on autoland, go to chapter 22.
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DC DISTRIBUTION -GENERAL DESCRIPTION
General
The dc distribution system divides the electrical power dc buses into sections. This permits better control over small groups of electrical loads. It also protects against a severe loss of power due to a single electrical power failure.
The ground handling TRU is the only component in the dc distribution system.
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DC DISTRIBUTION -GENERAL DESCRIPTION
