24-22-00
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MAINTENANCE MANUAL

 (k)  During a category III autoland, the main ac buses are prevented from being supplied by a common source. The autoland condition is started by the flight control computers (Ref 22-11-00). The computers provide a ground to the isolation request relay (K122). The isolation relay energizes, allowing a 28-volt dc signal to be sent to the bus power control unit (BPCU). (The autoland signal is inhibited if the dc tie relay is closed, tying both main dc buses together.) The 28-volt dc signal is also routed through each open BTB, which energizes the BTB's autoland relay. (Each BTB has its own autoland relay.) When the autoland relays energize, they open circuits to the close coils of their respective BTB. The BTB's are unable to close. If only one BTB is open prior to autoland, only it is kept from closing. If both BTB's are closed, the autoland lock-out will not occur. (An autoland relay cannot energize when its associated BTB is closed.) The autoland lock-out does not cause the ISLN lights in the BUS TIE switches to come on, nor will the L or R BUS ISOLATED messages appear on the EICAS display. The autoland lock-out cannot be removed by cycling the BUS TIE switches. Autoland is only removed when the flight control computers remove the ground from the isolation request relay (K122). The isolation request relay then de-energizes, restoring the system to normal.
 (4)  
During catagory III autoland, the dc tie relay is actuated by the captain's or first officer's instrument bus voltage sensing unit (IBVSU). When the IBVSU's sense a loss of power on either main ac bus, a ground is applied thru the autoland bus isolation relay to actuate the dc tie relay, which in turn restores the dc power. When the dc tie relay actuates, the TR UNIT EICAS message appears on the status and maintenance pages.

 (5)  
Auxiliary Power Breaker (APB) Control (Fig. 6)

 (a)  The APU GEN switch on M10063 controls the auxiliary power breaker (APB). The switch is a push on/push off type, which latches in when on. When latched, an ON indicator in the switch is visible. The switch works with the APU generator control unit (GCU) to open and close the APB. In the closed position, the APB connects APU generator output power to the ac tie bus. With the APB in the trip position, the circuit is open. The amber OFF light in the switch is on when the APB is open and: 1) The APU GEN switch is OFF (out position); or 2) The external power contractor (EPC) is open, and the APU is
 operating above 95 percent full speed.
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MAINTENANCE MANUAL

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Auxiliary Power Breaker Control Schematic
Figure 6

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(b)

(c)

(d)

(e)

(f)

(g)

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MAINTENANCE MANUAL

Along with the amber OFF light in the APU GEN switch, an APU GEN OFF message appears on the EICAS display. The APU must be running, with the failure existing, for the level C EICAS message to appear.The APU GCU contains separate APB close and trip relays. To close the APB, the close relay sends 28-volts dc to the APB close coil. The APB opens when the trip relay sends 28-volts dc to the APB trip coil.Normally, the APU GEN switch is ON (in). After the APU is started, the auxiliary power control unit monitors APU input speed. When the APU generator input shaft is at or above 95 per cent full speed, the control unit sends a 28-volt dc APU "ready" signal to the APU GCU. (Full input shaft speed is 12,000 rpm.) The GCU energizes the close coil of the APU generator field control relay (GCR). The APU generator then produces power (Ref 24-21-00).When APU power quality is acceptable, the APU GCU allows the APB to close. The APB closes when external power is not connected to the ac tie bus (external power contactor is open). An open GCB results in its bus tie breaker closing. (The BUS TIE switches are assumed to be in their normal (AUTO) position.) The APU generator supplies main ac bus power. The OFF light in the APU GEN switch should not come on as long as the APU GEN switch is ON (latched-in position).The APU generator can be used to supply main ac bus power before the integrated drive generators are on. The external power contactor, and the GCBs are then open. The BTBs are closed. Upon engine start, the BTBs open and the GCBs close. The IDGs then supply power to their main ac buses. The APB remains closed. Load shedding takes place when the engines are started with APU air, while the APU generator is supplying main ac bus power (Ref 24-23-00).The APB trips (opens) due to: 1) Pressing the APU GEN switch off (out); 2) pressing the EXT PWR switch on M10063 with external power available; 3) the APU generator input shaft falling below 95 per cent full speed; 4) pulling the APU fire switch on aft pilots' control stand P8; or 5) a protective trip by the APU GCU (Ref 24-23-00).
　
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