(2)
Should the control valve fail to close on schedule, compartment temperature rises to the maximum limit of the overheat switch. At this time, the switch closes to energize its relay and send power to close the override valve. On declining temperature, the overheat switch remains closed until temperature drops to the switch minimum limit before releasing the overheat valve to reopen.
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A
747-400
MAINTENANCE MANUAL
M47977
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747-400
MAINTENANCE MANUAL
(3)
When the AFT CARGO HT switch is set to off the override valve relay is de-energized and power is sent to the close the valve. Similarly if the control valve is open when the switch is selected off the control valve relay will de-energize and send power to close the control valve.
(4)
Two cargo overheat indications occur in the flight deck when the aft cargo compartment overheats:
(a)
The TEMP light on the AFT CARGO HT switch comes on
(b)
The TEMP CARGO HEAT advisory message shows on EICAS.
(5)
The overheat indications are inhibited until the aft cargo
temperature control valve opens.
(6)
When both aft cargo air conditioning and aft cargo heat are selected on, aft cargo heat temperature control is automatically transferred to the underfloor heat control switch. Regardless of aft cargo heat temperature select switch setting, the aft cargo heat system will operate to maintain underfloor temperature at 40°F (4°C) to 50°F (10°C).
(7)
CMC initiated BIT
(a) The Central Maintenance Computer (CMC) provides built-in testing for the aft cargo heat valves. When the AFT CARGO HEAT ground test is selected the aft cargo heat valves are checked via the environmental control systems miscellaneous card.
B. Control
(1)
Apply electrical power (Ref 24-22-00/201).
(2)
Provide pneumatic pressure (Ref 36-00-00/201).
(3)
Set AFT CARGO HT switch on P5 overhead panel to ON.
(4)
Set the temperature selector switch, located just inside the bulk cargo door, to the desired temperature.
(5)
The system will operate, as described above, to maintain compartment temperature to within |5°F (|3°C) of the selected temperature.
(6)
To shut down the system set AFT CARGO HT switch to off.
(7)
Remove pneumatic pressure if no longer needed (Ref 36-00-00/201).
(8)
Remove electrical power if no longer needed (Ref 24-22-00/201).
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747-400 MAINTENANCE MANUAL
AFT CARGO COMPARTMENT HEATING SYSTEM - ADJUSTMENT/TEST
______________________________________________________
1. General_______
A. This procedure has instructions to do these tasks:
-Cargo Compartment Heating System Ground Test
-System Test - Aft Cargo Compartment Heating System
-Calibration Check of the Aft Cargo Compartment Overheat Switch
(1)
The System Test makes sure the system valves are in the correct position at the set point of each temperature switch. The test also makes sure the system valves close when you arm the cargo fire protection system.
(2)
The Calibration Check of the Aft Cargo Compartment Overheat Switch is a scheduled maintenance task. It is the same as the overheat switch test in the System Test.
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