BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
C23428
A
747-400
MAINTENANCE MANUAL

 (3)  
The rotational speed of the air cycle machine is dependent upon the bleed air supply available at the turbine inlet. As flow is bypassed around the turbine through the turbine bypass valve, the machine slows down until, with the turbine bypass wide open, the machine ceases to perform its function in the system. Choking at the compressor inlet during high turbine bypass flows is prevented by a bypass line around the compressor. The check valve in this line automatically provides this bypass and prevents backflow when the compressor is running.

 (4)  
Normal pack flow is activated when the airplane is in cruise. On ground and during takeoff, climb, descent and landing the packs are in high flow. In high flow, the flow control and shutoff valve is fully open to provide the maximum flow of regulated air.

 (5)  
The pack dump valves are normally open. One valve closes when the main deck cargo fire arm switch is set to ARM. If pack 1 is on, the pack 1 dump valve will close and the pack 3 dump valve will stay open. If pack 1 is off or failed, the pack 3 dump valve will close and the pack 1 dump valve will stay open.

 (6)  
A small amount of bleed air is discharged through a nozzle to induce water flow through the line connecting the water separator drain with the aspirator. Ram air carries water through the heat exchanger where it is vaporized. This process provides a significant amount of additional cooling of the bleed air. The water supply is always available under hot-day, high-humidity conditions when the largest amount of cooling is required.

 (7)  
The compressor discharge overheat switch protects against excessive air conditioning pack temperatures resulting from an inadvertent blockage of the ram air duct. If the switch is closed by an overtemperature in the duct, the SYS FAULT light on the cabin temperature control panel is illuminated and the pack is shut down. To restart the pack, the PACK RST switch on the cabin temperature control panel must be pressed momentarily.

 (8)  
During flight, cooling air for the heat exchanger is forced into the ram air intake by the ram effect. On the ground, the ACM fan draws air through the same opening and the cooling air check valve closes to prevent backflow through the ram air inlet.

 (9)  
In case of improper sequencing between the turbine bypass valve and the ram air inlet door actuator or the exit door actuator, the pack overtemp shutdown relay will be energized and the flow control and shutoff valve will close to shut down the air conditioning pack. The SYS FAULT light will come on. The pack can be restarted by pressing the PACK RST switch if the pack controller has repositioned the ram air actuators and the bypass valve to the prestart (open) position. If shutdown was caused by an overheat condition, as indicated by the pack can be restarted by pushing the PACK RST switch after cool-down.

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 21-51-00
 ALL  ú ú 01F Page 12 ú Feb 10/95
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (10)
 If all three packs are operating, the number two and three packs will shut down if any zone, (main deck, fwd lower lobe, aft lower lobe) switch is set the ARMED position.

 (11)
 If the number pack one is not operating prior to arming:

 -
Number two pack is shutdown when the lower aft zone is armed.

 -
Number three pack is shutdown when the lower aft zone is armed.

 -
Number two pack is shutdown when the main deck is armed.

 B. Control
 (1)  Operation of the air conditioning packs is automatic. For air conditioning system startup and shutdown procedures, refer to AMM 21-00-00/201.
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 21-51-00
 ALL  ú ú 01F Page 13 ú Jun 18/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 AIR CONDITIONING PACK SYSTEM - ADJUSTMENT/TEST
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