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Air Inlet Duct Seal (Fig.1)
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 A.  The seal for the air inlet duct is located between the air inlet duct and the air inlet plenum. It provides the transition between the duct and the plenum. The seal is bolted to the lower flange of the air inlet duct. The seal for the air inlet duct is a mold of rubber that is made into a channel shape. When the APU is raised into place, the sides of the seal deform and press against the flange on the air inlet plenum to provide a double seal.
Air Inlet Door (Fig. 1)
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 A.  The APU air inlet door controls the airflow to the APU. The door is on the right side of fuselage between the horizontal and vertical stabilizer. The door is of fiberglass-honeycomb construction with steel fittings. It is attached to the air inlet duct and opens inward. The door is partially open for inflight operation and fully open for on-ground operation. When closed, the door fairs with the fuselage and provides a smooth aerodynamic surface on the exterior of the airplane. The door is operated by an electromechanical linear actuator.
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 49-15-00
 ALL  ú ú 01 Page 1 ú Feb 10/92
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

SEE  A 
BRACE 
SEE  B 
FWD  AIR INLET PLENUM BRACE  A  DUCT AIR INLET SEAL FOR THE
ACTUATOR SHAFT 
(OPEN) AIR INLET DOOR 
ACTUATOR CONTAINER ELECTRICAL PLUG DOOR FOR THE AIR INLET POSITION SWITCH 
AIR INLET DUCT  ACTUATOR B  V-BAND CLAMP 
APU Air Inlet Door, Ducting, Sealing and Actuator Locations Figure 1 

F58284
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 49-15-00
 ALL  ú ú 02 Page 2 ú Feb 10/96
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 5.  Air Inlet Door Actuator (Fig. 1)
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 A.  The air inlet door actuator is an electromechanical actuator. It consists of an electric motor, actuator rod, jackscrew, reduction gear train, two limit and one intermediate position switches, drive socket, and a housing. Mechanical stops located beyond the limit switches are incorporated in case of limit switch failure. The drive socket allows manual operation of the actuator. Access to the drive socket is through a port in the actuator container. The actuator is located on the air inlet duct with the rod end of the actuator attached to the air inlet door and the terminal end of the actuator connected to the actuator container. The actuator container is attached to a tube on the air inlet duct with a V-band clamp.
 6.  Air Inlet Door Position Switch (Fig. 1)
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 A.  The door position switch is located on the air inlet duct, above the door actuator, near the lower hinge position of the inlet door. The purpose of the switch is to detect that the inlet door has opened, working in conjunction with, but independently from the internal switches of the actuator. Should the door fail to open within 60 seconds, or open to a position less than the minimum 15-degree position, the electrical circuitry of the APU control unit is such that the APU will not start and the EICAS fault message APU DOOR will be displayed.
 7.  Operation
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 A.  Functional Description
 (1)  
When the APU master control switch is positioned to ON, power is supplied to the door open relay which activates and supplies power to the retract windings of the air inlet door actuator. The actuator retracts the door to the full open, 45-degree position and is stopped by the limit switch. If the door fails to reach the full open position within 60 seconds, the EICAS fault message APU DOOR will be displayed.

 (2)  
The EICAS message APU DOOR will show when the door is not in its commanded position. SB 49-2068 will allow the APU DOOR message to show even when the circuit breaker for the inlet door is open.

 (3)  
If the APU is operating when the airplane takes off, the ground safety relay is energized allowing power to the close windings of the actuator and retracts the door to the part open, 15-degree position. If the airplane lands with the APU operating, the door will move to the full-open position upon touchdown.
　
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