D.  
Only one FCU at a time is enabled to command a change in flap position for a given group of flaps. The FCU in control of each group of flaps is determined by a combination of FCU fault status and left/center/right FCU priority logic.

 E.  
The FCU in control operates in primary pneumatic mode or primary electric mode to command a change in flap position. Normally, LE flaps are moved in primary pneumatic mode. If no movement with a group of flaps under pneumatic power is detected, the FCU automatically switches to primary electric mode for that group of flaps. In primary electric mode, the flaps are commanded by the FCU to move under electric motor power. If alternate flaps are selected from the flight deck, the FCU control outputs are disabled and flap drive is under direct manual control.

 F.  
Each FCU continuously provides flap system failure protection while operating in the primary pneumatic mode or the primary electric mode. System failure protection includes uncommanded motion detection, and primary pneumatic and primary electric disable functions. Protection is provided separately for groups A and B LE flaps. The FCU's will disable their control and failure protection functions when the alternate system is engaged, but will continue to provide position information for flight deck displays and user systems.

 G.  
The FCU's automatically retract the group A LE flaps during reverse thrust operation in order to improve the structural life of the flap panels. Three air-ground signals are received from the PSEU. The signal is used to arm the automatic LE flaps retract function, enable the air ground power-up BITE test to execute, and enable CMC test functions when the airplane is on ground.

 H.  
BITE

 (1)  
Each FCU automatically runs a power-up test for approximately 2 seconds each time power is applied.

 (2)  
Each FCU independently runs a continuous built-in test to monitor for failures of internal components, and FCU inputs and interfaces.

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 27-81-00
 ALL  ú ú 01 Page 6 ú Oct 10/92
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (3)  A ground functional test may be initiated by the Central Maintenance Computer System while the airplane is on the ground. This test commands each FCU to check all LRU's for open/short circuits or failures, to check for subsystem power, to check ARINC 429 data busses for correct data format, and to check system discrete inputs. Faults internal to the FCU's are stored as shop faults.
 3.  Variable Camber Flaps
_____________________
 A.  The 11 variable camber flaps (Detail B, Fig. 3) in each wing provide more efficient lift characteristics by being warped to contour as they are extended prior to takeoff and landing. The flexible surfaces are made of fiberglass. The area of each flap is increased by a folding nose which is positioned at the flap aerodynamic leading edge as the flap is extended. This folding nose is rotated and positioned above the flap when the flap is retracted to the faired position. Each flap is supported by two flap linkages which extend and contour the flap and extend the folding nose. Each linkage attaches to the flap at three points and to the folding nose at one point. One support point also serves as the hinge point for the folding nose. The edges of the fiberglass panels are protected by a cap bonded to the surface. Adjustable seals are installed on the sides and aft edges of each flap panel for aerodynamic sealing. Mating seals are installed on wing structure except aft of flap panels 5, 7, 20 and 22. These seals are omitted for venting of the flap cavity.
 4.  Krueger Flaps
_____________
 A.  Each of the three Krueger flaps installed in each wing (Detail A, Fig. 3) is of fiberglass construction. Each has an attached folding nose which increases the effective area when the flap is extended and which folds above the flap when the flap is retracted to the faired position. The nose is hinged to the flap and positioned by links. The flap is supported by two hinges which attach to fittings in the fixed wing leading edge. The hinges also attach to rotary actuators which position the flap. Bulb seals are provided at the forward and lateral edges to eliminate gaps between flaps and between the flap and wing structure.
 5.  Power Drive Units
_________________
 A.  The four power drive units (PDU's) in each wing (Detail C, Fig. 3) provide the power necessary to drive the LE flaps. Primary pneumatic power is provided by the airplane pneumatic system, controlled by inputs from the flap control lever. Primary electric power is provided by 3-phase, 400-Hz, 115-volt electric motors, controlled automatically by the FCU's. Alternate power is provided by same electric motors, controlled manually by switches in the flight compartment.
　
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