(b)
Alternate antiskid - The normal antiskid valve driver supplies a signal for the alternate antiskid valve driver. This signal is compared with the normal valve driver for the other wheel in the alternate antiskid pair. The higher signal is used to drive the alternate antiskid valve for the pair.
(c)
Locked wheel protection - This prevents a wheel from becoming locked into a skid during brake operations at speeds higher than 25 knots. The speed of each wheel is compared with its tandem wheel. If the speed of one of the wheels in the tandem pair decreases to less than 30% of of the other, a full brake release signal is sent to valve driver of the slow wheel.
(d)
Touchdown/hydroplane protection -
1) This protects the aft wheels from becoming locked into a skid by hydroplane action or brake pressure being applied at touchdown prior to spinup. The main wheel card compares the ground speed signal from the IRS with the wheel speed signal from the transducer. When the wheel speed is at least 50 knots slower than the ground speed signal the brakes on the aft wheels are released. Hydroplane protection for the front wheels is supplied by locked wheel release.
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2) The IRS signal must be supplied to the main wheel cards for hydroplane protection and touchdown protection to operate. If the IRS signal is not available other antiskid functions will continue to operate.
(e)
Low speed dropout logic - No brake release signal is supplied by the antiskid system at speeds below approximately 8 knots.
(3)
System outputs
(a)
Antiskid valve driver 1) Current from the antiskid valve driver is from 6 mA to begin brake release to 55 mA for full brake release.
2) Antiskid valves release brake pressure as necessary to prevent skids.
(b)
Fault signals to the system BITE card
1) Each main wheel card monitors the operation of the antiskid transducers and antiskid valves connected to the circuits on the card. Antiskid circuits in the main wheel card are also monitored. Malfunctions are isolated to the transducer, the normal antiskid valve, the alternate antiskid valve, or the main wheel card. Fault messages are transmitted by the test circuits in the main wheel card to the system BITE card.
(c)
Antiskid indication
1) There is no antiskid indication on the flight deck when the antiskid system operates without malfunctions. Fault advisory messages come into view on EICAS when an antiskid malfunction occurs. No flight crew action is necessary for antiskid operation.
B. Functional Description - Autobrake system (Fig. 12)
(1)
Autobrake system inputs
(a)
Power to operate the autobrake system is supplied by 28 vdc Bus 4 through a circuit breaker on the P180 DC Power Distribution Panel. Power passes through the autobrake selector switch. The selection (1, 2, 3, 4, or MAX AUTO) made by the flight crew supplies power to the applicable circuit in the autobrake circuit card. At touchdown, the brakes will operate automatically and maintain the deceleration rate set by the autobrake selection.
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SOLENOID VALVE
CONTROL VALVE
AUTOBRAKE PRESS CONTROL MODULE RIGHT WING WW
BITE
CARDS
(2)
M80 BRAKE SYSTEM CONTROL UNIT (E9 CENTER EQUIP CENTER)
UNIT ALSO CONTAINS TORQUE LIMITER AND ANTISKID SYSTEM CARDS
BRAKE SYSTEM CONTROL UNIT
Autobrake System Schematic
Figure 12 (Sheet 1)
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