B.
Antiskid/Autobrake System Built-In Test Equipment (BITE) (Fig. 16)
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(1)
Extensive BITE on the control unit provides the system for both in-flight and on-ground testings. The BITE also provides system trouble shooting at LRU level. If the unit is the failed LRU, it isolates to the circuit card.
(2)
A BITE control and display panel and an instruction placard are on the front face of the unit. The panel face consists the following:
(a)
One alphanumeric display - provides readout and identifies the failed channel.
(b)
One BRAKE TEST/DISABLE rotary switch - allows selection of 1 thru 8, normal system operation test position, and an A/B brake test positions. In addition it disables fault indication on selected wheel. On airplanes dispatched with one wheel deactivated, the switch inhibits the inputs to the related channel, thus preventing the display of antiskid EICAS message and the illumination of the ANTISKID light.
(c)
One ENABLE/VERIFY momentary pushbutton switch - used with the VERIFY switch to enable a system test or brake operational test.
(d)
One VERIFY momentary pushbutton switch - performs a complete system test or brake operational test when used with ENABLE/VERIFY switch.
BRAKE TEST/DISABLEROTARY SWITCH
MEMORY RESET
SWITCH
WHEEL NUMBER
ASSIGNMENT
AS SHOWN
SYSTEM TEST
SWITCH
Antiskid/Autobrake Control Unit BITE
Figure 16
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(e)
One PRESS/TEST momentary, 3-position (PRESS/TEST-OFF-BIT) toggle switch - performs lamp test (switch in PRESS/TEST position) and recalls faults (switch in BIT position). The lamp test includes all segments of display, ANTISKID light and AUTOBRAKES light.
(f)
One RESET pushbutton - clears memory of stored fault
information.
(3)
Three levels of BITE test are provided. These are system test, continuous monitor test, and brake operational test.
(a)
System test
1) With both the ENABLE/VERIFY and VERIFY switches pressed, and the BRAKE TEST rotary switch in the NORM position, the control unit performs a complete system electrical test and sends its results to the BITE. When a fault is detected, the test identifies the failed LRU on the display. When the VERIFY switch is pressed and released again, subsequent faults will be shown until TEST END is shown, indicating no remaining faults.
(b)
Continuous monitor test
1) The control unit checks the valves and transducers for continuity and the control circuit for proper functions. a) The control unit checks the antiskid valves (normal and
alternate) by monitoring a small voltage which it applies to the valve torque motor. This voltage causes the antiskid valve torque motor armature to be biased slightly, away from the hydraulic return port.
b) With full system pressure available, the bias can be overcome and the return port fully blocked. This allows full pressure to the brakes.
c) With only accumulator pressure available, pressure against the armature may not be sufficient to overcome the bias. Pressure losses to return could occur, degrading brake performance.
d) Therefore, without right or left system pressure, the BITE monitor function of the antiskid valves is suspended.
2) The test stores following detected faults (continuous and intermittent): a) Failed transducers b) Control circuit failure c) Faulty valves d) Parking brake control/parking brake valve disagreement e) Wheel deactivation f) Loss of IRS signal
3) Faults are stored in non-volatile memory for readout during ground maintenance. The memory can store data for at least 250 hours without power.
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Memory recall for fault readout requires placing the PRESS/TEST switch at BIT and releasing; this shows the first fault. Subsequent BIT selections display any remaining faults until TEST END is shown. The RESET switch clears memory of all fault data.
(c) Brake operation test
1) The test checks the skid release feature and allows a visual brake check. The visual check is on a single wheel for the normal system, and on wheel pair for the alternate system. The test starts with the BRAKE TEST switch selected to one wheel and the ENABLE/VERIFY switch pressed. Pressing and releasing the VERIFY switch causes a one-time brake release and reapplication for the selected wheel (normal hydraulics) or wheel pair (alternate hydraulics).
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