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locked main wheels (tire blowouts) even if full brake pedal force is applied at touchdown.
Figure 2-16. Wheel Brake and Anti-skid System
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I-2-48 ORIGINAL
Wheel spin-up override is activated at 50 knots wheel speed to allow normal braking if the right
WonW switch fails.
Skid control is enabled when sensed wheel speed differs from what the anti-skid control unit
determines it should be (e.g., hydroplaning is detected). If the system detects wheel skid, anti-skid
limits the amount of HYD 2A pressure applied to both brakes as required to prevent skidding. If no
skid exists, full pilot-applied brake pressure is routed to the brakes.
If the speed of one wheel drops 50% below the other wheel, locked wheel protection dumps brake
pressure to both wheels until the speed of the slower wheel returns above 50% of the other. Locked
wheel protection is removed below 35 knots, so that full braking performance (including locking a tire)
is available for taxi and turning operations. Below 14 knots, anti-skid is completely disabled. Below 35
knots, judicious braking is required to avoid flat spotting tires.
NOTE
Hot brakes and/or melted wheel assembly fuze plugs can be expected
any time maximum effort braking is used at heavy gross weights with
or without anti-skid, e.g., aborted takeoff or heavy weight landing
(above 46,000 lb GW) with high taxi brake usage.
2.7.3.4.1 ANTI SKID Switch. The ANTI SKID switch, located on the lower left main instrument
panel, is used to manually disable the anti-skid system, e.g., for carrier operations or following an
anti-skid failure (ANTISKID caution displayed). The switch is lever-locked in the OFF position.
ON Anti-skid system enabled for use with normal braking.
OFF Anti-skid system disabled (SKID advisory displayed when the landing gear is
down).
2.7.3.4.2 Anti-skid BIT and the ANTISKID Caution. The anti-skid control unit performs two types of
BIT: initiated and periodic. IBIT is performed when power is initially applied to the anti-skid system:
(1) when the landing gear is lowered, (2) when the ANTI SKID switch is selected from OFF to ON
inflight, or (3) on the ground with the parking brake set. IBIT performs a complete test of the anti-skid
system 9 seconds after power is applied and runs for 4.5 seconds. With WonW, IBIT is inhibited with
the parking brake released, as wheel motion will cause a false BIT failure and brake pressure would be
dumped if brakes were applied. PBIT only performs a partial anti-skid test and runs whenever power
is applied and IBIT is not running.
If an anti-skid failure is detected by either BIT, the ANTISKID caution will be displayed at BIT
completion. If an anti-skid failure is detected by PBIT, cycling the ANTI SKID switch will command
an IBIT and a more complete test of the system. When IBIT is running, the ANTISKID caution is
inhibited or is removed if previously displayed. If the ANTISKID caution returns after IBIT, the
ANTI SKID switch must be placed to OFF in order to isolate the failure and make sure that normal
braking (without anti-skid) is available.
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I-2-49 ORIGINAL
For instance, assume the right wheel speed sensor has failed and an ANTISKID caution is displayed.
If the ANTI SKID switch is left ON during landing, touchdown protection circuitry will dump and
never restore brake pressure to both wheels. Normal braking will be lost. In this case, placing the ANTI
SKID switch to OFF will restore normal braking (without anti-skid), or pulling the EMERG BRK
handle will enable emergency braking (bypassing anti-skid).
· Do not cycle the ANTI SKID switch in response to an ANTISKID
caution immediately prior to landing. Cycling the ANTI SKID switch
removes the ANTISKID caution for up to 13.5 seconds as the system
performs IBIT even though the anti-skid system may still be failed
and, if the system is not failed, wheel motion at touchdown may cause
a false BIT failure and a dump of normal brake pressure when brakes
are applied.
· If the ANTI SKID switch is not placed to OFF with an ANTISKID
caution displayed, normal braking capability may be lost completely.
2.7.3.5 Emergency Braking. Emergency braking is enabled when either EMERG BRK handle is
pulled to detent. This action opens the emergency brake valve and applies backup hydraulic pressure
to the hydraulic servovalves. If available, backup pressure from HYD 2B is utilized through the aft
isolation and arming valves, which are open with WonW. If HYD 2 is failed completely, backup
pressure from both the brake and APU accumulators is used. Check valves are incorporated to prevent
the loss of accumulator pressure if HYD 2B is failed. With backup pressure applied, the servovalves
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NATOPS Flight Manual 飞行手册 1(54)
