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represented in the figure. In such a case, the pressure is regulated to keep an optimum
rate of braking with a certain amount of slip of the wheels determined by the
design of the system and the wheel do not lock even when the pedals are fully pressed.
However, if the switch A/SKID & N/W STRG that can be seen in the figure is moved to
the OFF position, the BSCU is disconnected and the servo valve is fully open, and therefore
any hydraulic pressure of the blue system called upon by the BDDV is transmitted all
along to the brakes. Under those conditions, the pilot must carefully watch the triple
indicator (also represented in the figure) to keep the pressure on each side or leg of the
MLG at 1000 psi as a maximum, because otherwise there is a risk to lock the wheels and
almost certainly burst them.
Technical report IN-062/2002
14
Figure 3. Schematics of the brake system. The alternate system is operating with the antiskid
system ON in the represented condition
When the alternate brake system is active, each pedal applies pressure to the auxiliary
low pressure system through a master cylinder that can be seen in figures 1.6.2.5.2 and,
in greater detail, 1.6.2.5.3. When there is no force F applied, the chambers C1 and C2
are communicated and full of hydraulic fluid because the valve is kept outside its seating
by the spring R2. When the pedal is pressed (force F) the sliding assembly is compressed
downwards against spring R2 and there is initially a dead travel. The hydraulic
fluid goes from chamber C1 to chamber C2 and comes out through port B. If the pedal
deflexion continues, when the dead travel ends, and the preformed packing moves with
the sliding assembly up to it is pressed against the valve, in such a way that the flow
between chambers C1 and C2 is cut, and pressure in chamber C1 starts raising. The sliding
assembly moves the valve and presses springs R1 y R2 and produces the reduction
of the volume of chamber C1 and the fluid flows out through port A towards the dual
valve (BDDV). The pressure in chamber C1 is proportional to the resistance opposed by
the hydraulic fluid in the external system connected to port A.
If the brake system is in normal mode (with green hydraulic pressure), the movement
of the pedals is electronically sent to the BSCU that processes the intention of the pilot
and the information of the antiskid system to command directly the normal servo valve
and to apply the corresponding pressure to the wheels. The dual valve does not work
in that condition.
15
Technical report IN-062/2002
Figure 4. Drawing of the master cylinder. When the pedal is pressed, a force F is applied in the
upper part of the cylinder
Technical report IN-062/2002
16
Figure 5. Detailed cutaway of the master cylinder
17
Technical report IN-062/2002
According to the Component Maintenance Manual (CMM) «Master Cylinder C24592020»,
dated 31 July 1992, the total length of the cylinder must be between 170.050 mm and
170.100 mm, and the dead band must be between 0.4 mm and 0.5 mm (there was a
typo in the CMM, page 703, in which it is mentioned «dead travel 0,4 to 05»).
1.6.3. Abnormal and emergency procedures related to the
brake system
1.6.3.1. Abnormal procedures
The Operations Manual prepared by the operator in Spanish language, Paragraph 3-
02.32, dated 20-3-2001, mentions two abnormal procedures related to the circumstances
of the event:
«BRAKES A/SKID FAULT» AND «BRAKES A/SKID NWS OFF»: (English translation) The first
warning appears in the event of antiskid failure and the second when the switch A/SKID
& N/W STRG is in OFF. In both cases, the maximum brake pressure must be 1000 PSI, and
the brake pressure must be watched in the triple indicator. The effect of brake application
is much higher than in normal mode and therefore the pedals must be pressed smoothly
without exceeding the mentioned pressure. The landing distance increases by 1.4.
«BRAKES RESIDUAL BRAKING» (dated 30-4-2002): (English translation) If the warning
appears on ground, immediately after engine start or during taxi, make a reset of the
BSCU selecting the switch A/SKID & N/W STRG to OFF. If the warning does not disappear
a maintenance action is required. NOTE: to make the reset of the BSCU it is needed
that the aircraft is still and the parking brake applied. The indication «ON BRAKE 1
(2, 3, 4, 5, 6, 7, 8») appears for the information of the crew when there is brakes residual
pressure affecting one or two wheels.
In this latter case that brakes residual pressure appears in any of the wheels (amber bar
on those wheels) there is no instructions about what the crew must do. There is no clear
　
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