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Total flying hours: 22265
Last 100-hours check: 20/05/1998
1.6.4. Engines.
Manufacturer: C.F.M.
Model: CFM 56-5A3 Turbofan eng.
Position: Nº 1 Nº 2
Serial number: 731350 731684
Total flying hours: 20739 16919
Total cycles: 9989 7864
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1.6.5. Braking and Anti-Skid System Description
The aircraft was designed with two braking systems; ‘Normal’ with
pressure supplied by the Green hydraulic system, and ‘Alternate’ with pressure
supplied by the Yellow hydraulic system. The Normal system provides ‘brake
by wire,’ Autobrake and Anti-Skid functions via the BSCU (Brake and
Steering Control Unit).
In the event that the Normal system is inoperative, Alternate braking
should be made available by a spring-biased changeover valve (Automatic
Selector Valve), which allows Yellow hydraulic pressure to the Alternate
braking system. This is a system whereby braking is achieved via foot pedal
master cyliders through a low-pressure auxiliary hydraulic line. Pressure is
ported to the Alternate servo valves via a Brake Dual Distribution Valve
(BDDV) and a Dual Shuttle Valve, with the anti-skid function being controlled
through the BSCU, if still operative. There is one servo valve for braking
pressure control on each wheel for the Green system plus another one for the
Yellow system. A triple indicator on the central instrument panel in the flight
deck shows the Alternate system pressure ported to the left and right wheels, as
well as the pressure in the accumulator.
In addition, a Parking Brake is also provided, which operates off the
Yellow system, backed up by a Brake Accumulator. Operation of the Parking
Brake handle applies unmodulated, reduced to 140 bars, Yellow hydraulic
pressure to the brakes by the Parking Brake valve
The full system description as presented in the Flight Crew Operating
Manual is shown in Appendix B, with a more detailed braking system
schematic presented in Appendix B Figure 1, page 18.
1.6.6. BSCU - Brake & Steering Control Unit.
The BSCU (Brakes & Steering Control Unit) is a computer that controls
the Normal braking, Autobrake function, Nose Wheel Steering and Antiskid
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(on both Normal and Alternate systems). It consists of two physically distinct
but functionally identical channels (1 and 2, or A and B) which have
independent power sources from No 1 and 2 electrical busses (See Appendix B
Figure 2).
The system is controlled by either one of the channels, whichever is
powered first at start-up, or No 1 if the computer is reset via the Antiskid and
nose wheel steering ON/OFF switch (“A/SKID & N/W STRG”) on the flight
deck, i.e. the main BSCU switch.
If a fault develops in the channel in control, (i.e. active), the design
logic decrees that the passive channel takes over. This becomes a non
redundant operation and the active channel cannot relinquish control.
Each of the two channels has a command or control function
(“command” or COM) and a monitor function (“monitoring” or MON); the
monitor function checks for agreement with the command function output
before any output is sent. If the monitor function does not agree with the
command function, a ‘disagree’ condition is logged within the BSCU and also
sent to the CFDIU (Centralised Fault Data Interface Unit).
If a disagree or any failure occurs in redundant mode, (i.e. the other
channel is still functioning) the system transfers control to the passive channel,
which then becomes active and operates in a non redundant mode. If a
disagree occurs in a non redundant mode, some or all functions may be lost, the
surviving and active channel at the moment of the failure provides the
remaining functions.
The Autobrake function controls the level of braking demanded by the
pilot by providing a given level of deceleration, for Autobrake ‘LO’ this is -
0.17g. The amount of braking to give the desired level is signalled to the servo
valves. The pilot pre-selects the Autobrake level via switches in the cockpit,
‘LO’, ‘MED’ or ‘MAX’.
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Normally, on touchdown the BSCU channel in command will open the
Normal Selector Valve, allowing Green pressure to the four servo valves of the
Normal system (“Normal Servo Valve”). The level of braking is then
controlled by the current applied to the servo valves at each wheel by the
BSCU. There are two solenoids in each of the servo valves, one controlled by
each of the BSCU channels.
The BSCU performs a functional test at Landing Gear down selection.
This firstly opens the Normal selector valve. Although the operation of this
　
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