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action diagrams corresponding to the braking system descriptions are shown in
Figures 1, 2 and 3. Predicate-action diagrams have a simple formal translation
into the temporal logic TLA [Lam94a].
Related Work. The Airbus approach to using computers in civil aircraft
design is presented in [Pot93]. Much fundamental research and analysis of algo-
rithms involved in flight control and management stems from SRI Computer Sci-
ence Lab, and may be found on theWWWstarting at http://www.csl.sri.com/ft-history.html
. A comprehensive introduction to safety analysis for systems involving com-
puters in general is [Lev95]. Aircraft engineering safety issues are discussed
in [LT82]. Standards for certification of flight-control systems are in [RTCA92].
Analysis and statistics of airplane accidents appear in [OSZ92]. Articles
concerning the DC-10 accidents appear in [FB92]. An account of fatal A320
accidents to date is in [Mel94]. Reports of facts and findings related to the
Warsaw accident are in [FI.93a, FI.93b, FI.93c], and the official report on the
Warsaw accident itself is [MCAAI94].
2 The Design of the A320 Braking System
The braking system design of the A320 is described in the A320 Flight Crew
Operating Manual [FCOM],repeated in [Lad95]. We give some sample entries.
Each page is identified by a section number (three sets of digits separated by
periods), a REVision number, a SEQuence number, and a Page number.
Landing Gear: Brakes and Anti-Skid (1.32.30)
1.32.30, REV 16, SEQ 001, P 1
[...]
Anti Skid System
[...]
The anti skid is deactivated when the speed is lower than 20 kts (ground speed).
An ON/OFF switch activates or deactivates the anti skid system and nose wheel
steering.
Principle
The speed of each main gear wheel (given by a tachometer) is compared with
the aircraft speed (reference speed). When the speed of a wheel decreases below
0.87 time [sic] reference speed, brake release orders are given to maintain the
wheel slip at that value (best braking efficiency).
2
1.32.30, REV 15, SEQ 001, P 3
Auto Brake
System arming
The crew may arm the system by depressing the LO, MED or MAX push button
switches, provided all the following arming conditions are met:
• Green pressure available
• Anti-skid electrically powered
• No failure in the braking system
Note: Auto brake may be armed with parking brake on.
System Activation
Automatic braking is initiated by the ground spoiler extension command (refer
to 1.27). Consequently in the event of an acceleration stop, if the deceleration
is initiated with the speed below 72 KTS, the automatic braking will not be
operative because the ground spoilers will not be extended.
System disarming
The system is disarmed by:
• Pressing the push-button switch or,
• Loss of one or more arming conditions or,
• Applying sufficient force to the rudder pedals when autobrake is operating:
– In MAX mode both pedals must be depressed,
– In MED or LO desarming [sic] may be accomplished by action on
one pedal only,
• Ground spoiler retraction (refer to 1.27).
• Flight condition since 10 seconds.
1.32.30, REV 15, SEQ 001, P 4
[...]
Normal Braking
Braking is normal when:
• green hydraulic pressure is available
• A/SKID and N/W STRG is ON
• PARKING BRAKE is not ON.
3
Anti-skid is operative and autobrake is available.
The control is electrically achieved through the BSCU:
• either via the pedals
• or automatically
– on ground by autobrake system
– in flight by setting the landing gear lever to the up position
Anti-skid system is controlled by the BSCU via the normal servo valves.
No brake pressure indication is provided.
[...]
Flight Controls Description (1.27.10: REV 18, SEQ 106, P 11)
Ground Spoiler Control
Achieved by the spoilers 1 to 5.
• Ground spoilers are armed when the speed brakes control lever is
pulled up into the armed position.
• Ground spoilers automatically extend:
– (At MLG touch down) OR (During T.O run at speed greater than
72 KT)
WHEN
(They are armed and all thrust levers are at idle) OR
(When reverse is selected on at least one engine (remaining en-
gine at idle)
• Ground spoilers retraction is achieved when:
– (All thrust levers are set at idle) AND (Speed brake control lever is
pushed down)
OR
– One thrust lever advanced
above 20◦
at least 3 sec between 4◦ and 20◦
3 Analysis and Critique
The specification of the braking system design includes some causal or temporal
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