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valve is not sensed, the BSCU then sends a current to open momentarily each
of the Normal servo valves, and monitors the consequent pressure rise. Full
release (i.e. brakes off ) corresponds to a maximum current value of 38mA.
The valves are then closed again, the Normal selector valve closed, and the
servo valves fully opened to release the pressure. This test cycle would have
occurred on the accident flight, at gear down selection after the failure of both
BSCU channels.
1.6.7. BSCU, internal disagreement logic.
The status of the autobrake selector pushbuttons is acquired
asynchronously by the command and monitor functions every 20 ms. Having
detected the pilot’s selection, the command channel then sends a signal to the
light in the Autobrake switch via a relay. Therefore it is possible that a short
switch operation can be detected by the command function and not by the
monitor function (or vice versa), causing a 'disagree' within one channel of the
BSCU, or in both channels.
After a six seconds confirmation time, this channel then logs a 'disagree'
fault with the BSCU, which is sent to the CFDIU, resulting in a BSCU failure
message on the ECAM screen. After four seconds it hands over control to the
other channel. The same input can also be detected by the other channel; this
also logs a failure message but as it is in a non redundant mode the design logic
dictates that it cannot relinquish control. It will be appreciated that in the event
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of a short button push, the same command/monitor disagree could occur in
both channels.
The disagree failure message remains in the channel which is still in
control. On the accident flight the Autobrake ‘LO’ switch was turned off and
on again; however this cannot break the disagreement, except when repeating
the primary fault – a highly improbable occurrence. Thus the
deselection/reselection of the AUTO/BRK LO switch had no effect; the only
way to clear and eliminate the fault would have been to reset the BSCU
(A/SKID&N/W STRG) OFF-ON switch, i.e. to switch it consecutively off and
then on.
On touchdown, four seconds after the spoilers deployment signal, the
Autobrake of the BSCU demands to the command function to apply the current
to open the normal selector valve. The monitor function senses that the
command is making a demand that is inconsistent and at this point the fault
takes effect due to the command/monitor disagree. The Normal selector valve
never opens, the Autobrake function is lost and the Normal braking system is
left inoperative.
This is recorded in the CFDIU as a failure occurring in the Normal
servo valves, which is then sent to the ECAM as a “BRAKES AUTO BRK
FAULT” warning message. During the landing, phases 8 and 9, until final
engine shut down, this failure message is inhibited and it is not displayed to the
pilots. However it is recorded in the post flight report (PFR), see section 1.11.3.
1.6.8. Warning and failure messages.
Most of the electronic systems interface with the Centralised Fault Display
System (CFDS), the main component of which is the Centralised Fault Data
Interface Unit (CFDIU).
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Failure messages and ECAM warning messages are recorded by CFDIU.
Only ECAM warning messages are displayed to the crew and some are inhibited
from display during critical phases of flight.
The data stored in the CFDIU, can be accessed via the Multipurpose
Control Display Units on the flight deck pedestal, and are presented in the form of
Post Flight or Last Leg Reports. Fault messages with more internal information on
the components are also stored and can be accessed by maintenance personnel in
order to trouble-shoot problems.
1.7. Meteorological information.
The METAR’s at 00.30 hours and 01.00 hours were the following:
Hour: 00.30 h. 01.00 h.
Wind: 010º/05 Kts 020º/04 Kts
Visibility: CAVOK CAVOK
Temperature: +18 ºC +18 ºC
Dew point: +14 ºC +14 ºC
Q.N.H.: 1.016 hPa 1.016 hPa
There were no significant weather phenomena and no significant change was
expected.
1.8. Navigation aids.
The aircraft conducted the ILS approach for Runway 24 (actual heading
244º) which was reported to be fully serviceable. The flight crew did not report
any abnormalities with the navigation equipment and the approach was
conducted normally.
All visual aids for approach and landing were operative and functioned
properly.
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1.9. Communications.
Communications of the aircraft with the various control centres along the
route and Ibiza Tower Control were normal and satisfactory at all times.
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