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jammed mechanism within the BDDV, caused by ice, represented a
dormant failure condition of a component that is maintained “on
condition”, ie does not receive regular, periodic inspections.
The presence of detergent in the water found within the BDDV
cover indicated that aircraft washes (probably using hose pipes) were the
cause. The water from the other failed BDDVs found as a result of the
weekly checks was not analysed; thus the link to aircraft washes was not
confirmed in these cases. Indeed, it seems unlikely that it would have
been a factor in every case. Nevertheless, a high pressure hose would
seem the most likely means to introduce water via the junction between
the valve body and the cover.
There is also a possibility that a contribution could be made by rain
impinging on the BDDV during the short period the belly door is open
each time the landing gear is extended or retracted, or even while rolling
on the runway with the doors closed, since water can filter in through the
slots between the doors when closed. In this way, detergent residues
dried on the exterior fuselage or exposed landing gear could have
become liquid again when combined with water spray and thus found
their way into the BDDV through the defective valve sealing.
The sealant bead between the valve body and cover probably
contributed little in the way of excluding water due to the lack of
adhesion between the sealant and the smooth external finish on the
surfaces. The internal components of the valve are machined to very
close tolerances and the use of sealant reflects the manufacturer’s desire
to exclude the risk of any contamination through any gaps between the
valve body and cover. The interim Service Bulletin counterproductively
involved increased risk of contamination by drilling a 6 mm diameter
hole. The final solution hopefully has eliminated this risk.
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2.2. Actions by the Crew.
2.2.1. Flight Deck Crew Procedures.
Regarding Autobrake Low selection, there was no reference in the
FCOM to a specific method of pressing the Autobrake selector button.
The crew member performing this selection may make a quick or a slow
button-push, and even the duration of the button-push might in some
cases be affected by a vibration or movement of the aircraft. A
minimum time of 1 second has been introduced by Temporary change
notice to the FCOM, make definitively by OEB nº 137/1 of September
98.
The only reference in the ATA 32, Landing Gear, warnings and
precautions section of the FCOM Vol 3 Section 2 regarding BSCU CH
1(2) FAULT indicated that such fault message was for crew awareness.
No other specific action was recommended, even though switching the
BSCU-A/SKID&N/W STRG switch OFF then ON again would have
reset the system logic and cleared the fault condition.
Regarding BSCU resetting (switching OFF and then ON the
A/SKID & N/W STRG selector-switch), the information appearing in the
FCOM, Volume 3, Section 4, Supplementary Techniques, indicates that in
the case of difficulties with the brakes and/or steering the BSCU may be
reset, in particular in the cases in which any of the following ECAM
warnings is displayed:
WHEEL N.W.STEER FAULT
BRAKES AUTO BRAKE FAULT
BRAKES BSCU CH 1 (2) FAULT
On the ground, reset with the aircraft stopped; in flight, reset with the
landing gear retracted; and no resetting is to be made in the event of an
AUTO BRAKE FAULT in order to avoid clearing an actual tachometer
fault. Also, there is an explanatory Note reminding that BSCU resetting on
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the ground with the aircraft moving is not recommended, although it may
be attempted if both channels, 1 and 2, are lost and the crew cannot keep
the aircraft within the runway and provided that care is taken to ensure
that during resetting the nose wheel is at neutral position, the rudder
pedals are at neutral, and the brake pedals are released.
The crew were not aware of the presence, in the Supplementary
Techniques Section 4 of the FCOM Volume 3, landing gear, LOSS OF
BRAKING: In a case of extreme emergency, and only if the pedals are
ineffective, with the BSCU switch in OFF, the aircraft may be stopped
with the Parking Brake (full pressure application will occur); ie the
possibility of using the parking brake in the event of extreme
emergency.
There was no reference whatsoever, in the FCOM Vol 3 Section 2
Abnormal and Emergency Procedures section regarding the crew action to
be taken in the event of a loss of braking, as was experienced on this
occasion. This action is now a 'Memory Recall' item and has been
included in the Abnormal & Emergency Procedures Section.
　
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