Airbus question from non-Airbus pilots
 

The following is an excerpt from a blog:

The question asked was why the system would deactivate the #4 brake. Having never flown an Airbus, I could only guess what was going on. Here's my theory:

When the wind lifted the wing, the #4 wheel came off the ground just enough for the anti-skid system to interpret it as a skidding tyre, and thus released the brake pressure. With conflicting signals from the air/ground sensing system nicely timed with the anti-skid logic, the system released the brake.

That's my explanation of why it was initially released. Now I'm wondering why it stayed released until maintenance reset the computers.

What do the Airbus experts say?

Edit: Seems pprune is preventing the blog address from being linked properly. Here it is:

flightlevel390.********.com/2012/03/approved-for-line-operations.html

******** = "blog spot" (without the space)

PS. can anybody tell me why the forum software is censoring a blog hosing website? Does the airline that shall not be named own that as well?

Have no idea, I've never managed to get the wind lift the wing on 319/20 after touchdown, even at the max demonstrated and I was never big on stick-in-the-wind technique. 321 is different? Sure, it's heavier. I especially have no idea how one lifts no4 tyre witho no3 remaining in ground contact. This occurrence actually might have happened (EDIT: could be antiskid computer malfunction that was wrongly attributed to normal operation when landing in bank) yet given the unintentional hilariousness of the rest of the blog, I have my doubts.

BRAKES AUTO BRK FAULT
Applicable to: ALL

L2 The AUTOBRAKE FAULT warning may be due to a failure of the autobrake mode itself, or to a brake released condition. The crew should,therefore, be prepared to counter a possible slight lateral drift at landing, by using the rudder.

we expect the airbus to pretty much do everything for us and pilots have become overly reliant on what it does when all is good and well. Thats why you have to know what the plane is doing and how to correct it, especially in critical phases of flight. Autobrake fault...no prob, simply put that landing gear on the runway and apply manual brakes...problem solved!!

As above, no doubt if the No4 wheel was the only one in contact with the runway then the BSCU would note a discrepancy between the Tacho on the individual wheels. As the other 3 would be similar the no 4 reading would be deemed unreliable and the 'Brake Released' which would have the knock on effect of the Autobrake becoming U/S.

I am not sure if the blog is correct though, for the ground spoilers to deploy it requires both main landing gear to be on the runway and the thrust levers to be in the idle position. You NEED the spoilers to deploy to activate the Autobrake. So the blog doesn't quite ring true when it says the Autobrake activated slightly before the spoilers deployed. The other wheels must of been off the ground for a considerable amount of time for this to be an issue.

I have had the Brake Released ECAM once, I was stationery on stand, released the park brake for push back and got a 'Brake Released' ECAM on the No 3 Wheel. Turned out to be a faulty TACHO on that wheel.

I read his blog fairly regularly and I've found a few things in there at times that I felt were at odds with the way Airbus say you should do things, but it's not a technical blog, it's written for readers with a variety of experience and very well written it is too.

Coincidentally, about a week after I read this blog entry, I had the same thing. We landed, a tiny dance on the main gear and we got the caution with the no. 1 brake failed almost immediately. No idea what caused it but it wasn't particularly exciting.

It did happen on take off run at low speed as well.
Airbus published following bulletin:

Description
Why conventional BSCU logic does not allow making the distinction between a simple auto brake fault and an auto brake fault combined with a wheel braking loss?

Further to pilot remarks, the operator would like to obtain some clarifications about BSCU behaviour regarding tachometer signal failure, compared to the 3 different configurations of Atheir fleet (conventional BSCU, EMM BSCU fitted on conventional A/C and full EMM BSCU and A/C).

The main point is about flight crew information during flight when an AUTO BRAKE FAULT warning is displayed. In this case failure can be originated by a tachometer failure (TACHOMETER (XX) GG OR BSCU) or another equipment of the auto brake system.

On conventional BSCU, a tachometer signal failure with BRAKE AUTO BRAKE FAULT warning leads to loss of the auto brake function but also loss of the braking on affected wheel. In this case the crew is not formally aware of the wheel braking loss.

On EMM A/C and BSCU, according to operator's experience this failure is also associated with warning "BRAKE RELEASED"

Solution
With the BSCU EMM, a specific discrete sent to the SDAC has been created to monitor the "brake release". This discrete (brake release fault) is true in case of a tachometer failure or servovalve failure without a normal brake fault. This discrete allows to indicate to the pilot an "autobrake fault" due to a tachometer or a servovalve failure.

The ECAM message is the alert BRAKES RELEASED if FWC std E4 (or following) is installed with the pin program "NORM BRK IMPROV OPTION INSTALL" grounded on pin AA11C.

With the BSCU OLD, the distinction is not realized with for example an autobrake fault due to a COM/MON disagreement or an ELAC failure.

However, the ECAM display will inform pilot that brake is released :

BRAKES AUTOBRK FAULT

BRAKE RELEASED

Hope that helps a bit.:ok:

Appreciate the info guys.