A320 Brakes: Norm + Altn Fault

[Citation2]

FCOM says : Parking brake only.
Why braking on accumulator is not available ? I would appreciate if anyone can shed light on this
(Considering an electrical failure and not HYd)

Also on Systems description , it says : alternate braking without antiskid on accumulator is fully controlled by the ABCU. Does that mean that in case of ABCU failure , accu braking is lost?

On old 320's , with antiskid nosewheel steering switch to OFF , braking on accumulator was available.

On new 320's with ABCU fault +Norm brake fault would I be able to brake on accu by putting anti skid and nosewheel steering switch to OFF?

[vilas]

Citation2
Latest 1.8 std. FCOM says what you say. I am unable to understand why. I checked another manual which gives procedures according to manufacturers serial numbers where Norm+Alt fault is not mentioned at all. I don't see any reason why accumulator braking should not be available.

[Drifter72]

I just checked FCOM (available Braking Schematic): "Depending on the brake pedals’ demand, the ABCU controls the alternate brake selector and the alternate servovalves."

Both valves are in line and supplied from yellow system pump and accumulators through the same line.
Only parking brake control valve bypass those 2 valves and can transfer either normal yellow pressure or accu pressure to the brakes.

It looks like that if you can't control those 2 valves, and you can't without ABCU (alternate braking fault), the only solution is parking brake. All that of course only in combination with Normal brake fault.

Following that, in case of ABCU and Normal braking failure you will not get much by putting nosewheel steering switch to OFF because everything, related to brakes, that it controls is already unusable.

[TyroPicard]

NORM brake fault ... it won't work.
ALTN brake fault .. It won't work.
NORM + ALTN fault .. Neither works.

Only switch which will work the brakes is PARK BRK.
This in not to do with hydraulics, but electronic or mechanical faults.

[LEVEL600]

On enhanced 320 you have only electrical signal from brake pedals feeding ABCU or BSCU. There is no other way to control normal servo valves or direct drive valves than signal from these computers. Accumulator as pressure source is still available for alternate braking controlled by DDV if ABCU works or for ultimate emergency braking via park brake. You are all correct,but probably not happy with this brake system design...

[Citation2]

Thank you all for this clarification. I would have expected more redundancies in the system design .

[Microburst2002]

If something fails, it fails. It doesn't work. If both normal and alternate are faulty there are reasons for both failures and they are not available. End of the story.

However, the system is designed in such a way that you can still use the "ultimate" means to brake the airplane if you still have some pressure in the ACCU, that is, with the parking brake. I think it is a good system!

[WhiteKnight]

Just out of interest.

Does anybody know, why you only have accumulator braking with emergency elec config? ABCU is powered, as well as yellow hydraulics. What's the problem giving us alternate brake without antiskid but with yellow hydraulic pressure?
Any ideas?

[LEVEL600]

Alternate braking uses both accu and yellow syst. pressure,whichever is higher regardless of electrical configuration, I think. During elec emergency you loose power for BSCU which perform antiskid and this is reason for pressure limitation to 1000 psi.

[TyroPicard]

WhiteKnight
You are making assumptions...
Where does it say ACCU PRESS ONLY? Where does it say NORMAL BRK INOP or ALTN BRK INOP?
All FCOM says is MAX BRK PR ... 1000PSI
and in INOP SYS......ANTISKID

Keep up at the back!

[vilas]

WhiteKnight
It is alternate braking without antiskid. It is not accumulator braking.

[Citation2]

This is absolutely correct , notice the difference in G+Y summary it states clearly : "BRK Y ACCU PR only" whereas in EMER ELEC it says : " Alternate without antiskid "

This could also happen in combined failures like G sys lo press + DC ESS bus. With G you would loose Normal brake , with DC ESS you would loose ALTN brake.