A320 Park Brake Query

[SimJock]

Anyone help with this tech query ?

Say A320 is parked on a slope, park brake ON, engines on, all systems normal (simulated world :) ) , then you lose the brake accumulator function due to say the bag inside the accumulator losing its nitrogen, after that you lose the yellow hydraulics system pressure. What happens next with regard to the brakes and indications ?

Could it be:

a) Accumulator pressure shows zero, brake pressure shows as brakes on due to yellow return line being isolated thus trapping some yellow pressure in the system then once park brake released, brake pressure goes to zero and unable to re-apply park or manual brakes.

or

b) Accumulator pressure shows zero, brake pressure shows zero, brakes are off with park brake handle in ON position and aircraft rolls away.

or

c) Something else entirely ??


Ta for your thoughts.

[c130jage]

If the diaphragm inside the accumalators popped you would notice very little, if any change as you would still have 207 bar of n2 / hyd pressure (the n2 would not just dissapear instantly)
However if the accumalators were to instantly lose all pressure for some strange reason = well thats a different matter.

[Cardinal]

I'm going with Answer A, mostly.
a) Accumulator pressure shows zero, brake pressure shows as brakes on due to yellow return line being isolated thus trapping some yellow pressure in the system then once park brake released, brake pressure goes to zero and unable to re-apply park or manual brakes <-except for that.

No parking brake available, but normal braking throught the green system would still be available. You'd roll only until you sood on the pedals.

[SimJock]

Yep, thanks Cardinal, accepted that green is available for manual braking at this point.

The only bit I'm not 100% about is whether the yellow pressure is trapped and the brake stays on once yellow system is depressurised as I've not seen that written down anywhere as such. Manuals suggest that the park brake will stay applied for 12 hours if the system is within leak tolerances but that presumes I suppose that the accumulator is good.

I guess it would be a rare occurence to have an accumulator failure anyway.

[c130jage]

You are correct accumalators very rarely fail however accumalator diaphragms failure is not a rare occurance. There is no methord to stop the yellow pressure escaping except in the case of a ruptured hyd pipe by means of a valve in the line which senses the escape of fluid and closes.

[St. Ex]

If the green hydraulic system were available, wouldn't that pressurize the yellow system through the PTU?

[hetfield]

As long as the park brake is set the aeroplane will start moving.

Park brake inhibits normal braking!!!

[bishop99]

And if there is no pressure in the acc or green system, then the cockpit voice recorder test button wont work. ;-). (not relevant at all but o'well).

[shortfuel]

Park brake On inhibits normal braking only if actual brake pressure > 35 PSI.
If A/C rolls away, you probably have no more braking, you will instinctively press the pedals and it will work. But there is no way to check actual brake pressure hence the procedure (old MOD), PARK BRK OFF to recover normal braking.

On newer A/C and modified one, if PARK BRK ON and A/C starts moving, press pedals to recover green braking.

If you lose yellow hyd (even LO LVL but depending where the leak is, cf. story below), yellow fluid is trapped between accumulator and available for PARK BRK or ALTN BRK w/o ANTI-SKID thanks to shuttle valves.

It is difficult to answer your question precisely. You have to consider first your A/C version and MOD.

I see only one real case where they lost ACC braking, alternate braking (and normal braking...!): they landed uneventfully, vacated.
While taxiing, G HYD LO LVL. Stopped the A/C on park brake. Whille carrying out ECAM...Master Warning: G+Y HYD LO LVL. The a/c starts rolling, they had nothing to steer the A/C except ENG thrust and one REV and no braking means whatsovever. They luckily steered the A/C on the grass to stop it.
What happened: on (normal) touchdown, left MLG braking unit shattered creating a dual leak in both G+Y hyd lines. They have been able to brake normally during landing roll, once they vacated, braking had worsen the leaks, sensors tripped the ECAM, G first then Y couple of minutes later.
That's a case where you lose HYD at the worst place, just before the brakes.

I digressed a bit just to show you how the weirdest thing can happen, even on a bus! Your initial question SimJock reminded me that event.