747-400 Park Brake

[NSEU]

Leafing through my training notes on the 747-400 Park Brake, I started to wonder why the Park Brake Valve is really required.

The notes tell me that the park brake valve is situated in the hydraulic return line attached to the Normal Antiskid Control Module/s. When antiskid is activated during a skid, the brake fluid is ported away from the brakes through antiskid valves, through the hydraulic return line and then through a normally open park brake valve, back to the hydraulic system. At the gate however (wheelspeed less than 8kts), as far as I can see, the antiskid control module will keep the fluid return ports closed and all the pressure coming from the antiskid module will be applied to the brakes.

Basically, is the Park Brake Valve only there as a backup, or have I overlooked something? Maybe the antiskid valves are, by design, slightly leaky?

Thanks.

Cheers.
NSEU

[unwashed]

The parking brake is a mechanical mechanism not reliant upon electrical input. The Antiskid module needs an electrical input to function. Simplistic as it seems that is how I have come to grips with it.

[NSEU]

The Antiskid module needs an electrical input to function.

Understood, Unwashed, but in the absence of electricity (or an antiskid control electrical signal), I would have thought that all the fluid going into the antiskid module would have been ported to the brakes(?). In the event of an antiskid failure, I assume that all pressure would be allowed to reach the brakes, rather than the opposite (no pressure at all).

Thanks.

Cheers
NSEU

[SNS3Guppy]

The parking brake is mechanical, and electrical. Mechanically, the parking brake lever is linked to the brake input on the rudder pedals. It also electrically operates the parking brake valve, which is located in the anti-skid return line. The parking brake valve prevents loss of accumulator pressure for fluid leaking past the antiskid valves.

[NSEU]

The parking brake valve prevents loss of accumulator pressure for fluid leaking past the antiskid valves.

Thanks, Guppy. So you're saying that leakage is normal and more a design thing rather than an aircraft age/condition issue?

I guess you could say that this is more a hypothetical situation. If the park brake valve failed to close, would the brakes still hold for a certain time period (or are these leaks monumental)?

Cheers.
NSEU

[NSEU]

I think I've found part of the answer to my original question.

The antiskid valves should be closed below 8kts wheel speed (system logic). However, the operation of the antiskid module is dependent on more than just electronic signals from the BSCU (Brake System Control Unit) and metered brake pressure (controlled by pedal input).

An active source of hydraulic pressure directly from the #4 system is also required to operate the normal antiskid valves. If this source is absent, the spools in the second stage of the antiskid modules may possibly reposition to allow fluid to escape into the return lines (feeding the Park Brake valve). If the park brake valve is open, then pressure cannot be held.

My initial tests on the aircraft (with no active source of hydraulic pressure) seem to prove this theory. There are no leaks per se, just no proper control of the antiskid valves. The accumulator pressure bleeds off very quickly if the Park Brake Valve is held open (by pulling the circuit breaker prior to parking the brakes)

My next tests will involve an active source of pressure. I'm hoping this will be more effective in holding pressure on the brakes (even though the Park Brake Valve is open).

Cheers.
NSEU

[EW73]

I recall the '74 parking brake system as follows...please correct if my memory fails me...

The individual anti-skid valves are closed until commanded open to release some small amount of pressure to get the skidding wheel to rotate again...

This small amount of pressure results in a small amount of hyd fluid being released back to reservoir, tracking via the park-brake valve (which would at this point be open).

At the gate (or anywhere else), when the park brake is set, the rudder pedals (one or both) are depressed and mechanically held there by selection of the park brake.
That allows hyd pressure (with accumulator pressure when the system is not pump pressurized) to be directed to the brakes selected by, one or both rudder pedals that are depressed.
The pressure is trapped between the park brake valve and the brake units to keep the brake applied.
If you release either the pedals (the pressure supply valves), or the pressure return (the park brake valve), the pressure between those two places is released, i.e. the brakes are released.
The problem here is, when you release the park brake to, say, enter and line up, and the rudder pedals release but the park brake valve does not (i.e. it stays closed), the park brake pressure will release and you will roll for the line-up to takeoff....BUT

..with the park brake valve still closed, any operation of the anti-skid valves to release any skidding wheel will have nowhere to go, that is, the anti-skid system will not work, and there is a flight deck indication when this malfunction occurs.
If it does occur, simply cycling the park brake will fix in most cases.

If it does occur, no real problem for takeoff (or landing), just keep in mind that you will have no anti-skid system available, and remind yourself how much runway in excess of that you really need, is available.

EW73

[NSEU]

Thanks EW73.

Just a few comments....

It's impossible to use only one pedal to park the brakes. If you push only one pedal, the unraised pawl on the un-pushed pedal mechanism interferes with the locking of the pedals (the Maintenance Manual even asks you to try one pedal parking.. if you can, something is broken)

Also, I proved today on one aircraft, that with an active #4 hydraulic system, the park brake valve can be wide open (c/o of a pulled Park Brake CB) and you can still park the brakes. I don't think it's a case of a constant supply of pressure overcoming leaky antiskid valves, I think the antiskid valves are completely shut, which prevents pressure bleeding into the return lines (and past the Park Brake Valve).

With only accumulator supply feeding the brakes and antiskid module, the antiskid valves can leak (so fast, in fact, within 45 seconds, the brake pressure indication is down to almost the nitrogen charge pressure on the accumulator).

Hope this makes sense.
Cheers.
NSEU.

[spannersatcx]

I think the antiskid valves are completely shut

go down and feel the hyd pipes that will tell you.

With the park brake valve open, due cb pulled, and with the brakes set and constant no4 hyd sys supply the brakes will be on as the input from the pedals IS applying the brakes, just as it you had your feet on them, if you released the sys pressure it would soon bleed away.

[NSEU]

go down and feel the hyd pipes that will tell you.

Thanks, Spanners, but what am I feeling (temperature? flow vibration?).

Do you mean the return line/s from the Antiskid Module?

Sorry, this is outside my usual category

Cheers.
NSEU

[spannersatcx]

if fluid is passing through the return lines will be hot as will the valves, you will get dirty mind as it's not your usual avionic bits!