How do jet aircraft parking brakes work?
 

Hello,

I am having a hard time understanding how jet aircraft parking brakes work. I do understand how the brake accumulator can store pressure…but I am having trouble figuring out the path hydraulic fluid follows when the parking brake is finally set.

I suppose an hydraulic actuator helps to push against the brake disks. But once the actuator reaches the desired position, is the system not de-energized, thus storing the hydraulic energy left in the brake accumulator ?

Any input would be highly appreciated.

Thanks !

As you guessed, airliner parking brakes are not like mechanical hand brakes on cars. (some small GA aircraft do have mechanical parking brakes, but most airliners don't). When airliner parking brakes are applied, hydraulic pressure presses the wheel brake packs from one end with hydraulic pistons, compressing the alternate 'layers' of friction material and metal/carbon discs together, preventing each braked wheel from turning.

When the system hydraulic line pressure fades away after the engines and electric pumps have stopped, pressure from the brake accumulator via a non return valve holds the parking brakes on. An hydraulic accumulator is a cylinder half filled with nitrogen gas under pressure, acting on a piston which separates the gas from the hydraulic fluid in the other half which is in turn is connected to the brake hydraulics via the non return valve and the parking brake valve. So the pressure of the compressed gas in the brake accumulator maintains pressure in the braking circuit of the hydraulic system - even with no pumps running. However, this pressure too eventually fades away, releasing pressure on the parking brakes, which is why all such aircraft have wheel chocks applied when they are not running.

After a period of standing, there may be no pressure left in the brake accumulator, so one of the tasks of the next pre-flight cockpit preparation is to check the brake pressure and run the electric hydraulic pump of the appropriate system if necessary to fully pressurise it again and recharge the brake accumulator. This is certainly what we do in Airbus FBW, I cannot remember now about Boeing 737 or other types.

Thanks for your answer Uplinker !

How does this pressure “fade” away? The hydraulic fluid previously stored in the accumulator runs out towards the brakes…but once the actuator has reached its position, ie. Compressing the brake disks, can’t the piston can remain in the given position, with the hydraulic fluid under pressure “blocked” inside the actuator? Any diagram would be highly appreciated. I might getting confused with where all this hydraulic fluid is going on.

Thanks !!

Erm, are we sure about that? My understanding of hydraulic brake systems is that the park brake pressure is locked in. IE even if the accumulater pressure were to drop, the locked in park brake presuure would remain.

For the OP
The B787 has electric brakes. The brake packs are pressed together by rotary electric actuators. When park brakes are selected they adjust themselves over time as the brake temperature drops, applying a continuous force.

Hope this helps.

That's where the NRV comes in.

Indeed. My thoughts too.

The way I understand it is - when you set the parking brake, hydraulic pressure pushes the brake pads out against the disks. The NRV then locks that pressure in the brake circuits even if the accumulator pressure drops over time.

Some newer aircraft like the 787 use electric actuators instead of hydraulics too. Regardless of the mechanism, it's storing braking force mechanically once engaged.

Well, on the Airbus documentation I have, (for A330), it shows a direct, open pressure path from the brake accumulators to the parking brake actuators when the parking brake handle is set to park brakes on. The alternate servo valves controlled by the BSCU are kept open, allowing accumulator pressure to bear directly on the parking brake actuators. The parking brake handle valve stays open when parking brakes are set on - it does not close to lock the pressure in.

PS, Airbus state that the accumulator will maintain parking brake pressure for "at least 12 hours", but aircraft are always routinely wheel chocked.
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Thanks for all the inputs. How can we explain the hydraulic “fade-out” over time if the actuators are blocked and the hydraulic fluid is “trapped”?

As I just posted, (we might have crossed), the hydraulic fluid is NOT trapped; the fluid path is open between the accumulator and the parking brake actuators. Trace the pipework in the diagram I just posted.

"fade out", i.e. pressure reduction, occurs due to micro leaks past the hydraulic seals. This is normal and not an issue when there is a constant supply of pressure from a pump, but if the only pressure supply is from a charged accumulator, that pressure will slowly diminish.

Thanks Uplinker. So if I get it correctly, there is constant hyd fluid presence in the duct from the accumulator to the parking brake actuator via the parking brake valve. Nevertheless, don’t we see a decrease in accumulator pressure as soon as we set the parking brake? ie. When the hydraulic fluid is released from the accumulator?

Finally, the faded out fluid due to leaks returns to the hydraulic tanks I suppose?

Thanks!

The accumulator is charged with full system pressure - of around 3000psi, (A330). This pressure pushes on the free dividing piston in the middle of the accumulator cylinder. This piston is free to move and holds nitrogen gas on one side and system hydraulic fluid on the other. So when the pumps are running, the hydraulic fluid pressurises the nitrogen gas to ~ 3000 psi.

When the hydraulic pumps stop, the nitrogen in the accumulator presses on the free piston to maintain ~ 3000 psi in the hydraulic fluid. When the parking brake is set on, this nitrogen gas 'spring' in the accumulator keeps the fluid in the park brake circuit pressurised.

If the brakes are repeatedly applied and released with the pumps off; the free piston in the brake accumulator will move a little each time until eventually it reaches the end of its travel inside the accumulator cylinder. At this point the accumulator cannot maintain the hydraulic system pressure. On Airbus, the brake accumulators can apply full brakes about 7 times before they become exhausted.

Yes, absolutely correct, my bad, I had a senior moment.

Thanks a lot Uplinker.

I am taking this opportunity to ask another question for the tripple. What could be the use of the source select valve and the accumulator isolation valve? Diagram gives the impression the brake accumulator will use the Normal Brake Metering Valve…

Update: unable to uppload FCOM screenshot as I am still a new user!
Landing Gear - System Description

This will have to wait until I can look in my FCOM tomorrow or you can post a picture, so I can see exactly which valves you are referring to.

Unless your the RCAF