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rjsampson
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Mechanics of the Parking Brake

Mon Feb 03, 2020 8:05 pm

I've always wondered how exactly the Parking Brake works, and suspect the mechanism could be different among aircraft types.

Unlike, say, a car's parking brake: Pilots are not using any mechanical force to engage so: I would assume they are hydraulically actuated? If in fact that is the case: How is that hydraulic pressure maintained when the aircraft is cold and dark?

As for the brake itself: Is it basically the equivalent to pushing the brake pedals to the floor (ie, all of the same calipers, rotors lock the in same way), or is there a separate mechanism specific to the parking brake?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
GalaxyFlyer
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Re: Mechanics of the Parking Brake

Mon Feb 03, 2020 8:24 pm

Exactly, press the pedals, pull on the handle to trap the hydraulic pressure holding the pads against the rotors. Accumulators are used when there’s no pumps operating. Yes, they’ll bleed down after awhile, hence chocks.
 
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Starlionblue
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Re: Mechanics of the Parking Brake

Mon Feb 03, 2020 10:58 pm

On the A330, the park brake works off the blue hydraulic system, while normal braking is off the green system. (Alternate braking is also off the blue system.) So you don't really need to hold the pedals down. It is good practice, though, since it ensures you are completely stopped and prevents jerking. (Also look out the side window to see if there is any residual motion.)

If the blue system is unpowered, the park brake runs off the accumulator.

The A320 system pictured here is identical apart from running off the yellow system instead of blue.

Image
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rjsampson
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Re: Mechanics of the Parking Brake

Mon Feb 03, 2020 11:22 pm

So, given that the brakes don't necessarily need to be held down (and this being an Airbus): I'm guessing that when you turn the parking brake knob: This is in effect "parking brake Set by Wire?" And how long would the accumulator be able to hold hydraulic pressure (not that aircraft stay on the ground for very long these days).
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
Flow2706
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Re: Mechanics of the Parking Brake

Tue Feb 04, 2020 7:09 am

The accumulator will hold the brake pressure for a minimum of 12 hours. However setting and releasing the parking brake will deplete the accumulator rather quickly (the accumulator is able to provide 7 brake applications in the unlikely event that both green and yellow systems are inoperative).
 
Tristarsteve
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Re: Mechanics of the Parking Brake

Tue Feb 04, 2020 10:14 am

Until recently there were two types of park brake.
First B737 and A320 are different. B737 has a single set of brake pistons, with a shuttle valve to primary or standby supply.
A320 has two sets of brake pistons, One primary, and one standby
The B737 uses the foot brakes. You push the pedals down, then pull the park brake lever to lock the pedals down. The pressure is trapped in the lines and the brakes stay on for a few hours (not overnight). Most older Boeings are the same.
The A320, the park brake is a rotary switch on the pedestal that opens the valve and connects the accumulator to the standby brakes. This stays pressurised longer than overnight. Other later airbuses are similar.
B787 is different.
 
Dogbreath
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Re: Mechanics of the Parking Brake

Tue Feb 04, 2020 3:13 pm

The 787 has electric brakes and therefore no hydraulics or brake accumulator. When the Parking Brake is set and Thrust Levers are at Idle the EBA's (Electric Brake Actuators) only use a 50% clamping force. If a Thrust Lever is set to above Idle the clamping force increases to 100% to allow for an engine run scenario.
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basspaul
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Re: Mechanics of the Parking Brake

Tue Feb 04, 2020 4:08 pm

Dogbreath wrote:
The 787 has electric brakes and therefore no hydraulics or brake accumulator. When the Parking Brake is set and Thrust Levers are at Idle the EBA's (Electric Brake Actuators) only use a 50% clamping force. If a Thrust Lever is set to above Idle the clamping force increases to 100% to allow for an engine run scenario.


Though I don't the logic of operation, the CSeries has electric braking as well.
 
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fr8mech
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Re: Mechanics of the Parking Brake

Wed Feb 05, 2020 1:50 am

While the parking brake will hold for 12 or so hours, it is rarely used that way...at least at the places I've worked. After the aircraft has parked and is chocked, the parking brake is released. The most common reason I've heard is so that the disks won't warp as the brakes continue to heat and then cool.

An added concern on the MD11 is loading of fueling with the brake set. As the weight increases, the center gear tends to creep forward. If the brakes are set damage could occur. In fact, the AMM has an inspection that is to be performed on the center gear if it is fueled/loaded with the brakes set.
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Max Q
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Re: Mechanics of the Parking Brake

Wed Feb 05, 2020 4:04 am

Dogbreath wrote:
The 787 has electric brakes and therefore no hydraulics or brake accumulator. When the Parking Brake is set and Thrust Levers are at Idle the EBA's (Electric Brake Actuators) only use a 50% clamping force. If a Thrust Lever is set to above Idle the clamping force increases to 100% to allow for an engine run scenario.



Interesting,


What if the 787 is parked with APU off and no external electric power connected

How does the parking brake remain engaged?
The best contribution to safety is a competent Pilot.


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zeke
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Re: Mechanics of the Parking Brake

Wed Feb 05, 2020 4:32 am

Max Q wrote:
What if the 787 is parked with APU off and no external electric power connected

How does the parking brake remain engaged?


Maybe its like a normally closed solenoid, requires power to open.
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fr8mech
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Re: Mechanics of the Parking Brake

Wed Feb 05, 2020 5:49 am

zeke wrote:
Maybe its like a normally closed solenoid, requires power to open.


I suspect the valve and/or brake system controller are connected to the Hot Battery Buss.

Just a guess, if in fact, you can set the brakes with power off.
When seconds count...the police are minutes away.
Unless it's expressly prohibited, it's allowed.
You are not entitled to a public safe space.
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Flow2706
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Re: Mechanics of the Parking Brake

Wed Feb 05, 2020 6:47 am

fr8mech wrote:
While the parking brake will hold for 12 or so hours, it is rarely used that way...at least at the places I've worked. After the aircraft has parked and is chocked, the parking brake is released. The most common reason I've heard is so that the disks won't warp as the brakes continue to heat and then cool.

An added concern on the MD11 is loading of fueling with the brake set. As the weight increases, the center gear tends to creep forward. If the brakes are set damage could occur. In fact, the AMM has an inspection that is to be performed on the center gear if it is fueled/loaded with the brakes set.

An other reason is to allow for faster brake cooling (especially on aircraft without brake fans). The SOP is to set the parking brake before the walk around, as the brake pins (indicating brake wear) are not accurate with the parking brake off.
One of my former companies had an incident where two A330s collided due to some genius pulling the chocks without verifying that the parking brake was set...after that event it was not allowed anymore to release the parking brake once parked at the gate (for both A320 and A330 Fleets).
 
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jetmech
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Re: Mechanics of the Parking Brake

Wed Feb 05, 2020 10:10 am

Max Q wrote:
What if the 787 is parked with APU off and no external electric power connected. How does the parking brake remain engaged?

I suspect it's to do with the geometry of the gear train which has a ball-screw and nut to generate the required braking force.

https://www.flightglobal.com/787-specia ... 20.article

If the reduction ratio is high enough (ball-screw pitch fine enough) the system is irreversible without input from the motor.

Regards, JetMech
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Tristarsteve
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Re: Mechanics of the Parking Brake

Wed Feb 05, 2020 10:54 am

Not all airlines release the brakes at at the gate.
At British Airways, the crew set the brakes on arrival, and leave them on. They release them on departure when the ground crew tells them to do so for pushback.
It was decided that this was the safest option. There were enough incidents of aircraft moving with the brakes off. It was instigated about 10 years ago, and works.
 
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DarkSnowyNight
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Re: Mechanics of the Parking Brake

Thu Feb 06, 2020 6:34 am

fr8mech wrote:
zeke wrote:
Maybe its like a normally closed solenoid, requires power to open.


I suspect the valve and/or brake system controller are connected to the Hot Battery Buss.

Just a guess, if in fact, you can set the brakes with power off.


This is exactly the case. The 787 requires pwr to release brakes. It is actually noted on the Battery Change Task Card as well.



jetmech wrote:
Max Q wrote:
What if the 787 is parked with APU off and no external electric power connected. How does the parking brake remain engaged?

I suspect it's to do with the geometry of the gear train which has a ball-screw and nut to generate the required braking force.

https://www.flightglobal.com/787-specia ... 20.article

If the reduction ratio is high enough (ball-screw pitch fine enough) the system is irreversible without input from the motor.

Regards, JetMech


Correct, yes. It will lock into place.

Dogbreath wrote:
The 787 has electric brakes and therefore no hydraulics or brake accumulator. When the Parking Brake is set and Thrust Levers are at Idle the EBA's (Electric Brake Actuators) only use a 50% clamping force. If a Thrust Lever is set to above Idle the clamping force increases to 100% to allow for an engine run scenario.


Not only this, but as well, when parked and brakes are still hot, you will have 30% clamp force available -this is still more than sufficient to keep a stopped 787 from rolling. It does, however, gradually increase to a full 100% as the brake cools. This is pwr'd by the Hot Battery Bus, so the process will continue, even if the aircraft is fully pwr'd down.
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Max Q
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Re: Mechanics of the Parking Brake

Thu Feb 06, 2020 8:06 am

DarkSnowyNight wrote:
fr8mech wrote:
zeke wrote:
Maybe its like a normally closed solenoid, requires power to open.


I suspect the valve and/or brake system controller are connected to the Hot Battery Buss.

Just a guess, if in fact, you can set the brakes with power off.


This is exactly the case. The 787 requires pwr to release brakes. It is actually noted on the Battery Change Task Card as well.



jetmech wrote:
Max Q wrote:
What if the 787 is parked with APU off and no external electric power connected. How does the parking brake remain engaged?

I suspect it's to do with the geometry of the gear train which has a ball-screw and nut to generate the required braking force.

https://www.flightglobal.com/787-specia ... 20.article

If the reduction ratio is high enough (ball-screw pitch fine enough) the system is irreversible without input from the motor.

Regards, JetMech


Correct, yes. It will lock into place.

Dogbreath wrote:
The 787 has electric brakes and therefore no hydraulics or brake accumulator. When the Parking Brake is set and Thrust Levers are at Idle the EBA's (Electric Brake Actuators) only use a 50% clamping force. If a Thrust Lever is set to above Idle the clamping force increases to 100% to allow for an engine run scenario.


Not only this, but as well, when parked and brakes are still hot, you will have 30% clamp force available -this is still more than sufficient to keep a stopped 787 from rolling. It does, however, gradually increase to a full 100% as the brake cools. This is pwr'd by the Hot Battery Bus, so the process will continue, even if the aircraft is fully pwr'd down.



Thanks for the information


Interesting and a very clever design
The best contribution to safety is a competent Pilot.


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