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Parking Brakes On Airliners  
User currently offlineAF1624 From France, joined Jul 2006, 654 posts, RR: 0
Posted (1 year 7 months 2 weeks 3 days ago) and read 7269 times:

Hello everyone,

A small question regarding parking brakes on airliners (A3xx, B7xx mostly).

I noticed that when you set the parking brakes on these airliners, the brakes are placed in use at almost their maximum capacity. You can see that with the brake pressure gauge on the Airbus 320, 30, 40, 80 series cockpit - it just shoots all the way up with parking brakes set.

I believe the behaviour is very similar with Boeings as well.

My question is: why? Surely you don't need that much brake pressure to keep the aircraft at a stand still. I do know that very often the parking brake is disengaged when the chocks are in, but still !

Why do the parking brakes apply more braking pressure than, say, what's needed during landing? What's the logic behind that?

Thanks !


Cheers
42 replies: All unread, showing first 25:
 
User currently offlineGlobalMoose From United States of America, joined Aug 2012, 28 posts, RR: 0
Reply 1, posted (1 year 7 months 2 weeks 3 days ago) and read 7230 times:

I'm not too spun up on the workings of large pax transport but in all of the aircraft that I have flown (everything from small prop jobs, business jets, to heavy jet transports) the parking brake is not a separate brake system like most of us are used to in a car.

Every time we would want to apply the parking brake, we would fully depress the brake pedals (the tips of the rudder pedals) and then activate the parking brake which held the normal wheel brakes on while we desired to keep the aircraft stationary. The parking brake 'catch' would not engage until the brakes were fully depressed.

This might help to explain why you see the brake px increase to max, but I'm still interested in the design aspect myself! Hopefully there are some engineers on the forums.



When it absolutely positively has to be there ... at some point.
User currently offlinehorstroad From Germany, joined Apr 2010, 245 posts, RR: 0
Reply 2, posted (1 year 7 months 2 weeks 2 days 22 hours ago) and read 7185 times:

as said above the parking brake is just the very same brakes as the normal brakes, you just lock the hydraulic pressure inside... so when 3000psi do the job not worse than let´s say 1000psi, why would you want to reduce pressure for the parking brake, when 3000psi are available anyways?

most important is that the extra pressure is to compensate internal and external leakage. over a certain amount of time you might lose some % of pressure through the valves or the brake cylinders. if you have some extra pressure this is irrelevant (almost).



something interesting: many big aircraft brakes have two hydraulic systems powering them, or at least two separate circuits in the brake. on the A340 (don´t know about other A or B) you never power both. if you do so you will compress the brake so much that the cylinders won´t retract anymore... so basically on these brakes you never apply more than 50% of the possible brake force


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 3, posted (1 year 7 months 2 weeks 2 days 22 hours ago) and read 7182 times:

Quoting AF1624 (Thread starter):
My question is: why? Surely you don't need that much brake pressure to keep the aircraft at a stand still.

It's easy. It doesn't hurt the brakes or wheels and it doesn't require any metering...anything different would require some other regulated component to drop hydraulic pressure down to desired brake pressure.

When it's no longer easier (i.e. electric brakes) they don't bother...a 787 with parking brakes set only has the brakes at 30% power.

Tom.


User currently offlineDaleaholic From UK - England, joined Oct 2005, 3207 posts, RR: 13
Reply 4, posted (1 year 7 months 2 weeks 2 days 21 hours ago) and read 7143 times:

Who's to say the chocks won't be removed? It does happen from time to time when impatient people will take chocks from a parked aircraft. Very naughty!

Also, some gates can be on a slope... It might only be slight but to an aircraft it can feel quite steep so the parking brake + chocks ensures the aircraft won't move.



Religion is an illusion of childhood... Outgrown under proper education.
User currently offlineAF1624 From France, joined Jul 2006, 654 posts, RR: 0
Reply 5, posted (1 year 7 months 2 weeks 2 days 21 hours ago) and read 7132 times:

Quoting GlobalMoose (Reply 1):
Quoting horstroad (Reply 2):
Quoting tdscanuck (Reply 3):

Thanks ! Makes a lot of sense really.



Cheers
User currently offlinezeke From Hong Kong, joined Dec 2006, 8880 posts, RR: 75
Reply 6, posted (1 year 7 months 2 weeks 2 days 21 hours ago) and read 7135 times:

Quoting AF1624 (Thread starter):

I noticed that when you set the parking brakes on these airliners, the brakes are placed in use at almost their maximum capacity. You can see that with the brake pressure gauge on the Airbus 320, 30, 40, 80 series cockpit - it just shoots all the way up with parking brakes set

On Airbus FBW aircraft the pressure that is indicated on the triple indicator is the brake accumulator pressure on the top, and the alternate brake hydraulic pressure. During normal brake pedal use, the brake pressure gauge is supposed to read zero.

The park brake on the Airbus FBW aircraft uses the alternate brake system, it is a backup to the normal system. The gauges for each system are used during emergency procedure where normal braking is not available. The PM calls out to the PF the pressure, with the idea of trying to exert a constant 1000 psi into the brakes. The accumulator pressure, which is indicated at the top of the triple indicator maybe the only hydraulic pressure available.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlinen901wa From United States of America, joined Oct 2009, 457 posts, RR: 0
Reply 7, posted (1 year 7 months 2 weeks 2 days 19 hours ago) and read 7020 times:

Something else to consider. The Parking Brakes are also used for Engine runs and Hi Power Eng runs. When I do a after eng change run ( pre-Flight Check ), we set the Parking brakes with a certain amount of fuel to perform the checkout. The fuel load changes from acft to acft. The 757 at takeoff power is quite a handfull if too light, and I have seen a 757 drag the wheels on a wet runway. The one Aircraft that has to have the brake Pedals pushed and not use the Parking brakes is the A320. The Parking brakes are not using the full brake accumulator pressure, but a lower metered Pressure, and in some manuals you need a special Big Chocks to put on the mains, when doing a high power run in a A320. The most impressive high power run that had me worried, was a MD-11 & 767 that Pratt had a special plug for the EEC that took the P&W 4000 to 110%. On the wing eng we had to take the other side eng to 80% N1 to keep the plane from going round. I'll never forget those runs.
Just some info that might help  


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (1 year 7 months 2 weeks 2 days 18 hours ago) and read 6985 times:

Quoting n901wa (Reply 7):
Something else to consider. The Parking Brakes are also used for Engine runs and Hi Power Eng runs.

This is why the 787 parking brakes run up to 100% as soon as the throttles come off idle.

Tom.


User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 9, posted (1 year 7 months 2 weeks 2 days 17 hours ago) and read 6921 times:

On most types....Setting the Parking brakes is like deppressing the Brakes & locking it in that position.....If hydraulics is switched off as in long halts.....it could require reapplication with Hyd pr to compensate for any leaks.


Think of the brighter side!
User currently offlineAF1624 From France, joined Jul 2006, 654 posts, RR: 0
Reply 10, posted (1 year 7 months 2 weeks 2 days 1 hour ago) and read 6667 times:

Wow that's all very interesting, I hadn't thought of the high powered runs.

Also interesting to know that the 787 has electric brakes!

Random question, does anyone know of any aircraft having air brakes? Not as in "spoilers", but as in "truck style" brakes.



Cheers
User currently offlineboeing767mech From United States of America, joined Dec 2000, 1025 posts, RR: 3
Reply 11, posted (1 year 7 months 2 weeks 1 day 23 hours ago) and read 6633 times:

Quoting AF1624 (Reply 10):
Random question, does anyone know of any aircraft having air brakes? Not as in "spoilers", but as in "truck style" brakes.

I talked to a mechanic here at my place of employment he said the Fokker F-27 had pnuematic brakes.

David



Never under-estimate the predictably of stupidty
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 12, posted (1 year 7 months 2 weeks 1 day 22 hours ago) and read 6601 times:

Quoting AF1624 (Reply 10):
Also interesting to know that the 787 has electric brakes!

There was considerable skepticism about those (on a.net and elsewhere) back in the day. During early build there was also press about having to revise the brake software, although I can't find any evidence that that actually led to any delays. However, I'd say they're fantastic. They don't bleed down, so parking brakes are actually parking brakes, they eliminate a ton of hydraulic components and tubes, they don't leak, they're much easier to control, and you can deactivate individual actuators (as opposed to entire brakes) so you've got a lot more MEL options. I'm a fan.

The only big "gotcha" I've seen on them so far is in heavy maintenance...if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Tom.


User currently offlinezeke From Hong Kong, joined Dec 2006, 8880 posts, RR: 75
Reply 13, posted (1 year 7 months 2 weeks 1 day 17 hours ago) and read 6482 times:

Quoting AF1624 (Reply 10):
Random question, does anyone know of any aircraft having air brakes? Not as in "spoilers", but as in "truck style" brakes.

Pretty sure the de Havilland Dove and Heron had pneumatic gear, flaps, and brakes.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineTristarsteve From Sweden, joined Nov 2005, 3982 posts, RR: 34
Reply 14, posted (1 year 7 months 2 weeks 1 day 16 hours ago) and read 6456 times:

Quoting boeing767mech (Reply 11):
I talked to a mechanic here at my place of employment he said the Fokker F-27 had pnuematic brakes.

The F27 had no hydraulic system, it was all air driven.
The accumulators were charged by an engine driven pheumatic pump, but it took a while. Gulf Air used to operate the F27 BAH-DHA-BAH , with flight times of around 10 mins each way. We had to recharge the air accumulators from a bottle every BAH turnround.

Quoting HAWK21M (Reply 9):
On most types....Setting the Parking brakes is like deppressing the Brakes & locking it in that position.....If hydraulics is switched off as in long halts.....it could require reapplication with Hyd pr to compensate for any leaks.

Thats the Boeing way. Airbus is different, you just turn the park brake from off to on. And Airbus brakes are still on the next morning!!


User currently offline737tdi From United States of America, joined Sep 2007, 795 posts, RR: 1
Reply 15, posted (1 year 7 months 2 weeks 1 day 12 hours ago) and read 6366 times:
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Quoting tdscanuck (Reply 12):
There was considerable skepticism about those (on a.net and elsewhere) back in the day. During early build there was also press about having to revise the brake software, although I can't find any evidence that that actually led to any delays. However, I'd say they're fantastic. They don't bleed down, so parking brakes are actually parking brakes, they eliminate a ton of hydraulic components and tubes, they don't leak, they're much easier to control, and you can deactivate individual actuators (as opposed to entire brakes) so you've got a lot more MEL options. I'm a fan.

The only big "gotcha" I've seen on them so far is in heavy maintenance...if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Tom.

Kinda like jacking a 737 without pulling the correct breakers, you end up with a helluva mess if you have to swing the gear. LOL.


User currently offlineJAGflyer From Canada, joined Aug 2004, 3501 posts, RR: 4
Reply 16, posted (1 year 7 months 1 week 6 days 10 hours ago) and read 6140 times:

On the 737NG the brakes are depressed (the same way you'd push them to stop the plane when it's taxiing) then the lever is pulled. This lever will cause the brake pressure to not release. If the hydraulic systems are shut off (ie. when the APU is turned off) will the pressure stay in the brakes to hold the plane? I would think the parking brake is just simply a valve which closes to keep the fluid pressure from escaping.


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User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 17, posted (1 year 7 months 1 week 6 days 3 hours ago) and read 6015 times:

Quoting boeing767mech (Reply 11):
I talked to a mechanic here at my place of employment he said the Fokker F-27 had pnuematic brakes.

Those Pneumatic brake systems on that type was a tough snag to troubleshoot.

Quoting tdscanuck (Reply 12):
if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Normally the precaution prior to Fuselage Jacking.

Quoting Tristarsteve (Reply 14):
Thats the Boeing way. Airbus is different, you just turn the park brake from off to on. And Airbus brakes are still on the next morning!!

Interesting....



Think of the brighter side!
User currently offlineMax Q From United States of America, joined May 2001, 4384 posts, RR: 19
Reply 18, posted (1 year 7 months 1 week 6 days 3 hours ago) and read 6011 times:

Quoting tdscanuck (Reply 12):

There was considerable skepticism about those (on a.net and elsewhere) back in the day. During early build there was also press about having to revise the brake software, although I can't find any evidence that that actually led to any delays. However, I'd say they're fantastic. They don't bleed down, so parking brakes are actually parking brakes, they eliminate a ton of hydraulic components and tubes, they don't leak, they're much easier to control, and you can deactivate individual actuators (as opposed to entire brakes) so you've got a lot more MEL options. I'm a fan.

The only big "gotcha" I've seen on them so far is in heavy maintenance...if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Tom.


Tom, what happens if and when the 787 loses all electric power at the gate ?

For example, external power trips off and APU / Engine generators all off.


Does the parking brake remain set using battery power ?



The best contribution to safety is a competent Pilot.
User currently offlinetitanmiller From United States of America, joined May 2006, 89 posts, RR: 0
Reply 19, posted (1 year 7 months 1 week 6 days 2 hours ago) and read 6009 times:

How long does it take the hydraulic pressure to bleed off if an aircraft sits for an extended period of time with the parking breaks set?

User currently offlineDarksnowynight From United States of America, joined Jan 2012, 1347 posts, RR: 3
Reply 20, posted (1 year 7 months 1 week 5 days 23 hours ago) and read 5970 times:

Quoting horstroad (Reply 2):
so when 3000psi do the job not worse than let´s say 1000psi, why would you want to reduce pressure for the parking brake, when 3000psi are available anyways?

Often times you don't want any pressure on them. The Airbus 330 & 340s I used to work specifically called out for breaks release after chocks are set. On short turns, this is necessary to allow adequate break cooling.

Quoting horstroad (Reply 2):
most important is that the extra pressure is to compensate internal and external leakage. over a certain amount of time you might lose some % of pressure through the valves or the brake cylinders. if you have some extra pressure this is irrelevant (almost).

Chocks generally solve that concern.

Quoting n901wa (Reply 7):
The one Aircraft that has to have the brake Pedals pushed and not use the Parking brakes is the A320.

DHC-8 100, 200 & 300 all work that way as well.

Quoting tdscanuck (Reply 12):
The only big "gotcha" I've seen on them so far is in heavy maintenance...if you jack the airplane with the brakes on (not very unusual) then depower the airplane (also not unusual) you can't dejack without powering back up because you have no way to release the brakes.

Any chance they'll ever put a manual release feature for just that contingency. I understand that should be an avoidable encounter, but shit, as they say, happens.



Posting without Knowledge is simply Tolerated Vandalism... We are the Vandals.
User currently offlinem1m2 From Canada, joined Dec 2011, 84 posts, RR: 0
Reply 21, posted (1 year 7 months 1 week 5 days 18 hours ago) and read 5865 times:

I've done many run-ups on the Dash 8 100 and 300 using the parking brake. Are you saying you have to hold the toe brakes to do a run-up? I do keep my feet on the pedals just to be safe though, 4600 horsepower is something to be respected.

User currently offlineyeelep From United States of America, joined Apr 2011, 650 posts, RR: 0
Reply 22, posted (1 year 7 months 1 week 5 days 17 hours ago) and read 5859 times:

Quoting JAGflyer (Reply 16):
On the 737NG the brakes are depressed (the same way you'd push them to stop the plane when it's taxiing) then the lever is pulled. This lever will cause the brake pressure to not release. If the hydraulic systems are shut off (ie. when the APU is turned off) will the pressure stay in the brakes to hold the plane?

Yes. In a proper functioning system the parking brake will hold for a minimum of eight hours with "B" hydraulics off.

Quoting JAGflyer (Reply 16):
I would think the parking brake is just simply a valve which closes to keep the fluid pressure from escaping.

On the NG, when the parking brake lever is set, the rudder/brake pedals are mechanically held down which keep the metering valves open and pressure applied to the brakes. At the same time there is a parking brake shutoff valve that closes the return line from the normal antiskid valves, otherwise there would be unacceptable leakage through them.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 23, posted (1 year 7 months 1 week 5 days 17 hours ago) and read 5844 times:

Quoting Max Q (Reply 18):
Tom, what happens if and when the 787 loses all electric power at the gate ?

For example, external power trips off and APU / Engine generators all off.

Does the parking brake remain set using battery power ?

Basically, in the event of an external failure, they'll go to battery. In the event of a battery failure they'll just stay where they were.

The brakes run off the battery bus, so you can use the brakes just by turning the battery on. If the brakes were already set when the power went off, they'd just stay set. The brake actuators don't backdrive; the brakes can't back off without electric power to the motors. The only way you could end up without full braking is if you set the brakes right on arrival then immediately lost power...as the brake stack cooled the clamping force would drop off. As long as you have power (even battery power will do) the brakes will adjust to the cooling for a while after setting the parking brake (something like an hour, I think).

Quoting titanmiller (Reply 19):
How long does it take the hydraulic pressure to bleed off if an aircraft sits for an extended period of time with the parking breaks set?

Depends on how well the hydraulics have been taken care of. Typically hours.

Quoting Darksnowynight (Reply 20):
Any chance they'll ever put a manual release feature for just that contingency. I understand that should be an avoidable encounter, but shit, as they say, happens.

As a single-step release, I doubt it. That's a lot of work for a rare (and preventable) occurence. If you're really stuck you could just remove the actuators or "hotwire" them by opening the connector at each actuator and applying DC directly to the actuator motors.

Tom.


User currently offlineKC135Hydraulics From United States of America, joined Nov 2012, 291 posts, RR: 0
Reply 24, posted (1 year 7 months 1 week 5 days 10 hours ago) and read 5784 times:

Oh exciting, brakes! Anyone wanna know how C-17 and KC-135 parking brakes work?! Totally different systems. 50s design and modern design... what a contrast. I love me some brakes.

25 tdscanuck : I still refuse to admit that C-17 landing gear can work at all, let alone anything attached to said gear. Rube Goldberg would have been horrified. To
26 Darksnowynight : It was company SOP, & on the run-up sheet. Not sure how "technically" necessary it was, but that's what we did. And yes, we also set the parking
27 yeelep : Yes!
28 KC135Hydraulics : It's amazing, that gear. All the doors are mechanically linked to the gear, so there is no requirement for sequence valves or additional hydraulics.
29 HAWK21M : Thats thru Accumulator pressure which leaks off over time,hence the need to reset the pressure & reapply brakes in 8 hrs.
30 yeelep : No need to reapply brakes. Simply pressurizing "B" system hydraulics will restore brake pressure.
31 DashTrash : You can set the parking brake on the Dash without touching the toe brakes. The parking brake handle is also the emergency brake, as in you can stop a
32 wingscrubber : Can't speak for all aircraft, but some of the designs I've worked on use an accumulator to compensate for park brake leakage, to ensure brakes remain
33 Post contains links BEG2IAH : I was lucky enough to see and touch B788 brakes still dirty from all the carbon dust when I visited my then client: http://www.safranmbd.com/activite
34 tdscanuck : There's three on each wheel. They go right through the wheel on the inside half of the hub...there's no way to see them on an installed wheel because
35 Post contains images BEG2IAH : Tom, thanks! I could almost bet who would respond first.
36 Post contains images HAWK21M : Not normally visible from the outside,although it depends on the type we are referring to.....The fuseable surface is visible in the pic above.
37 Post contains images n901wa : Hi HAWK21M, Nice picture. What kind of main wheel is that? Looks real Familiar. I was thinking 727 but the valve stem is on the inner half. Early 737?
38 m1m2 : Hi DashTrash, Have you ever applied the Dash parking brake while in motion? I have been told to have your hand on the glareshield if you do because it
39 Darksnowynight : Yup, I knew that. What I mean when I said "work that way" was as in one needs to maintain pressure on the pedals during an engine run-up. I have to s
40 DashTrash : I have. Not in an emergency however. You have to meter the hell out of it manually. If you ease it on VERY slow, it is not an abrupt stop. If you yan
41 HAWK21M : sure looks B727....maybe someone who worked on the type could state so.....
42 m1m2 : To DashTrash, mmm, glareshield! Bet that tastes awesome. Thanks for the relies, happy taxiing.
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