Parking Brakes On Airliners

AF1624 · Mon Dec 17, 2012 11:27 am

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 !

GlobalMoose · Mon Dec 17, 2012 12:16 pm

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.

Horstroad · Mon Dec 17, 2012 1:42 pm

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

tdscanuck · Mon Dec 17, 2012 1:51 pm

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.

Daleaholic · Mon Dec 17, 2012 2:39 pm

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.

AF1624 · Mon Dec 17, 2012 2:40 pm

Quoting GlobalMoose (Reply 1):

Quoting horstroad (Reply 2):

Quoting tdscanuck (Reply 3):

Thanks ! Makes a lot of sense really.

zeke · Mon Dec 17, 2012 2:44 pm

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.

n901wa · Mon Dec 17, 2012 5:15 pm

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

tdscanuck · Mon Dec 17, 2012 5:51 pm

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.

HAWK21M · Mon Dec 17, 2012 7:19 pm

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.

AF1624 · Tue Dec 18, 2012 11:12 am

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.

boeing767mech · Tue Dec 18, 2012 12:21 pm

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

tdscanuck · Tue Dec 18, 2012 1:39 pm

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.

zeke · Tue Dec 18, 2012 6:42 pm

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.

Tristarsteve · Tue Dec 18, 2012 7:44 pm

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!!

737tdi · Wed Dec 19, 2012 12:07 am

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.

JAGflyer · Fri Dec 21, 2012 1:48 am

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.

HAWK21M · Fri Dec 21, 2012 9:02 am

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....

Max Q · Fri Dec 21, 2012 9:15 am

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 ?

titanmiller · Fri Dec 21, 2012 9:24 am

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?

DarkSnowyNight · Fri Dec 21, 2012 12:47 pm

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.

m1m2 · Fri Dec 21, 2012 6:15 pm

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.

yeelep · Fri Dec 21, 2012 6:43 pm

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.

tdscanuck · Fri Dec 21, 2012 7:07 pm

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.

KC135Hydraulics · Sat Dec 22, 2012 2:11 am

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.

tdscanuck · Sat Dec 22, 2012 4:50 am

Quoting KC135Hydraulics (Reply 24):
Anyone wanna know how C-17 and KC-135 parking brakes work?!

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.

Tom.

DarkSnowyNight · Sat Dec 22, 2012 3:44 pm

Quoting m1m2 (Reply 21):
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.

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 break.

You're right too. That kind of HP is no joke.

yeelep · Sat Dec 22, 2012 5:38 pm

Quoting KC135Hydraulics (Reply 24):

Yes!

KC135Hydraulics · Sun Dec 23, 2012 12:50 am

Quoting tdscanuck (Reply 25):
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.

Tom.

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. The only hydraulic actuation on the C-17 MLG is the retract cylinder, the downlock actuator, and the uplock actuator. That's it! The 135 on the other hand.... lock actuator, side strut actuator, walking beam actuator, door actuator, sequence valve, door control valve, door shutoff valve, valves, restrictors, check valves, rabbit ears... so much crap.

HAWK21M · Sun Dec 23, 2012 10:35 am

Quoting yeelep (Reply 22):
Yes. In a proper functioning system the parking brake will hold for a minimum of eight hours with "B" hydraulics off.

Thats thru Accumulator pressure which leaks off over time,hence the need to reset the pressure & reapply brakes in 8 hrs.

yeelep · Sun Dec 23, 2012 11:02 am

No need to reapply brakes. Simply pressurizing "B" system hydraulics will restore brake pressure.

DashTrash · Tue Dec 25, 2012 9:28 pm

Quoting Darksnowynight (Reply 20):

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

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 moving Dash 8 with that handle.

wingscrubber · Wed Dec 26, 2012 5:40 am

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?

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 engaged if the aircraft is parked overnight. Setting maximum pressure is desired, as this fully charges the accumulator.

I think the worse-case scenario is when you park the airplane in the hot desert sun, but then have a very cold night-time temp, the temperature change causes the accumulator to loose a lot of pressure. Brakes will still be 'on', but maybe not clamped as firmly.

BEG2IAH · Wed Dec 26, 2012 6:32 am

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

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/activites/p...eurs/article/boeing-787-dreamliner

Walking around the MLG ready to be installed on a new aircraft made my day. Speaking of, I asked the guys where is the fuse plug and they couldn't find it. I bet it does exist, but where is it exactly?

Thanks.

tdscanuck · Wed Dec 26, 2012 7:48 pm

Quoting BEG2IAH (Reply 33):
Speaking of, I asked the guys where is the fuse plug and they couldn't find it. I bet it does exist, but where is it exactly?

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 the brake stack is in the way. If you were to drill straight into the center of the tread headed for the axle, where you'd hit the wheel is about where the fuse plugs are.

Tom.

BEG2IAH · Wed Dec 26, 2012 8:03 pm

Quoting tdscanuck (Reply 34):
There's three on each wheel.

Tom, thanks! I could almost bet who would respond first.

HAWK21M · Thu Dec 27, 2012 11:12 am

Quoting tdscanuck (Reply 34):
They go right through the wheel on the inside half of the hub

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.

n901wa · Thu Dec 27, 2012 1:25 pm

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? I know I have built up that type wheel, but can't remember right now ( spent 2 years in the Wheel Shop) I must be getting old

m1m2 · Thu Dec 27, 2012 5:13 pm

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 stops in a hurry. Just curious as I haven't had to do it.

DarkSnowyNight · Thu Dec 27, 2012 6:57 pm

Quoting DashTrash (Reply 31):

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 moving Dash 8 with that handle.

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 say I do like the Dash's park brake best of all from the other types I've worked. Big, obvious & yellow (well, bee-striped at our company). And you can grab it to set or un set w/out having to sit all the way down up front.

Quoting m1m2 (Reply 38):
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 stops in a hurry. Just curious as I haven't had to do it.

Can't speak for DT, but I've done that twice; though never at more than about 15kts. Yes, you'll stop in a hurry. It applies a very "binary" brake force, with no modulation.

DashTrash · Thu Dec 27, 2012 7:23 pm

Quoting m1m2 (Reply 38):
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 stops in a hurry. Just curious as I haven't had to do it.

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 yank it on as though you were setting it, you will probably taste the glareshield.

HAWK21M · Thu Dec 27, 2012 7:25 pm

Quoting n901wa (Reply 37):
What kind of main wheel is that

sure looks B727....maybe someone who worked on the type could state so.....

m1m2 · Fri Dec 28, 2012 2:08 pm

To DashTrash,

mmm, glareshield! Bet that tastes awesome. Thanks for the relies, happy taxiing.

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