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Viscount724
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Aircraft Brakes Question

Sat Feb 20, 2010 3:47 am

I'm curious why air brakes, as opposed to hydraulic brakes, aren't used on aircraft. On motor vehicles like buses and trucks, it seems that once they reach a certain size, air brakes are used. Why not also for large aircraft?
 
411A
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Joined: Mon Nov 12, 2001 10:34 am

RE: Aircraft Brakes Question

Sat Feb 20, 2010 5:18 am

Quoting Viscount724 (Thread starter):
I'm curious why air brakes
, as opposed to hydraulic brakes, aren't used on aircraft

They have been in the past...Fokker F.27/Fairchild FH227, for example.
 
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Web500sjc
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RE: Aircraft Brakes Question

Sat Feb 20, 2010 5:20 am

Because the rest of the movable places are hydraulic?

The only part with high pressure air is the pressurazation system, why make a diferent loop for the breaks rather just run off the hydraulics which are right there going to the flaps.
 
411A
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RE: Aircraft Brakes Question

Sat Feb 20, 2010 8:39 am

Quoting web500sjc (Reply 2):
The only part with high pressure air is the pressurazation system

Depends on the specific type.
In the aforementioned F.27/FH227, pneumatics were used for landing gear operation, nose wheel steering, brakes.
Flaps were electrically operated.
Pneumatic air was generated by compressors on each engine accessory gearbox.
A good arrangement that worked reasonably well.
 
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Faro
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RE: Aircraft Brakes Question

Sat Feb 20, 2010 12:51 pm

Very interesting question. The main reason for use of air brakes in heavy vehicles is to maintain braking power in situations where a hydraulic braking systems may develop a leak. I wonder have there been any aircraft incidents/accidents caused by insufficent braking power due to hydraulic leaks affecting the braking system?

Faro
The chalice not my son
 
CosmicCruiser
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RE: Aircraft Brakes Question

Sat Feb 20, 2010 1:24 pm

Quoting faro (Reply 4):
caused by insufficent braking power due to hydraulic leaks affecting the braking system?

Most big jets have brake accumulators that will hold one long brake application.
 
EMBQA
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RE: Aircraft Brakes Question

Sat Feb 20, 2010 10:48 pm

Because the forces to compress fluid are much less then the force to compress a gas.
"It's not the size of the dog in the fight, but the size of the fight in the dog"
 
roseflyer
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RE: Aircraft Brakes Question

Sun Feb 21, 2010 6:12 pm

It comes down to what is the lightest and most reliable system. IIRC the 787 uses electric brakes along with electric steering.

Pneumatics tend to be the least efficient power source on airplanes with the greatest losses. It depends on what type of configuration is run, but either electric or hydraulic power is most efficient.

Quoting faro (Reply 4):
. I wonder have there been any aircraft incidents/accidents caused by insufficent braking power due to hydraulic leaks affecting the braking system?

Each tire has separate hydraulic lines from the wheel well to the brakes on the wheel. There are two different primary hydraulic systems that go to the brake metering valves that control braking pressure. Planes can land with inop brakes. It depends on how many wheels are on the plane to determine how many brakes can be inop.

Also, on an airplane like a 737, a Flaps 40 landing on a 10,000 ft runway can be accomplished with almost no braking at all.

Quoting CosmicCruiser (Reply 5):

Most big jets have brake accumulators that will hold one long brake application.

Accumulators will provide one hard stop with antiskid dumping a lot of pressure. They are also designed to be able to hold a parking brake for 8 hours.
If you have never designed an airplane part before, let the real designers do the work!
 
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HAWK21M
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RE: Aircraft Brakes Question

Sun Feb 21, 2010 8:03 pm

Quoting Viscount724 (Thread starter):


I'm curious why air brakes, as opposed to hydraulic brakes, aren't used on aircraft.

Can be real troublesome to identify & rectify in case of a Leak.aka F27.
regds
MEL.
I may not win often, but I damn well never lose!!! ;)
 
MD11Engineer
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Joined: Sun Oct 26, 2003 5:25 am

RE: Aircraft Brakes Question

Sun Feb 21, 2010 9:12 pm

Quoting RoseFlyer (Reply 7):
Quoting faro (Reply 4):
. I wonder have there been any aircraft incidents/accidents caused by insufficent braking power due to hydraulic leaks affecting the braking system?

Each tire has separate hydraulic lines from the wheel well to the brakes on the wheel. There are two different primary hydraulic systems that go to the brake metering valves that control braking pressure. Planes can land with inop brakes. It depends on how many wheels are on the plane to determine how many brakes can be inop.

Since the brake lines are exposed where they run doen the landing gear leg and prone to be damaged by FOD, aircraft brakes normally have hydraulic fuses installed in these lines (still inside the wheel well) to cut off the line should a leak occur (they sense excessive flow rate, just like an electrical fuse, and close a valve). This is being done to protect the relevant hydraulic system from total fluid loss.

Jan
Je Suis Charlie et je suis Ahmet aussi

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