Alternate braking
 

I had a question on a School Test the other day which I cannot really motivate the answer to (I am actually the teacher).

59. Aircraft equipped with power brake systems usually have an emergency brake system. How is the emergency system activated?

a) Manually by controls in the cockpit
b) The system is automatically activated in the event of main hydraulic system failure system failure
c) Automatically if the anti skid system fails
d) Automatically up on landing when spoilers are deployed and nose wheel on the ground

The Question Bank claims a) to be the correct answer. I have searched the internet for info. about different aircraft braking system. I've seen both the 737 and A320 has an automatic switching of hyd. system if the primary should fail. Alternate braking in general, is it usually as simple as a shuttle valve which senses low pressure in the primary system and change source to the alternate system?

Then on the other hand, I've heard the A340 has this button "Alternate Braking" in the cockpit. What is it for? :O

An alternate braking system is the use of a different hydraulic source for brakes than the normal braking system. Whereas brakes are hydraulically powered, and there are no alternate brake assemblies, it follows that if the normal braking system is lost, then another hydraulic source will be required to pressurize the brakes. It's the same brakes being used, just a different hydraulic source.

With this in mind, if one system fails, having another automatically take it's place may be unwise; this may cause the loss of the reserve or alternate braking system automatically too. Therefore, it should be pilot-selectable, and generally is.

Usually the alternate brakes and main brake hydraulic sources arrive at a common point before the brakes, which is a shuttle valve. If reserve braking and normal braking pressure is applied to the valve, the source with the highest metered pressure will open the shuttle valve, block the other system, and pressurize or power the brakes.

Anti-skid is just the opposite of braking; anti-skid is the dumping of brake pressure. When an anti-skid system determines that a locked wheel condition might occur, based on the rate of an individual or pair of wheels slowing down, it releases pressure to the offending wheel(s), dumping brake pressure, and preventing the wheel(s) from skidding. It follows, then, that anti-skid has nothing to do with activing alternate, or reserve brakes. In fact, often in alternate brakes, the braking capability, and anti-skid capability, is reduced somewhat.

During a normal landing, ground spoilers are deployed, causing loss of lift and putting more weight on the wheels, enabling more effective braking. During a normal landing, the weight on the nosewheel may control certain functions (such as pressurization, or other systems)...but not the reserve brakes. Whereas on every landing the spoilers will be deployed, and on every landing the nosewheel touches down, it follows that the reserve brakes aren't going to automatically be activated on every landing. Remember, reserve brakes are only needed when the normal braking is not available. Thing brings us back full circle to the concept that the reserve brakes should only be put in play when the pilot decides he or she need them, which makes it a pilot-selectable item.

To return to your question, then, the only answer that applies is A.

Yeah, I follow you there. The answer I and some of the students were looking for was b). The reason, just as you explained, the function of the shuttle valve. When primary pressure is lost alternate pressure takes over. But then I realize the pilot should be able to stop that from happening in case of leakage or something I pressume? (so you don't loose all the Skydrol) Thanks for your time explaining.

The reasons for not automatically selecting a reserve braking source may vary, but yes, if you have a loss in one system, connecting another system to the place that's experiencing the loss may not be the best choice.

It's more complex than that, however. In our aircraft (B747) for example, a number of functions are tied to the tilt of the landing gear. The landing gear is tilted to get it into the gear wells during retraction, but tilting the landing gear employs switch logic which also tells many aircraft systems what to do; it puts the airplane into "flight mode." Among the features of the tilt logic are touch down protection; a feature that dumps brake pressure upon touchdown to prevent a skid when the gear first touches. Reserve brakes disable this tilt logic function.

In our case, normal braking comes from system 4, or system 1 (depending on which is selected by the flight engineer. A massive fluid loss in either case should activate a hydraulic fuse which limits fluid loss, but automatically applying yet another system isn't something we necessarily want to do. Additionally, where the normal braking applies antiskid to all the wheels, our reserve brakes only apply it to axle-pairs of wheels, reducing braking effectiveness and applying other considerations, too.

In any event, before we ever arrive at the need or reserve brakes, we have the option of using either one of two systems to operate the brakes normally. Either of those systems must be manually attached to the brake system by a switch on the flight engineer panel. Just as the normal systems must be manually selected and connected to the brakes, the reserve brakes must be also. Admittedly, our Juraissic Jet isn't state of the art any more, but it gives you a general idea to answer your question. Different airplanes and different systems operate...differently. The main point is that there's some complexity to the system, and the choice of employing it, therefore, is best left as a pilot decision.

Turboprops
 

Quite a lot of the turbo-props you come across utilise an emergency brake system that can be as simple as applying the park brake, which can use a seperate accumulator of fluid.

Generally speaking though your alternative braking system is manually selected.

Alternate is probably an unfortunate choice of wording 'cos if my memory serves me right the 757 has an automatically switched alternate brakes system with a selectable reserve brake function if it all goes wrong.

Refering to the A320Fam:
Do not mix up "emergency" brake system with "alternate" brake system. :=

If the question will be ask with "alternate" braking system answer b is right, but the quetion says "emergency" brake system.

The "emergency" brake system is the parking brake actuated manually by the crew, so answer a is correct. :ok:

prk brk on in air
 

park brk on air what will it happen ny idea

True. However alternate would refer to using system pressure and emergency to using accumulator pressure. Both of which, on any Airbus, is selected automatically.

I would imagine the question in post #1 is part of a specific syllabus which uses a specific aircraft or manufacturer as reference. This often the case and questions which appear general in nature are then often not.

If your refer to A320Fam this statement is not true.
Using the accu pressure with the prake pedals is called "Alternate braking without Anti-skid".
The parking brake lever controls the "parking/ultimate emergency braking system".

You 'forgot' to highlight 'ultimate'.

Highlighting the "ultimate" would not change your statement in a correct one. ;)

Not if the meaning of ultimate is not understood.

I think that "Jet Blast" is a better place for you. :zzz:

Again, this highlights the need to point to the specific aircraft under discussion, as the foregoing airbus comments have no relationship to Boeing products.

I did it:
Post #6 ... Refering to the A320Fam: ...

Post #9 ... If your refer to A320Fam ...

I wasn't talking to you, but to the original poster. Once again, perhaps even more clearly this time, and directed to the original poster...a discussion of an alternative braking system must refer to a specific airplane, as both terminology and technical design will vary with the airplane.

Not all aircraft have emergency brakes.

Per the original poster question, A is the proper choice.

What we have here is, I fear, a failure to gap the bridge between understanding and quoting verbatim from a manual.

Now, if you have ultimate emergency brakes, you must also have, by definition, emergency brakes. Otherwise you would only have ultimate brakes or emergency brakes, but not both.

There is also this to ponder: A braking system with 3 different sources of pressure, loses both the primary and secondaty source. Now, what is left is a last and very finitie source (7 applications I hear you say). Lose this source and all braking is lost. What would a system relying on such a source be called?

Which is why the parking brake is called the parking/ultimate emergency braking system in the AMM, and not the parking/emergency braking system, and have been since 1972.

"failure to gap..." is on purpose. In case you wondered.

Guppy, you can't really answer the question unless, as you say, you know which aircraft or manufacturer the question relates to. As a stand-alone question it is poorly thought out, as there are two correct answers.

The 767 system uses the right hydraulic for normal braking, a pressure loss automatically activates alternate brakes from the center system. If those fail, a reserve brake system can be manually selected and last is the accumulator pressure if all else fails.

The question, copied right off the test, did not mention any specific aircraft-type. In other subjects (like Performance and Mass & Balance) the JAA syllabus is based on the 737-400, not sure if this is the case in ASP though which is more general aircraft knowledge.

Sorry I wasn't more specific but in order to get an answer to the Q in post 1, I could not modify it. Then of course, you learn that the world is never as simple as you want it to be.

Without a specific aircraft it hard to get the correct answer since both a and b are correct depending on the aircraft. Also Boeing makes more than one back up brake available.