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PostPosted: Wed May 09, 2018 10:10 pm 
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Hello everyone. I'm a beginner.

On P.52 of the C172 manual, under BEFORE STARTING ENGINE checklist, number 4 says Brakes --- TEST and SET. Could anyone please kindly let me know how I can test the brake before having engine started?? I am supposed to be stationary before engine start. So it seems that it is impossible to test the working of the brakes...

Thanks everyone and have a nice day.


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PostPosted: Thu May 10, 2018 3:32 am 
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Hello and welcome to the community.

I think what is meant in the manual is depress the brake pedals and check for correct feel. If the brake system is working properly you’ll feel a resistance in the pedals. If there’s something wrong, like no brake fluid, then the pedals would go to there stop without resistance. But of course you’re right. You can’t perform a test if the brake is holding when the plane is stationary.

Hope this helps

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PostPosted: Thu May 10, 2018 6:48 am 
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You can't year it in sim, but it is possible IRL The set part refers to setting the parking brake.

See my post about the about the pre taxi accident for my thoughts and experiences on parking brakes.

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PostPosted: Thu May 10, 2018 10:50 am 
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Piper_EEWL wrote:
Hello and welcome to the community.

I think what is meant in the manual is depress the brake pedals and check for correct feel. If the brake system is working properly you’ll feel a resistance in the pedals. If there’s something wrong, like no brake fluid, then the pedals would go to there stop without resistance. But of course you’re right. You can’t perform a test if the brake is holding when the plane is stationary.

Hope this helps


Thank you :D


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PostPosted: Thu May 10, 2018 11:13 am 
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Oracle427 wrote:
You can't year it in sim, but it is possible IRL The set part refers to setting the parking brake.

See my post about the about the pre taxi accident for my thoughts and experiences on parking brakes.


Just checked that out. Looks like parking brake isn't something reliable. How does parking brake actually work? Sounds like the parking brake is not giving full 'braking power'... Is that within the same braking system as the one you use after landing or is it a separate system like hand brake on cars?

Sorry if this question is too simple and thanks for replying me :D


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PostPosted: Thu May 10, 2018 11:34 am 
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The parking brake on the 100 model Cessna's and the Cherokee series is a lever that pulls on the same cables that actuate the brake cylinders going to the main wheels.

The problems I find are that the lever is poorly secured on the Cessna aircraft. The lever does not use a ratchet to lock the lever in position and instead relies on a metal tab in a slide lock style mechanism. It is very similar to the slide lock on this pipe clamp. https://www.amazon.com/Bessey-BPC-H34-4 ... B0012YNJRO. If you are familiar with pipe clamps, the right half of the clamp relies on the friction from the metal tab sitting at an angle against the pipe. It is very easy to bump the Cessna style slide lock and disengage the parking brake, but I see that they slide free on their own, especially when they aren't firmly applied to begin with

On the Cherokee, the system uses a ratchet with a release button. It is better, but still not that great thanks to poor visual/physical feedback and the risk that the ratchet can wear or be poorly engaged. I could barely tell the difference in the lever position between an engaged brake and disengaged brake.

In both cases, the system should hold secure, but with wear and tear and a non-robust set of materials, these things quickly loose their performance. I wouldn't even trust them on a factory new aircraft. :(

The brake check is very important. I've seen pilots lose one cylinder during their test before start. It would be a shocker to discover the issue after start!

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PostPosted: Thu May 10, 2018 4:39 pm 
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Also remember that even on cars, the "Parking Brake" is just that - to keep the car parked. This means that it is only designed to provide sufficient braking force to keep the car from rolling under its own weight under most conditions. In the airplanes, they actually actuate the brakes to set them, but they are usually set to less than 50% of full application force, even on aircraft with fully hydraulic (i.e. no cable) parking brakes. At least that's better than on your car where it's typically a separate drum on the left rear of the car with its own friction pad that is the actual brake and it doesn't actuate your actual braking system (which is why you don't see the brake pedal "sag" when you apply the brake as you do on aircraft which have manual/non-boosted brakes).

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PostPosted: Fri May 11, 2018 3:51 am 
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There are couple of ways to make a hydraulic parking brake. In 737 for instance, you'll depress the pedals and pull up the parking brake lever which mechanically locks the pedals in place. It is released by depressing the pedals further, thereby releasing the latch. I don't like this system that much. The other kind of system consists of a handle completely separate from the pedal brakes, which controls a separate parking brake valve (sometimes combined with emergency brake system). This applies accumulator pressure to the brakes. Control may be electrical or mechanical.

Applied parking brake is strong enough to lock the wheels. This has been demonstrated a couple of times in an embarrassing way as a result of in-flight application.

-Esa


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PostPosted: Fri May 11, 2018 5:10 am 
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Thanks everyone!! :D

Btw, 'This has been demonstrated a couple of times in an embarrassing way as a result of in-flight application.'
Does that mean landing with parking brakes on?... Sounds horrible.. What would happen? Would the plane drift and flip? :cry: :shock:


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PostPosted: Fri May 11, 2018 5:21 am 
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pangkinglim wrote:
Does that mean landing with parking brakes on?... Sounds horrible.. What would happen? Would the plane drift and flip? :cry: :shock:
Nah, a busted set of wheels and tyres, and some tedious paper work to slow down the re-inflation of ego. These have happened in airliners some times. In small airplanes (SEPs, MEPs) the parking brake is generally such that setting it takes action deliberate enough to make it all but impossible in flight by mistake.

-Esa


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PostPosted: Sat May 12, 2018 11:20 am 
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CAPFlyer wrote:
.....where it's typically a separate drum on the left rear of the car with its own friction pad that is the actual brake and it doesn't actuate your actual braking system....
.

I never realised that. Must be a new thing - the handbrake on my 3rd Gen Trans Am (1982) actuated both rear drums.

At least on most UK vehicles the handbrake/ e-brake/ parking brake actuates both rear wheels with a cable. Handbrake turns are fun and actually work :twisted: Some go for an electronic switch and motor which is crap (google Vauxhall handbrake fails). On some 4x4s , notably Land Rovers, the handbrake actuates a drum brake on the rear of the transfer gearbox - a transmission brake. Handbrake turns are a No-NO. :shock:

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PostPosted: Sat May 12, 2018 12:19 pm 
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In what comes to car parking brakes, there are local requirements. Around here, the system needs to be separate from the main braking system, justifying a cable actuation or similar. Also, it should affect two wheels by common sense. Think about jacking the car up to change a blown tire, if it happens to be one braked by the parkin'. Yet, if at any inconvenient incline, I'd certainly chock the thing like I would an airplane. :)

-Esa


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