Date: 08 Sep 97 02:03:40 From: Chris Dahler <email@example.com> Organization: gte.net References: 1 Followups: 1 2
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> 1: what exactly needs to cool down after such a heavy brake application? > Is it the hydraulic fluid or the brake discs and pads themselves? I dont > understand why hot brake surfaces should work less effectively than cold > ones, unless they melt or something You should see some of the video footage of Boeing doing aborted takeoff tests on heavy aircraft for the certification process. The brakes glow red hot. It isn't the hydraulic fluid, it is the brake surfaces that get so hot. This can produce two major problems: the brakes are not nearly as effective when so hot, and the tires can blow due to their close proximity to this heat. Brakes lose effectiveness when they get hot because the friction properties of the metal changes. The metals get soft, they can lose their symmetry (causing the shuddering you felt later), and in effect they get polished to a mirror-like smoothness, and the two surfaces can't grab each other as well (this causes the loud groaning sound you hear). If you want to see the effects of this yourself, go out in your car, accelerate to 60 mph, and do a panic stop without locking the wheels. Do this a few times in a row to heat the brakes up pretty good, and you'll notice the third or fourth time that your brakes won't work nearly as well as they did the first time (I *am* kidding here - this works as advertised, but I'm not seriously telling you to go out and do this!) > 2: Given the shuddering and groaning of the brakes, what would need to be > done to them in the way of repairs? A quick brake change solves the problem, but while it doesn't take much time to do this, it's kind of expensive. It will depend on whether the brakes have permanently reduced effectiveness (in which case the brakes must be changed) or whether the noise is just annoying but the brakes still work fine (in that case, it's more the airline not wanting to annoy or frighten passengers than a brake change being required).