Aircraft Brakes — Page 136, Lesson 179

Right, let's pick this up with the wear-checking methods, because that's where we left off in the brake maintenance story. So, we've got the retraction pin and its spring housing. The first method of checking wear uses a wear gauge to make sure that retraction pin hasn't moved too far within that spring housing. Think of it as a physical measurement of how much the pin has travelled as the brake linings wear down. Now, if the system doesn't have retraction pins fitted at all, we use an alternative method. We apply the brakes, and then we measure the clearance between the back of the pressure plate and the brake housing. That gap tells us how much wear has occurred. And if we're dealing with a single disc unit, the check is slightly different again. We apply the brakes and measure the distance between the disc and the brake housing. We make sure that distance is no less than a minimum value — that's the remaining brake lining material. So in every case, we're measuring a physical gap that shrinks as the friction material wears away. Now let's move on to the actual operation of the system. When the pilot operates the brake pedals on the flight deck, hydraulic fluid under pressure moves small pistons. Those pistons move the pressure plate, which forces the stator pads against the rotor plates. The resultant friction between the stators and rotors slows the plates down — that's the braking action. Here's the key distinction. On a small aircraft, the hydraulic pressure generated directly by your foot on the brake pedal may be enough to arrest the aircraft's progress. But on a large aircraft, foot power alone is obviously insufficient. We need another source of hydraulic power, and that's supplied by the aircraft's main hydraulic system. So the pedal is just a command input on a big aircraft; the muscle comes from the main hydraulics. Now, let's talk about brake modulating systems, because this is where things get interesting. Optimum braking is critical on modern aircraft — they have high landing speeds, low drag, and high weight, especially when operating from short runways in bad weather. The problem is that the pilot simply cannot sense when the wheels lock. So the first requirement of a brake modulating system is to provide anti-skid protection. Here's the physics. Whenever braking torque is developed, there must be only a degree of slip between the wheel and the ground. A skidding wheel provides very little braking effect — it's actually worse. So in all brake modulating systems, the deceleration of the individual wheels is taken as the controlling parameter of braking torque. That means the system watches how fast each wheel is slowing down, and uses that to regulate how much braking force to apply. That's the core principle — we control torque based on wheel deceleration to prevent the wheel from locking and skidding. That figure shows you a typical multi-plate brake unit, which ties together everything we've just discussed — the pressure plate, the stators, the rotors, and how they're all stacked together in the wheel.