Aircraft Brakes — Page 136, Lesson 177

I want to walk you through the aircraft brake system now, and I'm going to start with the modern design of the brake plates themselves. Recent technological advancements in heat dissipation have changed the design of the brake plates. Instead of a continuous rotating single plate, we now have a plate constructed of many interconnected individual segments. That segmented construction greatly improves the heat dissipation properties, and that in turn increases brake efficiency. The reason is simple: more surface area and better airflow between segments means the heat generated during braking can escape more effectively. Carbon is also used for manufacturing brake units, because it has much better heat absorbing and dissipating properties than steel. Carbon brakes are also much lighter than equivalent steel units. The disadvantage is their increased cost and shorter life, so they tend to be fitted only to aircraft where the weight saving is worth the extra cost — long haul aircraft, for example. Now, there's a critical limit to all this heat absorption. If the brakes become too hot, they will not be able to absorb any further energy, and their ability to retard — that is, to slow down — the aircraft diminishes. This phenomenon is termed Brake Fade. So brake fade is the loss of braking effectiveness caused by the brakes reaching their heat absorption limit. Let me show you the typical multi-plate brake unit so you can see how these components fit together. Now let's talk about Brake Release. When the pilot releases the pressure on the brake pedals, the brake adjuster assemblies will move the pressure plate away from the stators and rotor assemblies, thus allowing them to move slightly apart. The internal construction of the brake adjuster assemblies allows them to maintain a constant running clearance when the brake is off, thereby automatically compensating for brake wear. So as the friction material wears down over time, the adjuster keeps the gap between the plates consistent. But there's a failure mode here. If the return spring inside the adjuster assembly ceases to function, or if the unit is wrongly adjusted, then they could be the cause of a brake not releasing correctly. This is termed brake drag. Brake drag will generate a lot of heat, and it can be responsible for Brake Fade occurring sooner than it otherwise would. So you see the chain: a stuck return spring causes drag, drag generates heat, and that heat brings on fade prematurely. Let me show you the brake adjuster assembly so you can see the return spring and how it works. Now for Brake Wear. Aircraft brakes are designed to give good retardation, while at the same time avoiding excessive wear of the brake lining material. It is important that the thickness of the brake lining material is carefully monitored. Why? Because too little brake lining material remaining may mean that the disc of a single disc brake system may become excessively worn or grooved. Or, on a multiple disc brake, the remaining material overheats and erodes extremely fast. So the lining is the sacrificial layer — once it's gone, the metal disc itself starts to suffer. There are several methods of determining the amount of brake lining material which remains on the brake unit. On multiple disc brake systems, the most popular method is by checking the amount that the retraction pin — or the indicator pin, as it is sometimes called — extends from, or intrudes within, the spring housing, with the brakes selected on. So you apply the brakes, and then you measure how far that pin sticks out of the housing. The position of that pin tells you how much lining material is left. That's your wear indicator. So to tie it all together: segmented plates and carbon improve heat dissipation, brake fade is the loss of retardation when the brakes can't absorb more heat, brake drag is a failure to release that generates extra heat and hastens fade, and you monitor lining wear via the retraction pin on multi-disc systems.