Aircraft Brakes — Page 141, Lesson 189

We're starting a new topic now: aircraft brakes, and specifically the energy problem that braking creates. Let me walk you through this. When you apply the brakes on an aircraft, you're converting kinetic energy—the energy of motion—into heat. And I want you to appreciate just how much energy that is. A heavy aircraft rolling down the runway at speed has an enormous amount of kinetic energy, and the brakes have to absorb all of it in a very short time. That energy is released as heat, and that heat must be dissipated—got rid of—or the components will fail. Here's the critical limitation: the brake packs, the wheel assemblies, and the tyres are each capable of absorbing so much heat and no more before they fail. So there's a hard ceiling on how much thermal energy these components can take. Exceed it, and you're looking at deflated tyres, damaged seals, or worse—a wheel fire. So the question becomes: how do you know how much energy the brakes have absorbed? Because that determines what precautions you need to take after an aborted take-off, after a landing, or even just after moving the aircraft around the airfield. You need a method of determining the absorbed energy so you can make those decisions. That method is the brake kinetic energy graph. Let me explain how it works. You enter the graph with two primary values: the all up weight—that's the total weight of the aircraft—and the brake application speed, the speed at which you applied the brakes. Then you factor that result for several conditions: the head or tailwind component, the number of serviceable reversers—that's the thrust reversers that help slow you down—and the airfield altitude. The end result gives you the amount of kinetic energy absorbed, expressed in KE units. But more importantly, the graph places your situation into one of three zones, and each zone determines the course of action you must take. This is the real purpose of the graph—not just to give you a number, but to tell you which drill to run. Let me show you the system layout first. Now, Figure 6.7 is a reproduction from an aircraft operations manual, and it outlines those three zones and the drills to be carried out when the kinetic energy in the brakes is above a certain level. Let's go through each zone in detail, because these are operational drills you need to know cold. First, the Danger Zone—that's above 29 KE. This is the worst case. The drill is: clear the runway as soon as possible and alert the fire services. Use the minimum necessary footbrake pressure—and note, the tyres will probably deflate. The parking brake must not be used unless essential. Shut down engines not required. If the tyres remain inflated, they must be approached with caution from the front or rear—never from the side, because that's where a tyre can burst outward. Unless a brake or wheel assembly is in flames, allow the brakes to cool without applying extinguishant—you don't want to thermally shock a hot brake. If a brake or wheel assembly is on fire, apply dry powder extinguishant, and then retire from the vicinity for at least 15 minutes. Finally, allow a cooling period of 2 to 3 hours, unless cooling air is used. And the wheels and tyres must be changed—they're considered unserviceable after this. Next, the Caution Zone—that's 17 to 29 KE. The drill here: park the aircraft but do not apply the parking brake. Do not approach the wheel assembly for at least 30 minutes. Before take-off, check the brake wheel assembly for damage and apply brake pressure to check for brake seal leaks. Operate the brakes and check that pressures are maintained. And allow a brake cooling time of 5 minutes for each 1.0 KE in excess of 5.0 KE. Then the Normal Zone—that's 5 to 17.0 KE. Here you allow a brake cooling time of 5 minutes for each 1.0 KE in excess of 5.0 KE. Same formula as the caution zone. And finally, below 5 KE—no brake cooling time is necessary, and there are no special instructions. Let me make sure you understand that cooling formula, because it appears in both the caution and normal zones. For every 1.0 KE unit above 5.0 KE, you add 5 minutes of cooling time. So if you absorbed 10 KE, that's 5 KE above the 5.0 baseline, so you'd need 25 minutes of cooling. If you absorbed 17 KE, that's 12 above baseline, so 60 minutes. The point is, the hotter the brakes, the longer you wait before you touch them. So the whole picture is this: braking converts kinetic energy to heat, the components have a heat limit, the graph tells you how much energy you absorbed and which zone you're in, and the zone dictates your drill. The danger zone is about protecting life and property—fire services, minimum brake pressure, stay away from the wheels. The caution zone is about inspection and cooling before you trust the brakes again. The normal zone is just a cooling wait. And below 5 KE, you're fine. That's the complete energy management picture for aircraft brakes.