Aircraft Brakes — Page 136, Lesson 183

I want to walk you through the anti-skid unit — the ASU — and its protective functions, because this is where the braking system starts to think for itself. First, let's set up the context. We just talked about how the ASU releases brake pressure when a wheel starts to skid. Now here's the clever part: the system is designed so that when it re-applies pressure after releasing a wheel, it doesn't just slam the pressure back to where it was. The brake pressure applied immediately after a wheel is released following an ASU operation is deliberately lower than the pressure that was applied before the ASU released it. Why? To prevent an immediate return to the conditions that caused the ASU to release the pressure in the first place. In other words, if you instantly restored full pressure, you'd instantly recreate the skid. So the system eases the pressure back in at a reduced level. Now, the ASU has several important functions, and I want to take you through each one carefully. First is touchdown protection. This prevents the brakes from being applied before the aircraft actually touches down. Here's how it works: the electronic anti-skid controller monitors two things — the wheel speed and the air/ground logic. If no signal is received, the brakes simply cannot be applied while the aircraft is airborne. On touchdown, the wheels "spin up" — they accelerate from zero to landing speed — and that spin-up generates a signal to the controller. Once that signal arrives, the controller now allows the brakes to be applied. So the system uses wheel rotation as proof that you're on the ground. Second is skid prevention. The anti-skid controller monitors the deceleration rate of each individual wheel. If it determines that any wheel is approaching a skid — meaning it's decelerating too quickly — the controller will reduce the brake pressure to that specific wheel. Notice it's per-wheel, not all at once. Third is locked wheel protection. If a wheel locks up completely — say because of a wet patch or ice — the controller will release the pressure to that wheel entirely, all the way to zero. It holds the pressure off until the wheel spins up again, and then the pressure is re-applied. So this is a full release, not just a reduction. Fourth is hydroplane protection. This one is only on systems that have the facility, and it works differently. Here the controller monitors aircraft velocity and the wheel speeds of a complete bogie — that's the set of wheels on one landing gear leg. The scenario is this: if all the braked wheels hydroplane and lock up simultaneously, then the pressure to some of the wheels is released. The method varies from aircraft to aircraft, but typically, if all braked wheels lock, a number of brakes are released — for example, on a four-wheel bogie, two wheels would be released. The remaining pair will then provide locked wheel protection. Subsequently, the hydroplaned pair will spin up, and they in turn will provide locked wheel protection. And if hydroplane conditions still exist, then the other pair will be released. So it's a cycling, alternating release — the two pairs take turns being released and providing protection. Finally, there's the question of when all this protection switches off. To give the pilot full control of the brakes for taxiing and manoeuvring, the anti-skid system is deactivated — either manually or automatically — when the aircraft has slowed to below approximately 20 mph. At that speed, it's assumed there's no further danger of skidding, so the system steps aside and hands full braking authority back to the pilot. Let me show you the physical hardware we're talking about — this is a typical multi-plate brake unit, which is what the ASU is controlling. So to tie it together: the ASU is a protective layer on top of your normal braking. It watches wheel speed and deceleration, it prevents braking in the air, it eases pressure back in after a release, it handles individual wheel locks, and it manages the special case of hydroplaning across a whole bogie. And below 20 mph, it hands control back to you.