Aircraft Brakes — Page 136, Lesson 181

Right, let's pick this up with the anti-skid system and how it actually protects you on the runway. We've just covered the basic idea of anti-skid — preventing wheel lock-up. Now I want to look at the control logic behind it. The system works to a datum figure for wheel deceleration. That's a reference value, a target rate of slowing down for the wheel, which is deliberately selected to be greater than the maximum possible deceleration of the aircraft itself — that's of the order of 18 feet per second squared, which is about 6 metres per second squared. So the wheel is allowed to slow down faster than the whole aircraft ever could. When that datum figure is exceeded, meaning the wheel is decelerating too quickly and is about to lock up, brake pressure is automatically reduced or released. That's the core protective action. There's also a facility to "hold off" brake pressure. That means the system can deliberately keep the brakes off in two situations: in the event of a wheel bounce, so the wheel doesn't grab and skid when it briefly leaves the runway, or to prevent brake operation before touchdown, so you don't get braking while the wheels are still in the air. Now, these systems come in two flavours — mechanical or electrical. Mechanical systems have been in use since the early 1950s, but most aircraft today use electrical or electronic systems. Let's talk about the effect on performance, because this is a real operational point. An anti-skid system will reduce the braking distance on both take-off and landing. Conversely, an inoperative anti-skid system will increase the take-off and landing distances required. The performance data will be available to determine the runway length required in the event of a rejected take-off — that's an RTO, where you abort the take-off and need to stop. And here's a hard rule to remember: take-off is prohibited with an inoperative anti-skid system on a wet runway. Now let's look at the two types in detail. First, the mechanical anti-skid system. The basic principle here is the use of the inertia of a flywheel as a sensor of wheel deceleration. So you have a flywheel, and a wheel directly driven by the aircraft wheel is coupled to that flywheel by a spring. Here's the clever bit: any change in aircraft wheel velocity causes a relative displacement between the flywheel and the driven wheel. Because of its inertia, the flywheel resists changing speed, so when the wheel slows suddenly, the flywheel lags behind. That relative displacement is used as a control signal to operate a valve in the hydraulic braking system to release the brake pressure. The unit may be wheel rim mounted or axle mounted. Then we have the electronic anti-skid system. The response rates of the flywheels in mechanical systems are low when compared with electrical signalling, and furthermore the modulation — that's the way the brake pressure is varied — does not always conform to the true runway conditions. It's also much easier to alter the response rates and system biases of electronic circuitry to suit different aircraft types, making it simpler to adapt the circuits to match the requirements of new aircraft types. The electronic system gives approximately a 15% improvement over the mechanical unit, with the advantage that it can be tested prior to use. The electronic system comprises three main elements. First, a sensor which measures wheel speed. Second, a control box to compute that wheel speed information. And third, a servo valve to modulate brake pressure. That's the basic control loop — sensor, computer, valve. Now, that basic loop offers few advantages over a mechanical system except that the cycling rate is much improved. The real refinement is something called the Adaptive Pressure Bias Modulation Circuit — that's the system that continuously adjusts the pressure bias to match the runway conditions, which is where the true performance gain comes from. Let me show you what a typical multi-plate brake unit looks like, so you can see where all this fits together. So to tie it together: the anti-skid system's job is to keep the wheel at the edge of grip, using that datum deceleration figure as the trigger. Mechanical systems use a flywheel and spring to sense it; electronic systems use a sensor, control box, and servo valve, and give you that 15% improvement plus the ability to test before use. And remember the operational consequence — if it's inoperative, your distances grow, and on a wet runway, take-off is simply prohibited.