Aircraft Brakes — Page 141, Lesson 185

I want to walk you through the aircraft wheel brake system now — and I want to start with the big picture of how the brakes are powered, because everything else hangs off that. The brakes are powered by one of the aircraft's hydraulic power systems — we call it system 1. And here's the clever part: if system 1 pressure drops low, there's an automatic switch-over to an alternate system, system 2. So you get redundancy without the pilot doing anything. Now, if both the normal and the alternate brake hydraulic sources are lost — that's the worst case — an accumulator is automatically selected to maintain parking brake pressure. An accumulator is essentially a hydraulic energy storage device, a pre-charged bottle of fluid under pressure, so even with both hydraulic systems gone, you still hold the parking brake. Let me show you the whole layout — this is Figure 6.4, the typical brake and anti-skid system. Now let's talk about anti-skid protection, because that's the heart of safe braking. The anti-skid valves receive hydraulic pressure from the normal brake metering valves, or from the autobrake valves. The anti-skid control unit provides electrical signals to those anti-skid valves to control braking during skid conditions. So you have a hydraulic side and an electrical side working together. The sensing comes from wheel speed transducers mounted in the axle. They transmit wheel speed inputs to the anti-skid control unit. Each wheel is provided individually with anti-skid protection when normal brakes are operative — so each wheel is looked after on its own. When skidding is initially detected, the anti-skid controller commands the respective anti-skid valve to reduce brake pressure, which protects the wheel from further skidding. So the sequence is: detect the skid, command the valve, release pressure, stop the skid. There's also something called touchdown braking protection. That's provided by comparing wheel speed to IRS ground speed — IRS is the inertial reference system, which gives you the aircraft's true ground speed. By comparing the two, the system knows if the wheel is spinning up properly on landing. Now, one important contrast: during alternate brake operation — that's when you're on system 2 — anti-skid protection is provided to wheel pairs rather than individual wheels. So you lose the per-wheel granularity when you're on the alternate system. Next, torque limiting. A brake torque sensor is provided at each wheel to detect excessive torque during braking. The purpose is to prevent damage to the landing gear — and this is more of a problem with carbon brakes, because carbon brakes can generate very high torque. When excessive torque stress is detected, a signal is sent to the anti-skid valve and brake pressure to that wheel is released. So torque limiting and anti-skid share the same final action — releasing pressure to that wheel — but they're triggered by different sensors: one by skidding, one by excessive torque. Finally, autobrakes. This system permits automatic braking when using the normal brake system during landing rollout, or during a rejected take-off — that's an RTO, when you abort the take-off and need to stop on the runway. There are a number of levels of operation of the autobrake system — and that's where we'll pick up next, the different deceleration levels you can select.