Aircraft Brakes — Page 141, Lesson 187

Let's pick up with the autobrake system's operating states, because those three words — armed, activated, operative — are the backbone of everything that follows. I want you to think of them as three distinct conditions. Armed means the system is ready for use, but certain conditions have to be met before it will operate automatically. It's set, it's waiting, but it's not doing anything yet. Activated is the next step — a system that is armed may become activated once those conditions have been met. And it may be activated in a number of ways, depending on the aircraft type. Operative is the third state — a system is operative if it is working as intended. An inoperative system will not accomplish its intended purpose and is not considered to be functioning correctly. So armed is ready, activated is actually triggered, operative is working properly. Now, a critical limitation right up front: the autobrake system is not available when you're using the alternate brake system. That's a hard rule. Depending on the aircraft, you can select three or five landing deceleration rates. And importantly, anti-skid protection is provided during autobrake operation — the two work together. Here's how a landing autobrake run works. You arm the landing autobrakes by selecting one of the deceleration rates on the autobrake selector. On touchdown, with ground mode and wheel spin-up sensed, the brakes will be automatically applied. They'll provide braking to a complete stop, or until the autobrakes are disarmed. And here's a nice touch — you can change the deceleration rate during autobrake operation without disarming, just by rotating the selector. Now let's talk about RTO — rejected takeoff. With RTO selected, maximum brake pressure will be applied automatically when all thrust levers are closed at ground speeds above 85 knots. Below 85 knots, autobrakes are not activated. So that 85-knot figure is your threshold — above it, you get max pressure; below it, nothing. There's a safety feature built in. The landing autobrake system disarms immediately if a fault occurs when the system is armed. When that happens, the selector will move to the disarm position, and a warning caption will be displayed. So you get a visual cue that something's wrong. Disarming will also occur if any of these crew actions are taken during autobrake operation. First, manual braking — if you touch the brakes yourself, it disarms. Second, advancing any thrust lever after landing. Third, moving the speed brake lever to the DN — that's the down — detent after speed brakes have been deployed on the ground. And fourth, moving the autobrake selector to Disarm or Off. In normal operation, the autobrakes are disarmed by the non-handling pilot or the flight engineer as the aircraft speed reduces to approximately 20 knots. So that's the routine handoff — around 20 knots, someone deliberately disarms the system. Now let's move to the parking brake. The parking brake handle operates a shut-off valve in the return line to the reservoir from the anti-skid valves. Here's the procedure to apply it: depress the foot pedals, apply the parking brake lever, then release the foot pedals. What happens is that hydraulic pressure is now trapped in the brakes because the return line from the anti-skid valves is closed. And this will be capable of maintaining the brakes 'on' for overnight parking if required. So it's a mechanical trapping of pressure — you build it up, then you seal the line. Finally, there's automatic brake application on undercarriage retraction. As the undercarriages are retracted, reduced hydraulic pressure is delivered to the wheel brakes. This brings the wheels to a stop, and it eliminates the adverse effects of gyroscopic forces produced by the spinning wheels. That's the key — spinning wheels create gyroscopic forces that fight the aircraft's motion, so stopping them during retraction removes that problem. Let me show you how all of this fits together in a typical system layout. That figure gives you the overall picture of a brake and anti-skid system. You can see the components we've been talking about — the autobrake selector, the anti-skid valves, the return line to the reservoir, and how the parking brake shut-off valve sits in that return path. So to tie it together: you have three operating states — armed, activated, operative. Autobrakes aren't available with the alternate system. Landing autobrakes arm by selecting a deceleration rate, activate on touchdown with ground mode and wheel spin-up, and can be disarmed by faults or specific crew actions. RTO gives you max pressure above 85 knots with all thrust levers closed. The parking brake traps pressure by closing the return line. And on retraction, reduced pressure stops the wheels to kill gyroscopic forces.