Flight Control Systems — Page 162, Lesson 213

We’re now into the flight control systems chapter, and I want to walk you through the speed brake and spoiler systems. These are the surfaces that do two very different jobs: they act as air brakes in flight, and they dump lift on the ground after touchdown. Let’s start with the speed brakes themselves. The speed brakes may consist of two distinct sets of surfaces: flight spoilers and ground spoilers. So on a typical transport aircraft you have spoiler panels on the upper surface of each wing, and some of them are designated flight spoilers, some are ground spoilers. The pilot’s speed brake lever controls a component called a spoiler mixer. That mixer positions the flight spoiler control units — which are the Power Control Units, abbreviated PCU — and it also positions a ground spoiler control valve. So the lever input goes to the mixer, and the mixer splits the signal between the flight spoiler PCUs and the ground spoiler valve. The surfaces themselves are actuated by hydraulic power. That hydraulic power is supplied to the PCUs, or to actuators on each surface. So the spoilers are hydraulically driven, not manually. Now, the key limitation: ground spoilers operate only on the ground. Why? Because there’s a ground spoiler shut-off valve that remains closed until the main landing gear operates a ‘weight on’ switch. So the valve stays shut in flight, and only opens when the gear struts are compressed by the aircraft’s weight on the ground. That’s the safety interlock that prevents you from deploying ground spoilers in the air. Let me give you the operating sequence. With the lateral controls — that’s the ailerons — in neutral, application of the speed brakes will cause the flight spoilers to rise equally. So both wings’ flight spoilers come up symmetrically, which gives you a pure drag effect with no roll. When speed brakes are applied on the ground, the ground spoilers will also rise, because the weight-on switch has opened the shut-off valve. Moving the speed brake control lever provides an input to the spoiler mixer via a mechanical system. The spoiler mixer then conveys the speed brake signals to the ground spoiler control valve and to the flight spoiler actuators. Now, the ground spoiler shut-off valve is fitted in the hydraulic system downstream of the spoiler control valve, and it’s operated by that ‘weight on’ switch. So the order in the hydraulic line is: spoiler control valve first, then the shut-off valve downstream of it. There’s also an automatic mode. Speed brake control may be applied by an electric speed brake lever actuator. When ‘armed’, the actuator will drive the speed brake lever to the ‘full up’ position, raising the flight and ground spoilers when the landing gear wheels rotate on touchdown. So on landing, the wheel rotation triggers the actuator, and the spoilers deploy automatically. But here’s the aborted landing protection: if the engine thrust levers are opened up again on the landing run, the actuator will sense the aborted landing and will lower the flight and ground spoilers. So if the pilot decides to go around, the spoilers retract automatically. Now let’s look at the typical flight spoiler system in more detail. Two flight spoilers are located on the upper surfaces of each wing. So four flight spoilers total, two per wing. The outboard spoilers are powered by one hydraulic system, whilst the inboard spoilers are powered by a second system. That’s redundancy — if one hydraulic system fails, you still have spoiler control on the other set. Hydraulic pressure shut-off valves are controlled by the two flight spoiler switches. So the pilots have switches that control the hydraulic supply to the spoilers. The flight spoilers are hydraulically actuated in response to movement of the aileron controls. So they’re not just speed brakes — they assist with roll control. A spoiler mixer, connected to the aileron control system, controls the hydraulic PCUs on each spoiler panel to provide spoiler movement proportional to aileron movement. So the more aileron deflection, the more spoiler deflection. And here’s the key asymmetry: flight spoilers rise on the wing with the ‘up-going’ aileron and remain retracted on the wing with the ‘down-going’ aileron. So when you roll right, the right aileron goes up, and the right wing’s flight spoilers rise to help push that wing down. The left wing’s spoilers stay retracted. That’s how spoilers augment roll control. So to tie it together: the same spoiler surfaces serve two masters. In flight, they’re roll assist devices that rise asymmetrically with the up-going aileron. As speed brakes, they rise symmetrically on both wings for drag. And on the ground, the ground spoilers join in to dump lift and improve braking, all gated by the weight-on switch.