Flight Control Systems — Page 162, Lesson 215

I want to walk you through the spoiler system on a large transport aircraft, and then the automatic ground speed brake control that brings the spoilers up on landing. This is a dense area, so let's take it piece by piece. First, spoiler operation. The pilot rolls the control wheel, and that rotation transmits a roll control signal to the aileron Power Flying Control Units through the captain's control cables. So the captain's cables carry the mechanical input from the wheel to the aileron power units. When that aileron Power Flying Control Unit moves, it does two things: it actuates the ailerons themselves, and simultaneously it sends a roll control signal to the spoiler mixer. The spoiler mixer is the device that takes that roll demand and distributes it to the spoilers. Now, rotation of the spoiler mixer's output mechanism actuates the spoiler control valves, and this raises the spoilers on the down-going wing. Think about that: when you roll left, the left wing goes down. The spoilers on that down-going wing come up, which is the wing where the aileron has gone up. So the spoilers assist the aileron by adding drag and killing lift on that side. Here's the clever part — the follow-up. Vertical rotation of the spoiler actuators provides the follow-up to cancel the control valves' input when the desired spoiler deflection has been achieved. In plain terms, the actuator moves the spoiler, and that movement feeds back to close the valve once the spoiler has reached the commanded angle. This follow-up is what allows the flight spoilers to be positioned at any intermediate angle between retracted and fully up. It's not just on or off — it's proportional. And in this manner, the flight spoilers assist the ailerons in providing lateral control. Now, the hydraulic power for the flight spoilers is controlled by electric motor driven valves situated in the flight control power system. These valves allow the spoilers to be isolated if required. Two spoiler switches individually control these valves. So you have two switches, each controlling one of those electric motor driven valves, giving you the ability to shut the spoilers down. Here's an important operational point: the flight spoilers continue to provide lateral control when used as speed brakes, and they will sum the two inputs of Roll and Speed brake. The system adds the roll demand and the speed brake demand together. The example in the text: with the speed brakes deployed in flight and a pilot's input to roll or turn to the left, then the spoilers move up on the down-going wing, and down on the up-going wing. So there is a combination action of the spoilers taking into account the effects of drag, adverse yaw, roll, and speed control demands. Adverse yaw, by the way, is the tendency of the aircraft to yaw opposite to the direction of roll — the spoilers help manage that. Now let's move to the automatic ground speed brake control operation. This is what brings all the spoilers up automatically on landing. The operation is a function of input signals from several sources. Let me list them. First, the speed brake lever selected to the armed position. Arming the speed brake lever places the flight and ground spoilers in the automatic lift dumping mode of operation. So "lift dumping" — that's the term for killing the lift on touchdown so the weight goes onto the wheels for braking. Second, anti-skid, specifically wheel spin up. The anti-skid system sends electrical power signals to the wheel speed relays for each wheel. A combination of wheel spin up signals on touchdown, through two parallel circuits, will energize the speed brake actuator to drive the speed lever to the up position, raising all spoilers. So when the wheels spin up on touchdown, that's the trigger. Third, air/ground sensing. If both anti-skid channels are inoperative on touchdown, the air/ground sensing circuits will actuate the system when the landing gear strut is compressed. So this is the backup — the strut compressing tells the system the aircraft is on the ground. Fourth, thrust lever positions. Retarding the thrust levers on touchdown will operate the speed brake lever to raise all spoilers. So pulling the throttles back is another input. Fifth, thrust reverser operation. The reverser system linkage mechanically raises the speed brake lever and energizes a relay which supplies power to the speed brake system, raising all spoilers. So deploying the reversers mechanically drives the lever up. Sixth, excess indicated airspeed protection — excess IAS protection. There is an automatic protection system to prevent deployment at excess IAS. So if the aircraft is going too fast, the system will not deploy the spoilers. Finally, the system has a go-around capability. A wheel spin up after slowing down, or the opening of the thrust levers, will retract all spoilers. So if you decide to go around — you open the throttles or the wheels spin up again — the spoilers come back down automatically. So to tie it together: the spoilers are a multi-function control surface. In flight they assist the ailerons for roll, they act as speed brakes, and on the ground they dump the lift for braking. The automatic system watches the speed brake lever arming, wheel spin-up, air/ground sensing, thrust lever position, and thrust reverser deployment — and it has protection against excess speed and a go-around capability to retract them.