Flight Control Systems — Page 162, Lesson 209

Let's start with the big picture. Most jet transport aircraft are fitted with high lift devices on both the leading edge and the trailing edge. Their job is to increase the lift coefficient, which we write as CL. That's the number that tells us how much lift a wing generates for a given airspeed and wing area. By raising CL, the aircraft can produce large amounts of lift at low speed, which is exactly what you need for take-off and landing. And because you can generate that lift at a lower speed, the stall speed comes down. That's the whole point of high lift devices — they let you fly slowly and safely. Smaller aircraft, by contrast, are usually fitted with just trailing edge flaps, not the leading edge devices. Now let's look at the trailing edge flaps themselves. There are various types of flap design, and they all increase both lift and drag, but in varying amounts. For light aircraft, the most popular type is the plain or camber flap. For large transport aircraft, the widely used design is the slotted Fowler flap. A Fowler flap is the one that extends rearward and increases the wing area as well as the camber, and the slots allow high-energy air to flow over the flap to keep the airflow attached. So you have the plain or camber flap on the small side, and the slotted Fowler flap on the big transports. Let me walk you through how the system actually operates. When the pilot moves the flight deck selector, that produces an input to the slat/flap computers. There are two of them, and they control, monitor, and test the operation of the flaps. So you have redundancy there — two computers. The computers command an electrically controlled hydro-mechanical power unit, and that power unit drives the transmission which physically moves the flaps. The position of the flaps is then indicated on the cockpit display, so the crew always knows where the flaps are. Now, the system has protections built in. The flaps are protected against four specific failure conditions: asymmetric operation, which means one side moving differently from the other; runaway, which is an uncontrolled continuous movement; uncommanded movement, where the flaps move without any pilot input; and overspeed. And to stop the operation if excess torque is sensed, torque limiting brakes are fitted. So if the drive system sees too much torque, those brakes stop the flap movement. There's also a dedicated system called the Flap Load Relief System, or LRS, sometimes called the load limiter. Here's what it does: if the airspeed exceeds a predetermined speed, the LRS retracts the flaps to an intermediate position. That protects the flaps from excessive aerodynamic loads at high speed. Then, if the airspeed drops back below its predetermined limit, the system automatically returns the flaps to the selected position. So it's a protective retraction and automatic re-extension. Finally, let's talk about what happens if the main control system fails. In that event, emergency operation of the flaps can be achieved by an alternate hydraulic supply, or by an electric motor. That motor drives the trailing edge drive unit, which is a gearbox, and that gearbox then operates the same gear train. So even with a main system failure, you still have a way to move the flaps — either through a separate hydraulic source or through that electric motor driving the same mechanical gear train. So to tie it all together: you have the flight deck selector feeding two slat/flap computers, which command an electrically controlled hydro-mechanical power unit, which drives the transmission to move the flaps. Position is shown on the cockpit display. You have protection against asymmetric operation, runaway, uncommanded movement, and overspeed, plus torque limiting brakes. The Load Relief System retracts the flaps at high speed and re-extends them when speed drops. And for emergencies, you have an alternate hydraulic supply or an electric motor driving the trailing edge drive unit gearbox.