Flight Control Systems — Page 162, Lesson 211

I want to walk you through the leading edge devices of a flight control system. We've already covered the trailing edge flaps, so now we're moving forward on the wing — literally to the front edge. Let's start with the three types of leading edge devices. We have slats, Kreuger flaps, and variable camber flaps. Some aircraft use just one type, but some use a combination. The Boeing 747, for example, uses Kreuger flaps for the inboard section of the wing and variable camber flaps for the outboard section. Now, what's the difference between these? A leading edge flap is a hinged surface that extends by rotating downward from the lower surface of the wing leading edge. So it swings down from underneath. A slat, on the other hand, is a section of the wing leading edge that extends forward — it moves out ahead of the wing. Depending on the trailing edge flap setting, the slat forms either a sealed or a slotted leading edge. That's an important distinction: sealed means no gap, slotted means there's a gap for air to flow through. How are these powered? Leading edge flaps and slats are operated by hydraulic power or by air turbine motors, and they're controlled by operation of the flap lever. Some systems use hydraulic motors that power screw jacks to move the surfaces. Now, here's a critical safety point: those systems require mechanical locks to prevent creep of the surfaces when hydraulic power is removed. Creep means the surface slowly drifts or moves on its own — you don't want that, so the locks hold it in place. Now let's look at the sequence of operation, because this is a coordinated dance with the trailing edge flaps. When the trailing edge flaps are retracted, the leading edge flaps and slats are also retracted. When the trailing edge flaps move into the intermediate position, the leading edge flaps extend fully, and the slats extend to the midway position — though that midway position depends on the aircraft type. And when the trailing edge flaps are fully lowered, the slats extend fully. The sequence is reversed when the flaps are retracted. So it's a staged progression: retracted, then intermediate, then fully down. What happens if the normal hydraulic system fails? There's an alternate system. For leading edge devices, the alternate hydraulic operation is a standby hydraulic system. But for those powered by air turbine motors, the alternate is an electrical standby system. When the alternate system operates, the leading edge devices will fully extend. Now, here's a limitation to note: depending on the aircraft type, it may or may not be possible to retract the leading edge devices using the alternate system. So on some aircraft, once they're out, they stay out until normal power is restored. There's also an autoslat system that may be incorporated. This automatically extends the slats from the intermediate position to the fully extended position. When does it operate? If the aircraft approaches the stall angle of attack and the slats are not fully extended. So it's a safety net — if you're getting close to a stall and the slats aren't already out, the system pushes them out to protect you. Finally, let's talk about indications. The pilot needs to know where these surfaces are. Typical indications for flap and slat or leading edge flap positions are shown in Figure 7.9 — on the left you have an electronic display, and on the right an analogue display from an older aircraft. So you can see the position of each surface on your instrument panel. Let me pull that together for you. The key points are: three types of devices — slats, Kreuger flaps, variable camber flaps; they're powered hydraulically or by air turbine motors; they move in a coordinated sequence with the trailing edge flaps; they have mechanical locks to prevent creep; there's an alternate system for failures; and there's an autoslat system for stall protection. That's the complete picture of leading edge devices.