That familiar airline ping and the Captain’s voice signs off, “Flight attendants, prepare for landing.” You’re on final approach, and, if you’re sitting in a window seat, you might be watching the flaps make adjustments on the edge of the wing. Even if you can’t see out the window, you know the disquieting rumble of the landing gear bays as they open and the whine as the wheel assembly descends. And when the plane touches down, everyone feels that breathless surge forward in the seats when the engines reverse to brake.
Anyone who flies is familiar with these sensations, but what you may not be familiar with is the series of devices that make this all possible, devices collectively known as actuators. Actuators are devices that convert electricity into pressure, temperature, or mechanical movements, and they are becoming increasingly utilized in aircraft construction. What they have over the hydraulic and pneumatic systems of the past is that they increase fuel efficiency, reduce emissions, lower operating and maintenance costs, optimize performance, reduce noise, and provide higher levels of safety.
Take, for example, those simple flap adjustments you might have seen from the window. Flaps, edge slats, and stabilizer trim are all driven essentially by hydraulic motors. Where the hydraulic systems previously required a centralized fluid feed and cooling systems, electrohydrostatic actuators are self-contained units that have no need for hydraulic pumping that uses extra power nor the cooling systems that further sap energy and create extra weight. They’re self-contained, so they offer fewer areas of potential leakage or failure.
In that same flap mechanism, the flap is moved by another type of actuator called a linear actuator, which converts an electric motor’s rotary movement into linear movement. This mechanism drives a stainless steel piston that can telescope outward or collapse inward to either push or pull the item to which it is attached. Another type of actuator commonly used in aircraft is the rotary actuator, which you might see used, for example, to help the nosewheel steering system pivot through the 360 degree arc required for precision turning.
Because actuators are compact in comparison to their counterparts, they become important components of the safety redundancies of aircraft. Important systems in planes require double and even sometimes triple mirror systems in case of primary failure. Jet engines are protected in the case of fire, for example, by actuators that block off the fuel supply, a safety system that requires redundancy. Duplicate actuators take up far less space and load than other comparable systems.
In fact, you can thank actuators for controlling aircraft velocity and engine speed, for increasing the angle of descent so that you can land, for opening landing gear bays and for powering the system that lowers the wheels, for that telltale reverse engine surge that says you’ve arrived and even for opening the cargo bay doors so that your luggage makes it to the conveyors. In short, actuators make your flights cost less and keep you safer from take-off to touch-down.
By Ivan Young