Apr 24, 2014 Fly by Wire: Fact versus Science Fiction

By Stephen Pope / Published: Apr 23, 2014

Related Tags: Jets, Aircraft, Fly By Wire

The idea that the flight-control computers in a modern fly-by-wire airplane are in charge all of the time still sparks debate among pilots more than 25 years after the technology became the de facto standard in Airbus airliners. The very notion that the computers can ignore the pilot’s inputs if they so choose but the pilot can never override the computers can be a hard concept to grasp. Still, as fly-by-wire control philosophies increasingly spill over into general aviation, we’ll have no choice but to learn about — and ultimately embrace — this new way of flying. Should we be worried?

Think back to when you were a fresh student pilot trying to understand the relationships between pitch, power, bank angle and a dozen other small but vitally important things related to making the airplane go where you wanted it to. If you started out in a typical light trainer, some of the difficulties you encountered early on were probably related to the fact that you were learning to control the airplane based on its pitch attitude and power setting. Once you graduated to instrument training under the hood, you quickly realized that precision control required a delicate sequence of adjustments of multiple controls and a complex scan of the instruments just to keep you from losing control. At some point you probably thought to yourself that there had to be a better way.

Wouldn’t flying be so much easier if we could devise a system of control that would allow us to fly a desired trajectory without all the instrument interpolation and endless control adjustments? In an ideal world, we could simply pull back on the yoke or stick to reach a desired climb attitude, let go of the stick once we reached that attitude, and gain altitude without losing a knot of airspeed or having to touch the pitch-trim wheel.

Description: Fly by Wire

This, of course, is what a fly-by-wire digital flight control system architecture linked to highly sensitive inertial sensors and autothrottle allows the pilot to accomplish. The computers command the flight control surfaces to move ever so slightly to keep the airplane on the chosen trajectory and target airspeed without ever straying outside the flight envelope. It’s the stuff of science fiction, but it exists today.

As the term implies, fly by wire (FBW) replaces conventional mechanical flight controls with an electronic interface. The pilot’s movements of the flight controls are converted to electrical signals, which are interpreted by the flight control computers. They, in turn, determine how to move the actuators at each control surface to make the airplane do what the pilot commands. FBW offers a variety of benefits, the most obvious being a marked reduction in mechanical complexity, as aircraft designers are no longer forced to route control cables through pulleys and cranks to the control surfaces way out on the wing or tail.

The trade-off, of course, is that they must instead devise another, incredibly complicated system that uses computer software to achieve the same results with no mechanical links. Given the difficulty, is it all worth it?

Switching from conventional mechanical flight controls to a fly-by-wire architecture allows aircraft designers to get creative with the design of the wing, trading natural stability for improved efficiency.