First and foremost, fly-by-wire means that the pilot's control commands are transmitted electronically to the hydraulic actuating cylinders of the tail assembly with the aid of cables ("wires") instead of through a mechanical rod system and cable controls. But there is also another key difference between the concept and conventional designs: There are always computers connected, as control systems, between the pilot and the tail assembly. These computers monitor all control pulses and correct them if necessary.
"Novices erroneously think of the autopilot, which can be switched on and off," explains Gerhard Hüttig of the Flight Simulation Center in Berlin. The autopilot is also a factor, but it is only one of many, and it guides the airplane along a course pre-programmed by the pilot. But Airbus's flight computers do a lot more. They are automatically activated if the aircraft enters a dangerous angle, loses too much speed or threatens to complete a violent rolling motion. The Airbus engineers christened the software, which is designed to keep the aircraft within a green zone at all times, "Flight Envelope Protection." "The computers intervene," explains Hüttig, "no matter how hard the pilot pulls on the controls."
The flight professor provides a live demonstration of how this happens -- not in the air, but safely on the ground in downtown Berlin, where his center has installed a state-of-the-art flight simulator. Hüttig sits down in the front left seat of an A330 cockpit. "This corresponds almost exactly to the Air France plane that crashed in the Atlantic," says the scientist.
He leaves the takeoff from Munich Airport up to his inexperienced co-pilot. Then, for illustrative purposes, he turns to his own control device. It looks like a joystick, hardly bigger than that of a computer game. "This is the most visible part of the fly-by-wire technology," says Hüttig.
He pushes the control device, known as a sidestick, all the way to the right. It immediately becomes apparent that the computer is indeed along for the ride. The flight simulator's hydraulic legs buckle, creating the sensation of banking gently to the right. "In fact, the airplane ought to have turned so sharply to the right that it would overshoot and, in the extreme case, would roll over," Hüttig explains.
But that is precisely what the computer is there to prevent. In an instant it has calculated, based on the position of the airplane reported by sensors, by how much the plane can turn without creating a hazard.
In the real world of aviation, the equivalent of this maneuver would be another plane approaching from the left. "If the pilot panicked and steered too far to the right to avoid a collision, the computer would prevent him from making a flight error," the aircraft engineer explains.
Boeing installed a similar system into its 777 long-haul aircraft in the mid-1990s. The Boeing 787, the company's latest model, will contain even more of these automated flight systems, and today's Boeing 737s already have various automated components.
But there is a difference between the American and European concepts. In the case of Airbus, the pilot is essentially prevented from disabling the flight computers. Unlike the autopilot, the flight computer can only disable itself, and only if its systems become so confused that it would otherwise malfunction.