Since its entry into commercial airplane service with the Airbus A320 in 1988, the fly-by-wire system has gained popularity, finally becoming the only system used on new airliners. Fly-by-wire systems are one way of sending signals from the cockpit to the other parts of the airplane. Those signals tell a part of the aircraft (i.e., flaps, horizontal stabilizer, rudder, engines) to move a certain amount. What is unique about fly-by-wire systems is that between the cockpit and the part, computers read and refine the signal before letting it continue. This helps the pilot in times of bad weather (turbulence) and some emergency situations. For a while, Boeing (the largest maker of airplanes) had purposefully kept the fly-by-wire system off of its aircraft because it thought that the computers might not allow the pilot to have the final say in an emergency. On Airbus (the second largest maker of airplanes) aircraft, the computers have the final say, and will never push an aircraft past its predetermined limits.
Boeing had a tough decision to make as it started production of its newest aircraft, the Boeing 777 (pronounced "triple seven" by pilots and other airline workers). It had to decide whether to stick with the traditional hydraulic system used on all previous jets or to follow Airbus and develop a fly-by-wire system. When serious studies on the 777 began in 1990 the Airbus fly-by-wire system had only been out for two years and it was hard to tell if the airlines preferred the fly-by-wire system or the hydraulic system. However, by the time that the flight deck was completed in 1993, it was clear that fly-by-wire was the way to go.
Boeing did not want the computers to have the final say over the pilots, though, so it had to develop a form of fly-by-wire that was substantially different from Airbus's system. Boeing tried to keep the feel and control of the aircraft as similar as possible to older models while still integrating the fly-by-wire computers to assist the pilot, make the cockpit neater (computerized Liquid Crystal Display screens instead of hard to read analog dials), and save weight.
Although it seems complicated, requiring lots and lots of lines of software code, the fly-by-wire process basically consists of five simple steps. The first step is pilot input. An example is a push forward on the yoke (see figures 1 and 2). This signal is an analog (pushing on the yoke is not the same as typing on a computer, which is a digital signal). Since a computer needs to read this signal, it is sent by wire to four boxes called ACE's, the acronym for Actuator Control Electronics. The second step is the ACE's converting the signal into a digital signal so that it can be read by the computers. The now digital signal is sent to three identical Primary Flight Computers.
The third step is the most important step that hydraulic systems don't have. Computers read and then refine the pilots signal. In this case, they will read that the pilot wants to go down. Although refining may never happen, it can be a vital part of the process. Let's say the the pilot pushes down slightly on the yoke, wanting to go down at a 3° angle to the horizon. If the plane is in turbulence that is causing it to already angle down 5°, the computer will probably change the pilot's signal to 2°s up, causing the plane to go to the pilot's intended 3° descent (see figure 3). In equation form, it would look like this: -5° (5° down) +2° (2° up)= -3° (3° down). A similar thing will happen if the pilot does not give a signal at all, but the plane angles down because of turbulence. The computers will realize that the plane is going down and automatically adjust so that the plane becomes level again.
"At each stage in this process, the PFC [Primary Flight Computer] can–and usually will–change the signals according to the circumstances the plane finds itself in. If the pilot moves the control column in such a way that the plane might be heading for a stall, the PFC will send feedback signals that produce a force in the column that pushes against the pilot's attempts. If the PFC gets a message that the plane is flying through turbulence and being made to change its pitch by the turbulent air, it may add something to the signal from the control column to adjust for that change." (Karl Sabbagh from his book 21st Century Jet, pages 156-157)
There are three computers to do this process in case one or two fail. Failing seems to always be a major threat with new technology, especially when 300 plus lives are at stake.
The fourth step is very similar to step two. The refined digital signal is sent to the ACE's to be converted back to an analog signal that the actuators can perform (moving a part is not a digital process). Now comes the actual moving of the part, step five. The actuator picks up the signal and moves the part. In this case, the actuator will move the elevator up a little, causing the plane to go down. The process is now complete with the signal having traveled all the way from the pilot's yoke, through the computers, and to the elevators at the end of the plane. On a 777, the distance between the yoke and the elevators is over 200 feet, yet the signal traveled this distance by wire in milliseconds. (See figure 4 for a diagram of all the steps.)
Safety is always an huge factor in designing an airplane, and it was an even larger factor in the design of the 777 since Boeing was trying to integrate a new technology (fly-by-wire) that some people had doubts about. The biggest fly-by-wire precaution was the use of 3 Primary Flight Computers, any one of which could keep the fly-by-wire system working by itself. Plus, each Primary Flight Computer has three subcomputers. So, there are basically 9 computers running the fly-by-wire system. Obviously, there are a lot of back of systems! Another safety precaution is cockpit power backup. Cockpit power is the power that runs the digital displays in the cockpit (LCD's). Usually, this power is taken from the engines. The chances that both engines would fail is minuscule, but Boeing did not want anything to go wrong, so they added two backups. The first is the APU (Auxiliary Power Unit) which is basically a much smaller third engine which supplies power when the main engines are not running. The second backup is a battery backup, which is the same as a battery backup on a CD player or clock.
The whole reason that both Boeing and Airbus both chose to use the fly-by-wire system is because of it many advantages over older systems. The oldest system is called direct muscular force. Basically, the pilot moves a control in the cockpit, and since that control is directly linked by a steel cable to a part, the part moves. Direct muscular force is the system that the Wright Brothers would have used. Direct muscular force systems not only fill the plane's belly with many bulky, heavy cables, they also cause the pilot to use a lot of muscle to move a part, tiring the pilot quickly. Fly-by-wire systems are very different from direct muscular force systems. Since fly-by-wire is the newest way of controlling planes, it doesn't use bulky cables – it uses thin wires – so the plane is lighter, saving fuel.
The middle system came about with the jet age. It is called a hydraulic system. While it is very similar to direct muscular force since it still uses steel cables, it uses hydraulic power to move the part. Once the force on the cable reaches the hydraulic part, hydraulic power moves the part. This is beneficial over direct muscular force because the pilot does not get very tired since the hydraulic systems are doing most of the work. Fly-by-wire systems are somewhat similar to hydraulic systems because both use hydraulic power to move the part. In fly-by-wire, the actuators use hydraulic power to move the part. The difference, though is that in fly-by-wire systems, the signal is sent by thin wire, not bulky cable, to a computer which refines the signal as necessary.
The fly-by-wire system seems like it will remain the favored system for a long time. The ability to have computer assistance makes the pilot's job a lot easier. As the three United Airlines 777 pilots that I have met have conformed, fly-by-wire was definitely the way to go on the Boeing 777.
Sources
Sabbagh, Karl 21st-Century Jet. New York, NY: Scribner, 1996 (Chapter 9 titled "Flying by Wire")
