Fly-by-Wire
Redefining flight and maintenance
By Jim Sparks
September 2001
One of my favorite toys as a youngster was a model aircraft equipped with a single-cylinder, air-cooled engine fueled with cigarette lighter fluid. The method of control was a simple handgrip. The aircraft was tethered to the grip with two wires (actually, heavy fishing line) one of which insured the toy would always circle the holder, while the second wire was linked to a movable horizontal stabilizer. The instruction book provided directions for successful operation involving a whole series of maneuvers including figure eight’s and landings. I was usually able to get the machine in flight, but my landing techniques were still far from those described in the directions. This was my first exposure to "fly-by-wire" technology.
Evolution of flight control systems
The evolution of flight control systems is the one primary factor that has brought us forward from the days of the Wright Brothers. As aircraft became larger and engine power increased, more force was required to manipulate the flight control systems. Soon control surfaces were assisted by aerodynamic tabs, pneumatic cylinders and hydraulic actuators. These conventional systems can often be a limitation in the initial design phase of a new aircraft and will affect wing size and shape along with location and size of stabilizers. With the onset of the age of electronics, it was only natural to anticipate the removal of push-pull rods and cables as the primary means of supplying commands to flight controls and replacing them with wires. Now the pilot’s actions are interpreted by an electrical signal and then the order is carried out by an actuating device. A fly-by-wire system would be lighter, and more responsive, to pilot input. In addition, military aircraft would be less vulnerable to battle damage. The end result is an increase in efficiency, safety and performance. Vulnerability of aircraft using long cable runs has unfortunately been demonstrated on several commercial airliners. Once in the mid-1970s, a main entry door blew off in flight and the resulting collapse of the floor caused loss of use of the control cables. What happened next requires no explanation. With the redundancy demanded in the certification of transport aircraft, the routing of wires to provide electronic flight control operations would be such that any one failure should not be able to disrupt safe operation of the aircraft. In addition to improved safety and handling, significant weight savings can be expected by using wires rather than cumbersome and complex cable or link rod systems.
Redefining flight and maintenance
Over the years, "fly-by-wire" has taken on a whole new meaning. Both Boeing and Airbus are utilizing computer technology on current production aircraft and Dassault announced its next business jet will also be fly-by-wire. Enhanced aircraft responsiveness and reduced pilot workload proven by supersonic military aircraft will no doubt make electronics the control method of choice. In fact, studies are under way to adapt these principles to general aviation aircraft. Just imagine working an inspection where flight control cables don’t require lubrication, replacement or even a visual check. In fact, the majority of the system can be completely tested by sitting in the flight deck and electronically interrogating a whole series of flight computers using onboard maintenance computer diagnostics. Flight control trim tabs will no longer be needed. External mechanical aerodynamic enhancements will be replaced with an optimized control surface that is specifically placed by a computer in a fraction of the time it would take a man to react. Airbus has already eliminated the stick and yoke and has gone to side mounted "joysticks" for both pilot and co-pilot. Boeing, on the other hand, still uses a conventional means of flight control input.
Although the concept may sound simple, the engineering challenges are immense. First of all, software has to be written and tested so that bug-free operation can be expected. This, in itself, takes tremendous hours of programming. Hardware also has to be developed that will implement the orders of the software using pinpoint timing and accuracy.
History lessons for the future
Even though this is still new technology, there is the need to include "lessons learned." That is, the design of the conventional control systems must incorporate protections and safeties as proven needed from history.
The thought of an entirely electronically-controlled aircraft has many people shaking their heads and wishing for the good old days. In all honesty, fly-by-wire is nothing more than the next step in Autopilot technology. Adapting to automation is never easy. In fact, trusting your own body senses to respond to a sensation is only natural. However, when the Concorde made its debut, many of the early flight crew members had a rough time adjusting to the 3 auto-throttle modes along with the 17 possible autopilot mode selections. Even in more conventional aircraft equipped with auto-landing systems, pilots have been seen hovering over the control systems in anticipation of the worst. The first autopilots were wing-leveling devices designed to give the pilot a break on long flights. This led to plans to hold altitude and even make coordinated turns. Eventually, systems were designed and installed that would allow the auto pilot to make precision approaches, and even landings, in situations where the flight crew is unable to see the runway before touchdown.
Electronics and maintenance
Many of us in the aircraft maintenance business already have first hand experience with electronic involvement. Engines now use Electronic Engine Controls (EEC) or Digital Electronic Engine Control (DEEC) and the majority of new aircraft incorporate a means of electronic interrogation to assist maintenance in resolving discrepancies. In fact, the electronic control of engines is one of the main features that makes total "fly-by-wire" a reality.
The concept here requires no direct mechanical link between the cockpit and flight controls. Instead, a series of computers will sense and process all inputs — including pilot request — then the computations are made that will determine precise and optimum control surface position. The results will be transmitted to actuators for the specific flight controls. Important functions such as the normal flight envelope can be programmed to ensure protection against stall, as well as the hazards associated with high-speed flight.
Is this new technology?
Not exactly. Almost 30 years ago, engineers at the Dryden Flight Research Center at Edwards Air Force base in California began discussions on how to modify an aircraft to achieve a Digital Fly-by-Wire (DFBW) testbed.
In May 1972, a modified F-8C Crusader became the first aircraft to fly depending on an entirely electronic flight control system. This program continued for 13 years and logged 211 flights.
The benefits of this research can be seen on most modern, high performance military aircraft and even extends to the Space Shuttle. For example, the side stick controller used on the Lockheed F16 Fighter is a direct result of the Dryden program. In addition, angle of attack limiters and maneuver-based flaps are devices that have immense value in the operation of a transport aircraft.
The triple redundancy, which was used on the F-8, prompted a concept called "Analytic Redundancy Management," where relationships between various sensors are used to detect any abnormality in any one sensor. In another series of tests, backup software was implanted to ensure survival and controllability — even if all three primary flight control channels developed malfunctions. For more on this and other research projects performed by the Dryden Flight Research Center, log on to www.dfrc.nasa.gov.
New vs. conventional
Making the new aircraft fly like a conventional machine is another significant challenge. In some cases, engineers install an artificial feel system, which gives the pilots the sensation that there is still a direct linkage with the flying surfaces. These systems may use nothing more than a spring or might have the complexity of varying control forces with airspeed. Conventional aircraft require frequent adjustments by the pilots to maintain stability. For example, as the engines consume fuel, depending on the location of the storage tanks, a change of center of gravity will be experienced. In a conventional aircraft, the pilot would make a small change to the flight control system — possibly by the use of a trim system. In a fly by wire aircraft, no trim system or even trim tabs would be needed. When the fuel levels change, the flight control system sensing slight changes in the airframe automatically makes the needed correction.
Tricks of the trade
Some aircraft include a device known as a stick shaker, which will vibrate the control column anytime an excessive angle of attack is reached. With fly-by-wire technology, the system has the ability to ensure the aircraft never reaches the attitude where stall could occur. Boeing engineers on the 777 gave pilots "just a little bit more," in that if the pilot really wanted to stretch the limits, they have a means of overriding the fly-by-wire system.
Using the Boeing method, whenever the pilot moves the control column, there are three "movement detectors," otherwise known as transducers. Any of these three sensors will observe and calculate the movement of the control input and deliver an appropriate electronic response to the Actuator Control Electronics, which consists of four independent microprocessors. These devices will, in turn, reconfigure the electronic input into a digital format. Once the digital signal is processed, it is then supplied to the Primary Flight Computers. These three boxes send nine identical outputs whose content consists of orders for the individual flight control servos.
Monitoring malfunctions
Comparators look for any discrepancy in the data supplied by the three flight control computers and should any error be found, the source computer will be identified and subsequently isolated from the system, while the two remaining computers continue to fly the aircraft. Likewise, should any of the four Actuator Control Electronics devices malfunction, it too will be automatically isolated and the system can still function even with multiple failures. In addition to observing pilot inputs, the Flight Control Computers also observe data from systems sensing airframe parameters such as airspeed and attitude.
This data is blended with the pilot request and the Flight Control Computers issue the orders for aerodynamic response. More often than not, the commanded action is in line with the pilot’s request; however if the demand from the flight deck exceeds the structural or aerodynamic limitations of the aircraft, the Flight Control Computer will do what is required to keep the aircraft operating within a safe envelope. Boeing, unlike Airbus, has designed a pilot override system so that in the event of extreme situations, the flight crew can initiate excessive commands in an attempt to get the aircraft back into a safe flight regime.
Benefits for maintenance
Obviously, this degree of sophistication will require sensors observing all means of entering pitch, roll and yaw commands as well as devices to measure flight control deflections and range of movement. Imagine the benefit of this technology for maintenance. For example, there will rarely if ever be a requirement to install rigging fixtures for flight controls or control columns as positions are monitored electronically, plus the before mentioned removal of mechanical controlling devices. Even the computer systems are self-monitoring and self-testing so should any malfunction exist, it should be adequately logged into a fault memory and then supplied electronically to appear as a fault message to flight crew and maintenance technicians.
Taken to the next step, fly-by-wire could be made to operate the entire aircraft with little or no human intervention. Taking into account that extraordinary things may occur and with this new technology, new problems might actually be introduced and manufacturers have elected to bank on qualified and experienced pilots to prevent the aircraft from being flown only on electronics, but still want to include sights, sounds and the age-old "seat of the pants" approach to provide continual feedback on how things are going.
As far as we who have made aircraft maintenance our profession and the ones who will be assigned the task to maintain this new technology, a degree of apprehension is often sensed. This is yet another challenge to learn new methods and what may at a distance appear to be more complex technology, in reality may make our jobs simpler.
I still think about my childhood airplane on a tether, but then I see children of today operating radio control models and I think "Wow, where was that when I was growing up?" Then reality settles in and I realize I don’t need the model, I’m dealing with the real thing.
Jim Sparks is Manager of Technical Information Support Services for Dassault Falcon Jet. He is an A&P and an Electrician, who began his aviation career as a technician in General and Business Aviation. Later, he became a Technical Instructor on Falcon aircraft and then a Field Representative.
