Fly-by-Wire: Redefining flight and maintenance

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...


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.

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