Digital electronic flight control systems, commonly known as fly-by-wire (FBW) flight controls, increasingly are being used aboard business aircraft because they reduce pilot workload, increase safety margins, and prevent structural and aerodynamic limits from being inadvertently exceeded. Most airframers also include many other proprietary high-level functions and features in their FBW controls to make the aircraft easier to handle during abnormal or emergency conditions and thereby gain an advantage over other manufacturers.

In addition to the foregoing, “Fly-by-wire offers redundancy above and beyond what's necessary for certification. It's a standout in that respect,” says Glenn Zwicker, chief engineer at Parker Aerospace, a leading provider of digital flight control hardware and software.

Airworthiness certification authorities typically require a one in 10 million probability of a catastrophic failure of a hydromechanical flight control system. FBW systems, in contrast, must meet a one in one billion probability of failure. Parker, among other leading FBW manufacturers, targets one in 10 billion probability of failure, notes Zwicker.

“This allows margin for common cause [catastrophic damage] associated with rotor burst or tire failure, along with bird strike or bomb blast. We design in multiple paths and multiple actuators to isolate local damage,” he says.

FBW control systems have no mechanical connections between the cockpit controls and the flight control actuators. Instead, as the name implies, FBW systems have electrical links between the cockpit flight controls and the power control actuators attached to the flight control surfaces. The cockpit hand and foot controls have force and/or motion sensors that measure pilot inputs. The inputs are transmitted as electrical signals to the FBW system computers. Those boxes then send electrical signals to command the movement of the power control actuators that move the control surfaces.

In the most basic FBW Direct Law mode, control inputs by the flight crew result in direct and proportionate movement of the flight control surfaces. The only FBW components required are the electrical cockpit control position or force transducers, actuator control units and the electrohydraulic or all-electric power control actuators attached to the flight control surfaces. There also is a feedback loop that senses when the desired control surface deflection has been attained to tell the system when to stop commanding more movement.

Direct Law, along with other FBW modes, also requires uninterruptable electrical and hydraulic power supplies as there are no backup mechanical links between the cockpit controls and flight control surfaces or power control actuators. If all electrical and hydraulic power is lost, the aircraft will not respond to stick, yoke or rudder pedal inputs.

Direct Law doesn't provide any of the higher level FBW functions that distinguish digital flight controls from conventional hydromechanical-powered flight controls. As with conventional flight controls, it's up to the flight crews to avoid inadvertent stall, or over-speed, over-control and over-stress of the aircraft.

Higher level FBW functions, commonly known as Normal Law or Alternate Law functions, require a second set of computers that are capable of using inputs from several sensors, such as angle-of-attack (AOA), flap/slat and landing gear position sensors, air data and IRS/AHRS, weight-on-wheels and perhaps also radio altitude, as well as cockpit controls. The computers then shape, smooth and calculate the best aircraft behavior in response to pilot inputs, speed and configuration changes and autopilot commands, as well as other factors. The higher level law computers team with the actuator control units to determine optimum flight control response.

However, the term “optimum” is subject to a wide variety of interpretations depending upon the FBW flight control design philosophies of each aircraft manufacturer. Some high-level FBW functions, however, are common to virtually all civil aircraft and include yaw, spiral, static and dynamic pitch stability augmentation to reduce pilot workload. Most FBW systems also have soft or hard limiting for maximum AOA, Vmo/Mmo over-speed and over-stress.

“Once you have the algorithms, it's not difficult for the control laws to accomplish such functions,” says David McLaughlin, Parker Aerospace's chief engineer for systems.

As noted, airframe manufacturers each add their own high-level FBW functions in order to differentiate their digital flight control systems from their competitors' designs.

Falcon 7X

Dassault Aviation's Falcon 7X, certified in 2007, is the first business aircraft to be fitted with digital electronic flight controls. Dassault borrowed liberally from the suite of military FBW technologies that it developed in the mid-1980s for its longitudinally unstable, highly maneuverable, Mach 2 Rafale fighter. It's a vintage design that has multiple components and several redundancies.

Dassault engineers believe that their extensive experience in developing military digital flight control systems gives them FBW design expertise not available to other business jet makers. Rafale's handling qualities, for example, are optimized for “carefree handling” and protection from overstress and loss of control.

Rafale's “gamma dot” FBW pitch function maintains the aircraft's velocity vector or flight path in a desired direction after the stick is released. The aircraft will hold flight path while compensating for changes in airspeed, c.g. and landing gear and high-lift configuration, among other variables. Envelope protection prevents the aircraft from stalling if AOA limits are exceeded, or from spinning in the event of excessive sideslip. It also guards against overstress if the pilot commands a higher g maneuver than the airframe can safely withstand. Many of the Rafale's FBW design features are carried over into the Falcon 7X.

In a move similar to Rafale's design, Dassault elected to fit the Falcon 7X with outboard sidesticks instead of control wheel yokes for pitch and roll command inputs. The sidestick configuration saves room in the cockpit. However, the controls are not mechanically interconnected and thus there is no motion cuing or tactile feedback between the two sides. To compensate, the sidesticks vibrate a warning signal if both pilots are attempting to control the aircraft at the same time. No such stick input conflict cuing is built into two-seat versions of the Rafale, but rather it sums the sidestick control inputs from both cockpits. The Falcon 7X and Rafale retain mechanically interconnected rudder pedals.

Adapting the Rafale's FBW functionality for the Falcon 7X wasn't much of a challenge for Dassault's engineers, for unlike its fighter sibling, the 7X is inherently stable and designed to be flown at subsonic speeds. FBW designers determined that the trijet's digital flight control system only needed a 50-millisecond response time, but they elected to retain the 12.5-millisecond response built into the Rafale's computers to assure there would be no detectable latency in the system. They also fitted the Falcon 7X with smaller, lower power, lighter weight control surface actuators than those on the Rafale because of the former's executive transport mission.

Similar to the Rafale, the Falcon 7X's digital flight control system maintains aircraft flight path with speed and configuration changes; it automatically trims the aircraft to neutralize primary control surface forces and provides stability augmentation. It also has low-speed and overstress flight envelope protection, plus it adds a high-speed envelope protection function not needed in a Mach 2 aircraft. Dassault designed the 7X for Mach 0.97 and 430 KIAS demonstrated dive speeds, but the FBW system limits the aircraft to Mach 0.94 and 405 KIAS.

The Rafale's FBW imposes no pitch angle limits, but the Falcon 7X's digital flight control system doesn't allow nose attitude to exceed 35 deg. nose up or 28 deg. nose down at speeds above 250 KIAS. Nose-up and nose-down pitch limits are reduced at slower speeds.

The Falcon 7X uses a radio altitude input to determine when to make the transition between ground and air modes. Above 50-ft. radio altitude, the aircraft will automatically trim the horizontal stabilizer to maintain flight path when the sidestick is released, assuming it is within the lower and upper speed limits. Below that radio altitude, auto trim is inhibited. At touchdown on landing, a weight-on-main-wheels command signals the horizontal stabilizer to start moving toward the pitch-down position. This causes a natural feeling, derotation of pitch attitude when the stick is released, helping the aircraft to transition to a three-point attitude with weight on the main and nosewheels.

The Falcon 7X has no hard bank angle limits, but the FBW system provides artificial spiral stability that automatically levels the wings if the stick is released and bank angle is less than 6 deg. The aircraft will hold bank angle if the stick is released at angles of 6 deg. to 35 deg. When the stick is released at bank angles greater than 35 deg., the aircraft automatically will roll back to 35 deg. Roll stability augmentation also prevents the aircraft from rolling off due to wing fuel imbalance or partial flight control system degradation.

Dassault installed three, single-channel digital computers plus a dual-channel analog computer aboard the Rafale to provide the required redundancy for such critical flight control functionality. The computers vote on control surface commands, so one or even two errant computers can be disqualified and excluded by the remaining computers.

If the Rafale FBW system has belt-and-suspenders redundancy, the 7X has belt-suspenders-braces-hooks-and-loops redundancy. This assures that it is controllable under all foreseeable abnormal or damaged conditions. It has three full-function, dual-channel main flight control computers (MFCCs) and three limited-function, single-channel secondary flight control computers (SFCCs). The MFCCs are capable of high-level normal, alternate (or degraded high level) and Direct Law modes while the SFCCs only can function in Direct Law modes.

Only one of those six computers is needed to fly the aircraft. All six send flight control-position commands to four actuator control and monitoring units (ACMUs) that essentially are FBW command signal quality assurance inspectors.

The ACMUs monitor the control surface commands coming from the main and secondary flight control computers. In the event of a disagreement between any of the MFCCs or SFCCs, the ACMUs can isolate and exclude that computer from the system. The design assures the system has the required 10-9 probability of failure.

But Dassault also installed a backup analog computer that provides an alternate means of pitch and roll control. The extra computer thus provides 10-10 redundancy similar to Parker Aerospace's current systems.

Embraer Legacy 450/500

The $20 million super-midsize Legacy 500 and $16 million Legacy 450 are the least expensive business aircraft yet to be fitted with full three-axis, digital FBW flight controls. Embraer chose FBW to reduce pilot workload, improve passenger ride comfort, enhance airport performance and save weight.

Similar to the Falcon 7X, the Legacy 450 and 500 have sidestick controllers that are not mechanically interconnected. Similar to the Rafale, the sidestick inputs are summed. The rudder pedals, however, are mechanically interconnected.

The Legacy 450 and 500, similar to the Falcon 7X and Rafale, have “gamma dot” flight path stability, a control function that maintains aircraft trajectory with speed and configuration changes so long as the aircraft remains within low- and high-speed flight envelope limits. Pitch trim is automatic.

The high-level control laws have both ground and air modes, but the transition doesn't use radio altitude. For instance, upon landing, the FBW system makes the transition from gamma dot flight path stability to speed stability after the landing gear and flaps are extended. Changes in speed cause nose-up or nose-down pitch changes. A trim reset button on the sidestick enables the flight crew to immediately retrim the aircraft for a new trim reference speed, thereby relieving the need to hold nose-up or nose-down sidestick pressure.

Embraer also included a heading and roll thrust asymmetry compensation control law that takes most of the work out of handling an engine-out emergency. The FBW system, though, retains enough sideslip to provide the crew with an unmistakable indication of which engine has failed. A flight director cue tells the pilots how much sideslip to add in the direction of the operative engine to optimize one-engine-inoperative climb performance.

Rather than taking Dassault's approach of developing the entire FBW system for its business aircraft in-house, Embraer subcontracted with Parker Aerospace and BAE Systems to save time and cost. Parker's and BAE Systems' hardware architecture for the Legacy 450 and 500 is much simpler than that of the Falcon 7X, but it delivers virtually identical benefits and system reliability.

The Legacy 450 and 500 have two, dual-channel, primary flight control computers furnished by BAE Systems that host high-level control law functions, such as stability augmentation, high- and low-speed envelope limiting and overstress protection. This is one of the latest quadruplex designs that uses four dissimilar channels, each one of which is capable of controlling the aircraft through all flight control surface actuators in all three axes.

The PFCCs send commands to three, multiple channel remote electronics units (REUs), also known as actuator control electronics (ACEs) boxes in other civil aviation sectors. The REUs, also supplied by Parker, only are capable of Direct Law flight control surface actuation solely in response to cockpit control inputs. The design architecture is similar to that on the Boeing 787, except that the jetliner has triple PFCCs and quad-redundant ACEs.

The PFCCs combine pilot control inputs from the REUs with inputs from various sensors, such as AOA, speed, configuration and vertical acceleration, among others, to calculate the appropriate flight control actuator commands based upon higher level Normal control laws. The PFCCs send back the Normal Law control response to the triple REUs that then command the movement of the flight control actuators.

Parker also furnishes most of the FBW software, along with the flight control surface actuators and other hardware, used aboard the aircraft.

Embraer's high-level Normal Law functions include both soft- and hard-limit protection. Within the normal flight envelope, there are +30-deg. /-15-deg. soft pitch and 33-deg. roll limits, along with 1.1 Vs minimum speed and Vmo limits, that can be overridden by maintaining lateral or longitudinal sidestick pressure. Beyond these soft limits, there are hard vertical acceleration, sideslip, maximum AOA and high-speed limits that cannot be overridden. There are no hard pitch or roll angle limits.

Embraer engineers believe the hard maximum AOA limits enable the designers to take advantage of lower takeoff and landing V speeds to improve airport performance. For users, this translates into a payload increase of up to 900 lb. when operating off of short runways, Embraer officials assert. AOA limit also enables flight crews to extract maximum performance from the aircraft during wind-shear escape or controlled flight into terrain (CFIT) avoidance maneuvers.

Gulfstream G650

Engineers in Savannah took a different FBW design approach than other business jet manufacturers, assuring that the G650 has high redundancy with the minimum number of components. Two dual-channel primary flight control computers (FCCs) are supplied by Thales. The FCCs host the high-level control laws. Each has two computing and two monitoring cards, eliminating the need for stand-alone monitoring computers.

Similar to Embraer's layout for the Legacy 450/500, the G650's dual primary flight control computers provide four dissimilar flight control command channels, any one of which can control all flight control surface actuators.

Unlike Dassault's and Embraer's FBW designs, Gulfstream elected to fit the aircraft with individual remote electronics units boxes, or ACEs, integrally mounted with each of the flight control surface actuators. The 16 hybrid REU/electrohydraulic system actuators (EHSAs) are also made by Parker.

The basic FBW design meets the 10-9 probability of failure required for certification, but Gulfstream wanted an additional level of redundancy to provide 10-10 probability of failure. So, the G650 has a fully autonomous, three-axis backup flight control system (BFCS) computer made by Thales that complements the dual primary flight control computers. It sends its commands to one of two REUs that signal the movement of the flight control surface actuators on the left and right ailerons, left and right outboard spoilers, left and right elevators and the rudder.

The G650 only has two hydraulic systems, rather than the three required for most FBW flight control systems. To meet the triplex power requirement, Gulfstream and Parker Aerospace teamed to create the aviation industry's first dual-mode flight control actuators.

These components are known as electric backup hydrostatic actuators, EBHAs for short. They function as conventional EHSAs if hydraulic power is available. However, in the event of hydraulic system failure, the actuators revert to a backup mode that uses tiny electrically powered hydraulic pumps mounted on the actuators. The actuator electric pumps generate the fluid power required to move the control surface. The design eliminates the need for a full-time, electrically powered third hydraulic system, thereby saving considerable system weight.

In the event of failure of both primary flight control computers and both hydraulic systems, the backup flight control unit sends commands directly to the EBHAs.

And if both engine-driven generators were to fail, the G650's 15 KVA ram air turbine (RAT) can be extended to generate emergency electrical power to supply the FBW system. Below 180 KIAS, 24 VDC emergency batteries take over from the RAT to assure an uninterruptable power supply to all electrical components in the FBW system.

Following Boeing's lead on the 777 and 787, Gulfstream elected to retain a conventional control yoke wheel for the G650 instead of fitting the aircraft with sidestick controllers. The yokes add weight, cost and complexity, but they are mechanically interconnected so that the pilot not flying can see and feel the pilot flying's control inputs. Gulfstream and Boeing engineers assert that the design promotes crew situational awareness and enhances crew resource management.

The G650's high-level Normal Laws, hosted by the dual primary flight control computers, are quite similar to those of the Boeing 787. The basic pitch law is similar to the 787's C*U design. C* means that fore/aft yoke movement commands pitch rate by means of a simple Direct Law mode on the ground and vertical acceleration, or g rate, in the air using several inputs to the FCCs. U means that the aircraft is speed stable, so the pilot must trim nose up or down with speed change.

Three-axis stability augmentation makes the aircraft easy to fly because it compensates for trim changes caused by control surface or landing gear extension or retraction. Similar to the 787, the G650's FBW system has a maneuver load alleviation function that progressively deflects the ailerons and outboard spoilers at 1.5 g's and above to reduce the lift produced by the outboard wing sections and thus the wing bending moment. Other high-level control functions include automatic retraction of the speed brakes under certain conditions, dynamic rudder travel limiting to prevent overstress of the vertical fin, and elevator split limiting to prevent overstress of the empennage.

Similar to Boeing and Embraer FBW designs, the G650 has no hard limits on pitch or roll angles. However, the system does have hard limit maximum AOA and high-speed flight envelope protections in the Normal Law mode.

If air data or IRS information is insufficient or not available, the FCCs revert to Alternate Law mode. The autopilot becomes inoperative and flight envelope protections are degraded. If all four channels of the primary FCCs are unavailable, the FBW system reverts to Direct Law mode in which the control surfaces respond directly and proportionately to cockpit flight control inputs.

The Future of FBW

Bombardier has announced that its two new ultra-long-range, high-speed business jets, the Global 7000 and Global 8000, will be fitted with FBW controls. Many components will be supplied by Parker Aerospace.

Many other general aviation manufacturers are likely to follow. Most early adopters will fit FBW controls to aircraft that otherwise would require conventional powered flight control surface actuators. But future light and medium business aircraft may feature some form of FBW because maneuver load alleviation and flight envelope protection may enable engineers to reduce structural weight, thereby improving fuel efficiency and increasing tanks-full payload.

Plainly put, FBW aircraft are easier to fly than aircraft with conventional flight controls. Optimum stability and control response characteristics can be written into the high-level primary flight control computer software codes, transforming a marginally stable, but aerodynamically superior airplane into the most docile handling ship in the air. Flight envelope protections enable pilots to fly the aircraft closer to stall and high-speed margins so that they can safely, consistently and confidently extract more performance out of the aircraft when needed.

But potential aircraft design and development cost reduction is the main incentive for airframe manufacturers. Future designs can be lighter weight, more aerodynamically efficient and even longer lasting because of FBW's stability augmentation, maneuver load alleviation and flight envelope protection capabilities. Flight test development programs should be shorter because FBW software will assure that aircraft consistently meet stability and control standards set by airworthiness certification authorities. Any shortcomings on the path to certification will be corrected by rewriting code in a few hours rather than having to paste on new aerodynamic bandages such as vortex generators, stall strips and wing fences.

In a few of decades, fly-by-wire on new turbine aircraft could become as common as throttle-by-wire, steer-by-wire and brake-by-wire on current aircraft. That development could make new models so much more efficient, as well as easier to fly than older models, that fleet replacement might accelerate at an unprecedented pace. BCA