Fly by Wire Technology

 - February 8, 2008, 4:57 AM

Considering the myriad benefits that digital fly-by-wire flight control systems provide, it’s somewhat surprising that only one business jet–Dassault’s Falcon 7X–has been certified with the technology.

Fighters built with digital flight controls have been roaring into the skies since the 1970s, primarily to compensate for their inherent aerodynamic instability but also to eliminate the danger of hydraulic system damage during combat. Airbus began incorporating fly-by-wire technology into the design of its airliners with the launch of the A320 in the 1980s as a way to save weight, increase reliability and cut cost. Boeing followed suit a decade later in the design of the 777 and is bringing fly-by-wire controls to the composite-construction 787 as well. Yet despite the widespread acceptance of digital flight controls among developers of military aircraft and airliners, business jet makers by and large have chosen to stick with traditional mechanical control systems using the same rod, cable and pulley techniques that have been applied since aviation’s golden age.

But that could be about to change. Dassault has made a major commitment to digital flight control technology that extends beyond the 7X to every future Falcon on its developers’ Catia design screens, beginning with its planned Rolls-Royce-powered super-midsize jet. It’s no surprise Dassault has embraced fly-by-wire (FBW) architecture considering its long fighter jet heritage and in-house expertise with digital flight control system design. But Dassault isn’t the only business jet maker interested in the technology.

Gulfstream has spent the last couple of years testing proof-of-concept designs as part of a so-called advanced flight controls (AFC) initiative. Announced in mid-2006, the AFC test program used a Gulfstream V modified with fly-by-wire and special actuator technology to evaluate the technical merits and configuration benefits of the systems. In the first phase of testing, Gulfstream evaluated the wing’s spoilers, which were commanded by electro-mechanical actuators. Test pilots also evaluated roll control, speed brake and landing operations, flying to the limits of the GV’s speed and maneuvering envelope.

The second phase of flight testing included making modifications to the airplane’s elevator control system by adding an electrical backup hydraulic actuator (EBHA) and other components. The hydraulically powered EBHA featured a self-contained reservoir and an electric pump backup mode that allowed it to operate even after a loss of aircraft-supplied hydraulic fluid. Gulfstream reported positive results from both rounds of flight trials. “The early testing has provided valuable data for us to consider the tradeoffs in flight control system design in future aircraft models,” said Pres Henne, senior vice president for program, engineering and test at Gulfstream. Whether that means the next all-new business jet from Gulfstream will necessarily incorporate FBW controls remains to be seen, but it’s clear that business jet manufacturers recognize the benefits of FBW.

Bombardier, likewise, has been investigating FBW technology. As part of the Canadian company’s Active Control Technology demonstration, engineers fitted a Challenger 604 with specially designed sidestick that sits on a pedestal in the cockpit next to the original control column. Bombardier admits it has been quietly working on the technology for the past few years, but most of the work appears related to bringing FBW to its next regional airliner family.

Embraer did just that during development of its E170/190/195 airliner line, cooperating on a joint FBW project with Honeywell. The controls in these airplanes are integrated with the Honeywell Primus Epic avionics system, which allowed designers to reduce the number of line replaceable units (LRUs) that were required, translating to less weight and lower cost. The “modules” used for the fly-by-wire systems in the Embraer jets are multi-redundant and can communicate with multiple actuator control electronics units, which tie directly into Primus Epic. As a result, the FBW systems use much of the same input/output and sensor data that is already available to the avionics.

Taming the Flight Envelope
Analog fly-by-wire systems have been around since the 1950s. One of the first airplanes ever to be equipped with digital fly-by-wire flight controls was a modified F-8 Crusader in 1972. NASA used the airplane to test the fledgling technology, which quickly evolved into systems for a new generation of fighters in the mid- to late 1970s, including the F-16 and Mirage 2000. Fly-by-wire made its civil debut in the form of an analog system aboard Concorde, which from 1976 through 2003 served as the world’s only commercially viable supersonic transport.

In the years since, world militaries have adopted FBW as the standard for all modern fighters, not by choice but by necessity. Because these airplanes fly on the edge of controllability, they require the assistance of computers to remain within the flight envelope. In essence, FBW provides artificial stability in airplanes where the engineers have purposefully moved the c.g. to enhance certain flight characteristics. Negative stability, for example, can produce significant gains in lift under certain conditions, but the tradeoff makes modern fighters virtually unflyable with conventional flight controls.

In modern airliners such as the Airbus A380 and Boeing 777, a primary design advantage of FBW is that it allows the engineers to use lighter wing and tail structures. And because the FBW system replaces the mechanical linkages between the control surfaces and the cockpit, it makes the control system inherently easier to build.

Fly-by-wire is the term used to describe any electronically managed aircraft flight control system, but such systems can differ greatly depending on their design. FBW systems can be fully digital servo-based flight controls that use advanced computers to make the aircraft easier to handle, or they can be limited to simpler control systems linked only to certain surfaces (in the Citation X, for example, the yaw damper on the rudder is fly-by-wire). But in basic terms, FBW systems replace the mechanical linkages between the cockpit controls and the moving surfaces with electrical wires and flight computers.

Fly-by-wire technology uses electronically controlled actuators that enable movement of an aircraft’s flight control surfaces, including spoilers, ailerons and flaps on the wings and the rudder and elevators on the tail. Because the computers monitor everything the pilot is doing and can provide control-surface input whenever needed, there is no need for a trim switch in the cockpit of a FBW airplane. In the Falcon 7X, for example, the pilot simply puts his velocity vector (shown on the HUD and on the flight displays) where he wants the airplane to go and the computers make sure it does what it is expected to.

For example, making steep turns in the 7X is a simple exercise of rolling into the turn, putting the nose on the horizon and letting go of the stick. The computers take care of the rest, maintaining proper bank and pitch angles throughout the turn at a constant altitude and rate of turn. The FBW system can also automatically (and quickly) compensate for turbulence or Dutch roll, features that 7X pilots and passengers are learning to appreciate.

At the heart of any FBW system are the flight computers, which convert the pilot’s commands into electrical impulses delivered to the control surfaces. When the technology was first brought to the A320, it was a major achievement for a number of reasons. The elimination of hydraulic systems reduced the weight of the aircraft, which in turn cut fuel burn and lowered operating costs for airlines. But fly-by-wire technology also worried many airline pilots who were reluctant to cede control of the airplane to the computers.

Video footage of an Air France A320 crashing into treetops and bursting into flames after making a low pass at a French airshow on June 26, 1988, only reinforced the opinion that FBW technology wrested too much control from the pilots. Accident investigators blamed the low altitude and airspeed and the captain’s tardiness in applying go-around power for the accident, in which three passengers died, but many observers pointed to the A320’s built-in flight-envelope protections as the real culprit. The airplane’s FBW architecture is designed to prevent the A320 from entering a stall, but to this day some still believe the system interfered with the captain’s ability to clear the trees at the end of the runway–in essence to squeeze more performance out of the airplane than the computers would allow.

Despite the controversies that have surrounded the technology, fly-by-wire has been proven to offer significant safety enhancements with the introduction of so-called “hard” envelope protections.

In airplanes where artificial envelope protections are incorporated, the pilot’s commands to the control surfaces are monitored to ensure the aircraft remains within a safety margin, or flight-protection envelope. Thus, the pilot should always be able to coax the maximum out of the aircraft in an emergency without running the risk of exceeding the flight envelope or overstressing the airplane. In essence, this means the pilot can push or pull on the stick as hard as he chooses without ever having to worry about breaking the airplane or straying beyond its lift and maneuverability capabilities.

Digital Flight Controls in the 7X
Dassault has adopted digital flight controls and sidesticks in the 7X to replace the conventional mechanical linkages and control yokes. Most pilots who have never before flown with sidesticks say they quickly become comfortable using them, and the majority of Airbus pilots say they have grown to prefer sidesticks because they are intuitive to use and take up less space in the cockpit.

Dassault designed its first digital FBW flight control system in the mid-1970s for the Mirage 2000 and since then has brought the technology to all of the various Mirage 2000 models to follow, as well as the Rafale. As a result, all components of the Dassault systems are designed and manufactured in-house, including the servoactuators and flight computers, a philosophy that is continuing in the design of the Falcon 7X and SMS.

Unlike Airbus, Dassault has decided to use a mix of hard and soft limits for the Falcon 7X system, blending the philosophies of hard limits controlled at all times by the computers and soft limits that the pilot can override. The hard limits in Airbus FBW jets prevent pilots from pitching up more than 30 degrees; pitching down more than 15 degrees; banking more than 67 degrees; or exceeding 2.5 gs during any maneuver. Dassault’s limits are not nearly as intolerant, permitting the pilot instead to make rapid roll changes and even to roll the airplane inverted–but not to stall the wing. The result is an airplane with almost fighter-like reflexes that can be hand flown with ease and complete precision.

Dassault recently decided to drop the term fly-by-wire from its internal lexicon, preferring the phrase digital flight control system instead. Whatever you call it, the technology is intended to achieve the same goals, namely to enhance performance and improve safety, said Olivier Villa, Dassault senior vice president for civil aircraft. “We strongly believed that if the system was well designed it would bring extra safety without any operational limitations,” he said. “The system protects the airplane from getting out of the flight envelope, meaning the pilots are more likely to reach the maximum maneuvering capability of the aircraft because they don’t have to worry about exceeding its capabilities.”

The major concern for Dassault when developing the 7X’s digital flight control system, said Villa, wasn’t determining the technology’s benefits (those were already well known internally), but rather figuring out how best to convey the benefits to customers and pilots. “We understood that we needed to make a system that would be well accepted by creating limits that were clearly not seen as [restricting] the pilots’ normal way of flying,” he said. “For example, we put in a limit for the rate of roll, but the limit is so high and gives you such a brisk roll that no pilot will ever complain that the system is limiting his ability.”

Dassault could have chosen to develop the 7X with conventional flight controls, but the designers wouldn’t have ended up with the same airplane, Villa noted. “Once you decide to go with a digital flight control system, you open the box to many other opportunities. The digital flight control system allows better use of the control system, to incorporate changes in the aircraft geometry, to reduce some stability margins, and basically you see that you are able to design a more efficient airplane.”

Villa explained that control-surface positioning in the 7X has been optimized to minimize drag, with the static stability margin reduced for a corresponding reduction in fuel burn. Flight-envelope limitations in the 7X include angle of attack/attitude, load factor and airspeed. The Falcon 7X, for example, is protected not just from abrupt control forces and stalls, but also against sustained flying above Mmo.

The changes made to the 7X design that resulted directly from the incorporation of digital flight controls included a more highly swept wing with a shorter chord and increased dihedral using sophisticated Catia design software. The changes provide improved handling characteristics and better overall efficiency, resulting in a lower fuel burn. Removing the mechanical linkages and replacing large control surfaces with smaller ones also means less weight, Villa said, likening the benefits of digital flight controls to a snowball effect.

“It gives more freedom to the designer,” he said. “You’ll find that you can incorporate a smaller empennage, for example, and the smaller empennage will reduce drag, and by reducing the drag you can reduce the size of the wing and also reduce the power needed. All these factors are good reasons for this technology to become immersed in any brand-new modern aircraft design.”