Fly by wire filtering down to bizjets

 - July 13, 2007, 11:15 AM

Fly-by-wire (FBW) flight controls have been commonplace in fighters and Airbus airliners for years, but the technology has remained out of reach for all but a handful of business jet pilots. The notable exception in business aviation is the Airbus Corporate Jetliner, a descendent of the A320, which in 1988 became the first airliner with fly-by-wire controls and sidesticks to enter production.

Yet as FBW continues to gain acceptance among pilots and operators and the cost of the technology comes down, experts predict a major shift in business aviation away from today’s cable-and-pulley or hydraulic flight controls to systems that are manipulated by servoactuators taking their cues entirely from computers.

Most of the early apprehension about FBW–primarily centered on the burning question of whether it was really wise to relinquish ultimate control of the airplane to the flight computers–has disappeared in the last decade as Airbus and Boeing have adopted the technology with excellent results in terms of safety and operating efficiency. Now the designers of new business jets are seriously considering FBW, and the enhancements to performance and handling that the technology can offer.

Dassault is busy developing the first FBW purpose-built business jet, the Falcon 7X, and Embraer and Honeywell recently achieved certification for the first FBW regional jet in the form of the Embraer 170 with Primus Epic avionics. As business and regional jet pilots gain exposure to these airplanes, it is likely that airframe manufacturers will bring certain FBW concepts (if not fully integrated FBW systems) to at least some of the next generation of clean-sheet airplanes.

Taming the Flight Envelope
To understand how FBW has evolved over time and where the technology is headed, a little history is useful. One of the first airplanes ever to be equipped with fly-by-wire flight controls was a modified F-8 Crusader in 1972. NASA used the airplane to test the technology, which quickly evolved into systems for a new generation of fighters in the mid- to late 1970s, including the F-16 Fighting Falcon and Mirage 2000. Meanwhile, 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 first and 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 inside the flight envelope. In essence, FBW provides artificial stability in airplanes where the engineers have purposefully moved the c.g. to improve flight traits. Negative stability can produce significant gains in lift under certain conditions, but the tradeoff makes fighters virtually unflyable with conventional flight controls–hence the need for FBW.

In modern airliners such as the Airbus A340 and Boeing 777, one of the primary design advantages of FBW is that it allows the engineers to use lighter wing and tail structures. And because the FBW system replaces all of 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. These systems can be fully integrated servo-based flight controls that use advanced computers to make the aircraft easier to handle, or they can be limited to simpler systems that control only 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 linkage between the cockpit controls and the moving surfaces with electrical wires.

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 to go and the computers make sure the airplane does what it is expected to do.

For example, steep turns in the 7X will be simple since all the pilot has to do is roll into the turn, put the nose on the horizon and let go of the stick. The computers take care of the rest, maintaining proper bank and pitch throughout the turn at a constant altitude and rate of turn. Also, the FBW system can automatically (and quickly) compensate for turbulence or for aileron droop on takeoff, features that 7X pilots and passengers are sure to appreciate.

At the heart of any FBW system are the flight computers, which convert the pilot’s commands into electrical impulses that are delivered to the control surfaces. When this technology was first brought to the A320, it was a major achievement for a number of reasons. First, the elimination of hydraulic systems reduced the weight of the aircraft. This in turn cut fuel burn and lowered operating costs for airlines.

Fly-by-wire technology also offered a significant safety enhancement with the introduction of “hard” protections. The pilot’s commands to the control surfaces are monitored to ensure the aircraft remains within a safety margin, called the flight-protection envelope. Thus, the pilot can always get the maximum out of the aircraft in an emergency without running the risk of exceeding the flight envelope or overstressing the aircraft. In essence, this means the pilot can push or pull on the stick as roughly as he wants without ever having worry about breaking the airplane or straying outside its lift and maneuverability capabilities.

Fly-by-wire technology has also made it possible for Airbus to develop a true family of aircraft, from the 107-seat A318 to the 555-seat A380, with near identical cockpit designs and handling characteristics. This makes crew training and conversion times shorter, simpler and more cost-effective for airlines–and it allows pilots to remain current on more than one type simultaneously.

Designing the Falcon 7X
Dassault has adopted a similar philosophy by commissioning Honeywell to develop the Primus Epic EASy cockpit for the Falcon 900EX, 2000EX and 7X–although only the 7X uses FBW flight controls and a sidestick. Replacing the bulky control yoke with a sidestick controller should be welcomed by pilots because of the extra cockpit room it will provide. Most pilots who have never before flown with sidesticks say they become comfortable using them in a very short time, and most Airbus pilots say they have grown to prefer sidesticks.

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

According to Olivier Villa, Dassault’s senior vice president for civil aircraft, reduced crew workload, better aircraft performance and enhanced safety should be the major benefits of FBW in the Falcon 7X when the first test airplane starts flying next spring.

“We are surely the only business jet manufacturer for which fly-by-wire is not a challenge,” said Villa. “We have mastered fly-by-wire technology for quite some time now, so really the biggest challenge is to make it compatible with the certification requirements” in Europe and the U.S.

He said the differences between the FBW controls in the fighters and the 7X have translated to a “significant” development effort, but stressed that the advantages in terms of safety, pilot workload and maintainability will reap benefits for customers once the 7X enters service.

It is likely that all new Falcons introduced after the 7X is certified will use FBW controls, he added. Convincing customers of the benefits of FBW has been a surprisingly simple exercise, probably due to the broad acceptance of the technology among airlines.

“We knew that there would be some pilots and customers who would love the idea, and some who are a little bit reluctant and would need to be convinced,” Villa said of Dassault plans for FBW in the Falcon 7X. “It was the right time to do it, the right airplane to do it in, and we actually didn’t have much difficulty convincing our customers.”

Dassault has decided to use a mix of hard and soft limits for the Falcon 7X system, blending the philosophies of Airbus’ hard limits and Boeing, which has designed 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 strict.

In the end, FBW is designed to enhance performance and improve safety, said Villa, explaining that control-surface positioning will be optimized to reduce drag, and the static stability margin will be reduced for a corresponding reduction in fuel burn. Flight-envelope limitations will include angle of attack/attitude, load factor and speed. For example, the Falcon 7X will be protected not just from abrupt control forces and stalls, but also against sustained flying above Mmo. Yet there will be no hard limits for banks, meaning that unlike Airbus jets, rolling the Falcon 7X will be unrestricted.

When Will the Rest of Bizav Catch Up?
All of the major manufacturers of business jets have explored FBW concepts, but other than Dassault none appears close to certifying–
or even announcing–a fly-by-wire-based airplane. Spokespeople for Gulfstream, Cessna and Raytheon said new technologies such as FBW are always being evaluated and discussed, although none pointed to active development programs to show FBW is being seriously considered for new aircraft.

A spokesman for Gulfstream said the company has been looking at a number of advanced technologies for its line of airplanes and plans to implement technology that allows the company to stay competitive. The spokesman added that its approach is to be responsive to customer needs and requests. This has led Gulfstream to certify an enhanced- vision system in a number of its airplanes and test concepts for a synthetic-vision flight display system.

Gulfstream is also known to be exploring ideas for a supersonic business jet (SSBJ), an airplane that would surely require FBW flight controls.

While saying that the reduced weight and sidestick controls of a fly-by-wire system would be pluses, a Cessna spokeswoman pointed to some related issues that the Wichita company believes still need to be settled. Chief among these is the maintenance required for such complex systems, which could force corporate flight departments to rethink the way they maintain their aircraft. Also, because FBW is still new to business aviation, she said Cessna would like to see the technology mature and evolve before making a major commitment to FBW.

Meanwhile, Bombardier for the past several months has been flying a Challenger (S/N 3991) with a developmental fly-by-wire system. Part of the Canadian company’s Active Control Technology demonstration, the airplane has been fitted with a specially designed sidestick that sits on a pedestal in the cockpit next to the control column. Bombardier admits it has been quietly working on the technology for more than a year, but observers say they don’t expect to see a fly-by-wire business jet from Bombardier anytime soon.

With four new or derivative models (the Learjet 40, Learjet 45XR, Challenger 300 and Global 5000) entering service this year, Bombardier has its hands full. More likely, say experts, Bombardier’s current efforts to develop FBW are probably centered on bringing a design to an all-new narrowbody airliner the company is developing. According to Bombardier, this airplane would have between 110 and 130 seats and be targeted initially at major and low-cost airlines in North America and Europe as a replacement for their aging Boeing 737s, DC-9s, MD-80s and Fokker 100s.

Honeywell and Rockwell Collins each have experience designing FBW-related systems and integrating digital FBW controls with avionics. Collins built key components for the FBW system in the Boeing 777, including the trim system and control-column-feel system and triplex autopilot, and Honeywell, as noted above, played a major role in certifying the Embraer 170. Honeywell was also recently named to develop the FBW system for the new Boeing 7E7.

In the Embraer 170/190/195, the FBW controls are integrated with the Primus Epic avionics system. This allowed Honeywell to reduce the number of line replaceable units (LRUs) that were needed, which translated 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 in the airplane can use much of the same input/output and sensor data that is available already to the avionics.

According to John Todd, Honeywell director of business development for business, regional and general aviation, the concept of sharing resources between the avionics system and the FBW controls is a relatively new one, but he added that the benefits of such a concept are clear.

“We have designed the system so that that computing resources and data are available to both the FBW and avionics systems,” Todd explained. “We have a module integrated into the Epic system that is a part of the fly-by-wire system, and that module communicates with the avionics system and has some processing capability. Allowing these systems to communicate gives us a lot of flexibility in terms of how we design the fly-by-wire system.”

Selling FBW’s Benefits to Airplane Manufacturers
Todd added that Honeywell has held talks with all of the major OEMs to discuss the advantages of fly-by-wire flight controls. Noting that aircraft performance is crucial when considering new, clean-sheet designs, Todd said Honeywell has tried to point out that with FBW the OEMs can start thinking outside the box to make performance improvements. Part of this effort has included convincing the OEMs that FBW controls provide real value, he said.

“The overall costs of the fly-by-wire systems in traditional terms have been pretty high,” Todd said. He added that as the technology matures and OEMs begin to realize the benefits, the acceptance of FBW will inevitably grow. “Clearly, the advantages of fly-by-wire will move it into business aviation,” he said. “Maybe not all of the control surfaces at first, but certainly to some surfaces where there is a clear advantage to the OEM. But I think we will see a progression as we move forward where more aircraft will have more surfaces that are fly by wire.”

Honeywell has also been testing its so-called “assisted recovery” safety system, which is designed to provide an additional layer of protection against controlled flight into terrain or buildings.

Assisted recovery builds on technology developed for the enhanced ground proximity warning system (EGPWS) by using the autopilot or fly-by-wire system to prevent aircraft from crashing into terrain or obstacles if pilots fail to heed warnings.

If pilots do not respond to warnings within five seconds, assisted recovery directs the autopilot or FBW control system to gently steer the aircraft away from the hazard. Honeywell has been developing assisted recovery to steer airplanes away from TFRs as well, which would require continuous data uplink of new TFRs as they are issued. In FBW airplanes, such a system could also be used to take over control of an airplane should hijackers appear to be intent on crashing it into buildings.

The EGPWS terrain database supplies the data necessary for assisted recovery to trigger aircraft control systems to avoid accidents. Airbus has been interested in an assisted-recovery-type system since the A320 was introduced, but terrain databases weren’t available at that time. Boeing is also said to be interested in assisted recovery. Honeywell thinks the concept can be refined and brought to civil airplanes in some form within the next 10 years.