Julian Moxon/PARIS
Any doubts about the safety and cost effectiveness of fly-by-wire (FBW) flight control systems on civil aircraft were long ago dispelled with the success of Airbus Industrie's single-aisle A320 and, later, the European consortium's twin-aisle models.
The justifications used by Airbus for introducing FBW were several, but centred mainly on the "carefree handling" aspects of aircraft pilotage, particularly in areas such as flight envelope protection. Because it protects the aircraft from entering dangerous flight regimes, FBW allows greater use to be made of the available performance, improving operating efficiency, and reducing fuel consumption.
The Airbus system also provides additional benefits such as gust load alleviation, for improved passenger comfort. For pilots, the use of sidestick controls means they get an uncluttered view of the instrument panel and a more spacious "office".
Justifying FBW for regional airliners has been less easy because the aircraft are less expensive, yet the system has to perform essentially the same functions as for a full-sized airliner. The weight and size of the FBW system, less of an issue in a large airliner, becomes a significant factor in a regional aircraft. On the other side of the coin, it is argued that regionals, which fly shorter sectors and spend much more of their flying time taking off and landing, would benefit from the simplification of the pilot's task, particularly in crowded airspace.
Until recently, the only regional aircraft to feature FBW was Indonesia's now-moribund IPTN N250 turboprop. Now Fairchild Aerospace has selected FBW for its planned 728JET family of regional jets. The supplier of both systems is Lucas Aerospace.
Compared with the Airbus system, they are relatively simple, essentially being electrically signalled substitutes for the traditional mechanical linkages, but they are the start of a trend.
Sextant Avionique flight control computers are at the heart of the Airbus system, and the company is studying the use of FBW for regional and business jets in its Montreal and Paris "centres of expertise" for the development of flight control systems. The Montreal plant, which opened two years ago, employs around 50 engineers and technicians and, besides working on new flight control technology, carries out all of the testing and validation of Bombardier's current regional aircraft flight control systems.
REGIONAL STUDY
It is no accident that the facility is in the same city as Bombardier, which has already contracted Sextant to produce the entire flight control systemfor the Global Express long-range business jet, and given it responsibility for the secondary flight control and stall protection systems of the CRJ-700 regional jet.
Alain Martres, deputy general manager for regional and business aircraft avionics at Sextant, insists that any regional aircraft FBW system that results from its studies will be aimed equally at other manufacturers. "We have a facility in Toulouse as well, but the fact is that Bombardier is bringing out a new aircraft every year", he says, "and we needed to be close to them. But our facilities and expertise in Toulouse ensure that we can meet the demands for other business and regional jet programmes as well."
Sextant is undertaking a "joint conceptual definition phase" with Bombardier to look at the trade-offs between performance and cost benefit of an FBW system for the manufacturer's planned Continental business jet. Similar trade-off studies are under way for the 90/110-seat BRJ-X regional aircraft.
REDUCING COST
"In four years, the size of a typical FBW system has come down by 30%," says Martres. He points to the far greater integration that has become possible with the advent of new microprocessor technology, but adds that this, in turn, brings its own problems. "We must preserve two essential features of FBW:dissimilarity and redundancy". In Airbus aircraft, the FBW system is based around five main flight control computers, split into two blocks, each responsible for a separate set of flight controls which can be used to control the aircraft in all phases of flight. Each computer is dual-redundant, incorporating command and monitor channels equipped with components sourced from different suppliers.
This level of dissimilarity and redundancy must be maintained in business and regional aircraft, but in a system that is more integrated and likely to be based around two main computers, instead of five. Sextant declines to reveal how this will be achieved, saying only that a decision on whether to use FBW for the Continental will be taken "later this year". Martres says the trade-off study is complex, "-because it involves not just the flight control system but the whole aircraft - hydraulics, avionics and other systems as well".
REGIONAL AIRLINERS
The arguments for the use of FBW in corporate aircraft are even less obvious than with regional airliners, since business jets tend to be used less frequently, on routes that avoid the busy airways, and with at least as much sensitivity to cost. The weight, maintenance and performance advantages may be beginning to change attitudes, however. Sextant says that interest in FBW for business aircraft is increasing. It adds that the trend towards manufacturers offering a complete range of aircraft, coupled with the increase in fractional ownership, militates in favour of FBW, which can be configured easily to provide similar, or even identical, handling characteristics across the different types.
The jury may still be out on FBW for small civil aircraft, but, with technology offering the possibility of lighter systems which occupy less space while providing the same reliability and functionality, it is probably just a matter of time before the argument for its use is carried.
BOMBARDIER READIES FLY-BY-WIRE DEMONSTRATOR
Graham Warwick/WASHINGTON DC
To help it decide whether fly-by-wire is the right choice for its future business and regional aircraft, Bombardier is to flight test alternative approaches, using a modified Challenger corporate jet.
Under the Active Control Technology (ACT) programme, the demonstrator is being prepared at the company's Montreal plant for a first flight in May. The one-year effort covers the evaluation of different control laws and sidestick controllers by multiple pilots, says programme manager Guy Bernard.
Bombardier stresses that the programme is a "technology readiness" effort timed to help it make a decision on whether its proposed BRJ-X regional jet should be FBW and, if the decision is yes, to enable it to draw up the specification for such a flight control system. "We want to be ready," says engineering vice-president John Taylor. "We do not want to be developing this in the middle of a programme." The aim of the demonstration is to take the technical risk out of a decision on FBW, he says.
"The cost of computers is coming down, the technology is becoming more acceptable and there are certain advantages," says Taylor, citing better fuel efficiency, handling qualities, envelope protection and maintenance and diagnostics. "You can make different aircraft handle similarly by modifying the control laws, but we have to look at it from a cost-benefit viewpoint against other systems."
Bombardier estimates that FBW should reduce fuel consumption by 5%. Reducing longitudinal static stability would decrease tail size and trim drag, allowing the aircraft to be resized. "We are moving towards a more unstable aircraft, but we have to be careful how we develop and apply the technology," Taylor says.
RECONFIGURABLE SYSTEM
The Challenger is being fitted with a research FBW system consisting of a sidestick and flight control computer driving new servo-actuators on all the control surfaces. The conventional mechanical linkages will be retained, for use by the safety pilot in the left seat.
The variable-feel sidestick, installed on the right side of the cockpit, is reconfigurable, Bernard says, enabling Bombardier to evaluate different control concepts, such as whether the co-pilot's sidestick should move in response to the captain's inputs. A similar sidestick has been installed in Bombardier's reconfigurable experimental flight simulator.
"We will try different philosophies on feedback, from Boeing's positive feedback to Airbus' more automated approach," says Bernard. The flight control computer will be reprogrammable within 2h with different control laws. The first phase of flight testing will look at "up and away" control laws, with the remaining time to be spent examining envelope protection. The demonstration is largely funded by Bombardier, Taylor says, with suppliers sharing cost. The company will not identify the suppliers with which it is working, except to say that they are not among those experienced with FBW, but are companies that want to get into the civil FBW market.
Following modification of the Challenger, a long-serving test aircraft that was to have been retired, there will be an "aircraft-in-loop" ground test phase during which the performance and safety of the FBW system will be tested. Flight testing will then begin, by the company's test and demonstration pilots.
Source: Flight International
