Fly-By-Wire Controls Extend Their Reach In Business Aviation

Credit: Dassault Aviation

Fly-by-wire controls have brought a quiet but sweeping evolution into business aviation since the Dassault Falcon 7X entered into service in 2007.

They have enabled higher-performance business jet designs. They have made the aircraft easier to fly for the pilot. Passengers in the cabin may not be aware their jet features such an advanced system but they would notice the difference in comfort, should they fly the same route in a more conventional aircraft.

And, while fly-by-wire control systems (or digital flight control systems, DFCS) have thus far been found on large and medium-size jets, the planets are aligning for them to take on the light segment.

On a conventional aircraft, the pilot provides input to a control device such as a column or stick, which directly drives the control surfaces through mechanical linkages.

With a DFCS, the pilot tells a sensor what he or she wants to accomplish and the electric signal goes to an actuator that moves the aerodynamic surface. Flight control computers in between augment the signal. The computer uses the aircraft’s movement as feedback information to further adapt control surface action to the pilot’s intention. The process is transparent for the pilot.

Fly-by-wire controls were born on combat aircraft. The idea was to make the fighter’s behavior the same whatever the loads it carries. In addition, designing a more agile fighter implies making it naturally unstable. This translates into difficult handling characteristics with conventional controls. Only fly-by-wire controls allow the pilot to safely handle the aircraft.

On a civil aircraft, a DFCS retains the advantage of making the aircraft’s behavior homogeneous throughout the whole flight envelope. Engineers can also design an aerodynamically “pure” wing, even though it could make the aircraft sometimes tricky to control – the DFCS will take care. In addition, a fly-by-wire system is lighter than a conventional one and its maintenance is easier.

A DFCS may include a flight envelope protection feature. In other words, the controls will prevent the aircraft from entering a dangerous attitude (measured in angle of attack) that could lead to a stall. Protection functions may also include over-speed protection and over-stress protection, not to exceed structural load limits. The flight control computer provides deterrents or hard limits to the control inputs.

Trims are useless on fly-by-wire aircraft. For example, to start a 3-degree descent, the pilot will push the stick and then release the effort. The stick will go back to its neutral position and the aircraft will continue on the 3-degree slope. The pilot may keep mental resources and time for other, higher-level tasks.

After a long experience with fighters such as the Mirage 2000 and the Rafale, Dassault introduced such controls to business aviation with the Falcon 7X, the first purpose-built business jet with a DFCS.
Since then, other manufacturers have followed suit. Gulfstream, Embraer, and then Bombardier launched full fly-by-wire aircraft. Cessna uses such controls, to a limited extent, on the Citation Longitude.

“The first fly-by-wire systems in civil aviation caused mixed feelings among pilots,” says Dominique Trinquet, president of business aircraft broker Boutsen Aviation. “They were wondering whether they had full control. But pilots have since realized the benefits. They are considerable in terms of flight envelope, passenger comfort and ease of flying.”

A DFCS has yet to become a factor in the acquisition process, according to Trinquet. Buyers are rather focused on the cabin, he says.

Moreover, the incontestable safety benefit does not transpire into accident statistics. Apples-to-apples comparisons between aircraft types are difficult and the number of accidents is very low anyway.

And, while a significant proportion of new aircraft deliveries are fly-by-wire designs, they still account for a small part of the in-service fleet.
Customers may nevertheless be convinced by the greater comfort and lower maintenance costs a DFCS translates into, says Trinquet. And they may understand the safety argument.

Thales is supplying flight control computers for the Gulfstream G500, G600, G650ER and G700, the Bombardier Global 5500 and 6500 and the Longitude’s rudder.

The Global 5500 and 6500 only have their secondary control surfaces – horizontal stabilizer, slats, flaps and spoilers – equipped with a digital system, as opposed to the full fly-by-wire Global 7500. The Longitude has fly-by-wire technology on the rudder, spoiler and braking systems.

“Flight control computers augment the system’s performance to reduce pilot workload, increase safety and improve comfort in turbulence,” says Andre Cleroux, Thales’ program director, flight control systems.

Rethinking control surfaces helps the tradeoff between low-speed and cruise performance. “Spoilers augment the action of ailerons, and ailerons and flaps are combined into flaperons – only fly-by-wire can make these evolutions possible,” says Cleroux. Control surfaces move more precisely.

As a result, drag is minimized and the overall aerodynamic performance of the wing improves. In addition, thanks to the system’s quick response, the wing and empennage can be shielded from some forces that would otherwise require extra design margins and accompanying structural weight, Cleroux explains.

All this may translate into a greater range, a key criterion for buyers.
Embraer joined the club early. Since 2014, the Legacy 450/500 (which has since evolved into the Praetor 500/600) have been the first and only medium-size business jets with a full fly-by-wire control system.

“We had a strategic intent for that product and fly-by-wire was an enabler,” says Ricardo de Paul Carvalhal, director of sales engineering. The medium-cabin segment had been linked to a certain airport category with a minimum runway length requirement. “We wanted to break this mold and bring the aircraft into airports where no other medium jet is operating today,” says Paul Carvalhal. The outcome is takeoff and landing distances that are not very different from those of a light jet carrying the same number of passengers, he emphasizes.

“At takeoff, the pilot can bring the aircraft to its limits without being afraid of the stick shaker,” says Paul Carvalhal. “Takeoff and landing speeds can be slower than on traditional designs, which means lesser runway requirements.”

In terms of comfort, the main advantage can be felt in turbulent conditions. “Sensors feel the aircraft 100 times per second and the system’s actuators make tiny corrections to keep the aircraft stable,” says Paul Carvalhal. Turbulence is still felt but the ride is smoother.

A DFCS alters the relationship between the aircraft and the pilot. Should flight crew members expect a major change when transitioning to a fly-by-wire aircraft type? Not necessarily.

“The primary intent in the Gulfstream design is to maintain the way pilots fly conventional aircraft as much as possible,” says Angel Barboza, Gulfstream’s vice president, engineering and advanced capabilities. “The forces on the control devices are carefully tailored to feel similar to those of a conventional aircraft and for the aircraft to have a similar and predictable response to the pilot’s inputs.”

In the beginning, the pilot tends to overcontrol the aircraft, acting too often on the stick, says Paul Carvalhal. He or she quickly understands the main qualitative difference with a conventional system – the stable flight path logic, he adds. “Pilots report this was their easiest type rating, as the aircraft helps pilot actions.”

A typical safety improvement can be seen in case one of the two engines fails during takeoff. The fly-by-wire system will automatically compensate for the asymmetric thrust. “However, the correction will not be 100%, so that the pilot remains aware of the problem,” says Thales’ Cleroux.

On the Longitude’s spoiler system, deployment is automated during the landing phase. The outboard spoiler panels also work as inflight speed brakes – the fly-by-wire control logic will ‘auto-stow’ the panels if a critical angle of attack is reached, a function that increases safety at low airspeeds, a Cessna spokesperson explains.

Each airframer designs its own flight control laws. “They govern flight dynamics and safety features,” says Cleroux. In other words, they are an indication of the values the manufacturer wants their design to be associated with – the importance it gives to landing performance, for instance.

Control laws also reflect the aircraft’s operational environment. A long-range Gulfstream can fly for hours over an ocean, far from a diversion airport. System redundancies should therefore be even greater. “We imagined multiple electric and hydraulic failures,” says Cleroux.

What’s next for fly-by-wire?

More integration, first. Dassault’s in-development Falcon 10X will add power control to its DFCS.

Second, fly-by-wire may become more widespread. The certification process of a new aircraft is increasingly difficult, says Cleroux. A DFCS makes it easier to avoid or mitigate some failures.

“Fly-by-wire knowledge tends to democratize,” Cleroux adds. “And civil aviation authorities master the technology.” Thales is working on a DFCS that will be suitable for a light business jet.

Thierry Dubois

Thierry Dubois covers French aerospace for Aviation Week & Space Technology.