Having gained many years of fly-by-wire (FBW) flight-control system experience with its combat aircraft, Dassault's decision to dispense with mechanical flight controls for its latest Falcon was hardly a leap into the unknown. The move has, however, given the manufacturer a clear lead in the technology race at the top end of the business-jet market and with it the associated benefits.

As an all-new aircraft, the Falcon 7X provided the perfect opportunity to integrate FBW into the overall design. While Dassault was certainly capable of introducing it on the Falcon 900EX and 2000EX, the many advantages FBW offers would have been minimised on these derivative aircraft. "Only with a brand-new platform is it possible to fully benefit from fly-by-wire," says deputy director of sales engineering, Brigitte Bonneville. "We wanted it from the beginning because you can do things you can't do with a conventional aircraft."

From the performance point of view, the most significant advantage of FBW was that it enabled Dassault to take full advantage of the three-dimensional Catia design technology applied to the wing, which is totally new and is thinner and longer than any of its forebears. This introduced significant issues with wing flexing and its coupling with the structure during flight – aeroelasticity – so the full performance could be realised only with FBW. The result in the 7X is that Dassault has been able to combine high- and low-speed performance as never before.

Also, Bonneville points out that FBW reduces pilot workload and improves passenger comfort as well as enhancing safety. The pilot benefits by not having to worry about trimming the aircraft, an action which is carried out automatically. More significantly, and in common with the Airbus FBW flight-control system, the aircraft is protected from exceeding flight-envelope limitations.

Faced with the need to fly the Falcon 7X out of a potentially dangerous situation using maximum climb or turn performance, all the pilot has to do is apply full power and pull the sidestick back to the limit. The aircraft does the rest, flying as close as possible to its maximum aerodynamic limits without stalling. The 7X is limited to a 3.5g pull-up, whereas Airbus FBW control laws only allow a 2.5g limit. Airbuses come with a 76° roll limit, but there is none on the 7X, which can achieve a 90° roll in 2s – giving pilots high manoeuvrability as well as flight-envelope protection.

Fly-by-wire also confers homogenous aircraft handling throughout the flight envelope, regardless of speed, altitude, weight or centre of gravity, and brings pilots the added benefit of sidestick control, which means they enjoy a clear field of view to the instrument panel and – again in common with Airbuses – free space in front of them for a pull-out table.

Dassault's FBW experience dates back to the original Mirage 2000, although in 1963 the company installed a prototype system on a vertical take-off version of the Mirage III, the Balzac, which transitioned to horizontal flight using FBW control of its engine nozzle. The Mirage 2000 is fully FBW-controlled, but uses four analogue computers, while the new Rafale has three digital computers and an analogue back-up.

Things have moved on. The Falcon 7X features three dual-channel main flight computers (MFC) and three single-channel secondary flight computers (SFC), all of which are fully digital. All channels are programmed independently. Even though it was not required by the certification authorities, Dassault elected to incorporate a digital back-up computer which, in the statistically infinitesimal likelihood of all the other computers failing, will keep the aircraft flying while the main computers are reset. It is not intended to be used for landing.

In normal operation, a single MFC operates, one channel performing control, the other carrying out parallel calculations that are compared with those from the first computer. Disagreement results in handover to MFC 2, then MFC 3. Failure of all three MFCs causes handover to the SFCs, which cross-monitor each other. Failure of the SFCs and the digital back-up computer results in manual reversion, with direct electric pitch trim through the horizontal stabiliser and electric control of the lower rudder. In the event of all three engines failing, basic electric power is supplied by a ram air turbine – the first time Dassault has fitted such a device.

Source: Flight International