's Falcon 7X, certified in 2007, is the first business aircraft to be fitted with digital electronic flight controls. Dassault borrowed liberally from the suite of military FBW technologies that it developed in the mid-1980s for its longitudinally unstable, highly maneuverable, Mach 2 fighter. It's a vintage design that has multiple components and several redundancies.
Dassault engineers believe that their extensive experience in developing military digital flight control systems gives them FBW design expertise not available to other business jet makers. Rafale's handling qualities, for example, are optimized for “carefree handling” and protection from overstress and loss of control.
Rafale's “gamma dot” FBW pitch function maintains the aircraft's velocity vector or flight path in a desired direction after the stick is released. The aircraft will hold flight path while compensating for changes in airspeed, c.g. and landing gear and high-lift configuration, among other variables. Envelope protection prevents the aircraft from stalling if AOA limits are exceeded, or from spinning in the event of excessive sideslip. It also guards against overstress if the pilot commands a higher g maneuver than the airframe can safely withstand. Many of the Rafale's FBW design features are carried over into the Falcon 7X.
In a move similar to Rafale's design, Dassault elected to fit the Falcon 7X with outboard sidesticks instead of control wheel yokes for pitch and roll command inputs. The sidestick configuration saves room in the cockpit. However, the controls are not mechanically interconnected and thus there is no motion cuing or tactile feedback between the two sides. To compensate, the sidesticks vibrate a warning signal if both pilots are attempting to control the aircraft at the same time. No such stick input conflict cuing is built into two-seat versions of the Rafale, but rather it sums the sidestick control inputs from both cockpits. The Falcon 7X and Rafale retain mechanically interconnected rudder pedals.
Adapting the Rafale's FBW functionality for the Falcon 7X wasn't much of a challenge for Dassault's engineers, for unlike its fighter sibling, the 7X is inherently stable and designed to be flown at subsonic speeds. FBW designers determined that the trijet's digital flight control system only needed a 50-millisecond response time, but they elected to retain the 12.5-millisecond response built into the Rafale's computers to assure there would be no detectable latency in the system. They also fitted the Falcon 7X with smaller, lower power, lighter weight control surface actuators than those on the Rafale because of the former's executive transport mission.
Similar to the Rafale, the Falcon 7X's digital flight control system maintains aircraft flight path with speed and configuration changes; it automatically trims the aircraft to neutralize primary control surface forces and provides stability augmentation. It also has low-speed and overstress flight envelope protection, plus it adds a high-speed envelope protection function not needed in a Mach 2 aircraft. Dassault designed the 7X for Mach 0.97 and 430 KIAS demonstrated dive speeds, but the FBW system limits the aircraft to Mach 0.94 and 405 KIAS.
The Rafale's FBW imposes no pitch angle limits, but the Falcon 7X's digital flight control system doesn't allow nose attitude to exceed 35 deg. nose up or 28 deg. nose down at speeds above 250 KIAS. Nose-up and nose-down pitch limits are reduced at slower speeds.
The Falcon 7X uses a radio altitude input to determine when to make the transition between ground and air modes. Above 50-ft. radio altitude, the aircraft will automatically trim the horizontal stabilizer to maintain flight path when the sidestick is released, assuming it is within the lower and upper speed limits. Below that radio altitude, auto trim is inhibited. At touchdown on landing, a weight-on-main-wheels command signals the horizontal stabilizer to start moving toward the pitch-down position. This causes a natural feeling, derotation of pitch attitude when the stick is released, helping the aircraft to transition to a three-point attitude with weight on the main and nosewheels.
The Falcon 7X has no hard bank angle limits, but the FBW system provides artificial spiral stability that automatically levels the wings if the stick is released and bank angle is less than 6 deg. The aircraft will hold bank angle if the stick is released at angles of 6 deg. to 35 deg. When the stick is released at bank angles greater than 35 deg., the aircraft automatically will roll back to 35 deg. Roll stability augmentation also prevents the aircraft from rolling off due to wing fuel imbalance or partial flight control system degradation.
Dassault installed three, single-channel digital computers plus a dual-channel analog computer aboard the Rafale to provide the required redundancy for such critical flight control functionality. The computers vote on control surface commands, so one or even two errant computers can be disqualified and excluded by the remaining computers.
If the Rafale FBW system has belt-and-suspenders redundancy, the 7X has belt-suspenders-braces-hooks-and-loops redundancy. This assures that it is controllable under all foreseeable abnormal or damaged conditions. It has three full-function, dual-channel main flight control computers (MFCCs) and three limited-function, single-channel secondary flight control computers (SFCCs). The MFCCs are capable of high-level normal, alternate (or degraded high level) and Direct Law modes while the SFCCs only can function in Direct Law modes.
Only one of those six computers is needed to fly the aircraft. All six send flight control-position commands to four actuator control and monitoring units (ACMUs) that essentially are FBW command signal quality assurance inspectors.
The ACMUs monitor the control surface commands coming from the main and secondary flight control computers. In the event of a disagreement between any of the MFCCs or SFCCs, the ACMUs can isolate and exclude that computer from the system. The design assures the system has the required 10-9 probability of failure.
But Dassault also installed a backup analog computer that provides an alternate means of pitch and roll control. The extra computer thus provides 10-10 redundancy similar to Parker Aerospace's current systems.