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.
The $20 million super-midsize Legacy 500 and $16 million Legacy 450 are the least expensive business aircraft yet to be fitted with full three-axis, digital FBW flight controls. Embraer chose FBW to reduce pilot workload, improve passenger ride comfort, enhance airport performance and save weight.
Similar to the Falcon 7X, the Legacy 450 and 500 have sidestick controllers that are not mechanically interconnected. Similar to the Rafale, the sidestick inputs are summed. The rudder pedals, however, are mechanically interconnected.
The Legacy 450 and 500, similar to the Falcon 7X and Rafale, have “gamma dot” flight path stability, a control function that maintains aircraft trajectory with speed and configuration changes so long as the aircraft remains within low- and high-speed flight envelope limits. Pitch trim is automatic.
The high-level control laws have both ground and air modes, but the transition doesn't use radio altitude. For instance, upon landing, the FBW system makes the transition from gamma dot flight path stability to speed stability after the landing gear and flaps are extended. Changes in speed cause nose-up or nose-down pitch changes. A trim reset button on the sidestick enables the flight crew to immediately retrim the aircraft for a new trim reference speed, thereby relieving the need to hold nose-up or nose-down sidestick pressure.
Embraer also included a heading and roll thrust asymmetry compensation control law that takes most of the work out of handling an engine-out emergency. The FBW system, though, retains enough sideslip to provide the crew with an unmistakable indication of which engine has failed. A flight director cue tells the pilots how much sideslip to add in the direction of the operative engine to optimize one-engine-inoperative climb performance.
Rather than taking Dassault's approach of developing the entire FBW system for its business aircraft in-house, Embraer subcontracted with Parker Aerospace and BAE Systems to save time and cost. Parker's and BAE Systems' hardware architecture for the Legacy 450 and 500 is much simpler than that of the Falcon 7X, but it delivers virtually identical benefits and system reliability.
The Legacy 450 and 500 have two, dual-channel, primary flight control computers furnished by BAE Systems that host high-level control law functions, such as stability augmentation, high- and low-speed envelope limiting and overstress protection. This is one of the latest quadruplex designs that uses four dissimilar channels, each one of which is capable of controlling the aircraft through all flight control surface actuators in all three axes.