Direct Law doesn't provide any of the higher level FBW functions that distinguish digital flight controls from conventional hydromechanical-powered flight controls. As with conventional flight controls, it's up to the flight crews to avoid inadvertent stall, or over-speed, over-control and over-stress of the aircraft.
Higher level FBW functions, commonly known as Normal Law or Alternate Law functions, require a second set of computers that are capable of using inputs from several sensors, such as angle-of-attack (AOA), flap/slat and landing gear position sensors, air data and IRS/AHRS, weight-on-wheels and perhaps also radio altitude, as well as cockpit controls. The computers then shape, smooth and calculate the best aircraft behavior in response to pilot inputs, speed and configuration changes and autopilot commands, as well as other factors. The higher level law computers team with the actuator control units to determine optimum flight control response.
However, the term “optimum” is subject to a wide variety of interpretations depending upon the FBW flight control design philosophies of each aircraft manufacturer. Some high-level FBW functions, however, are common to virtually all civil aircraft and include yaw, spiral, static and dynamic pitch stability augmentation to reduce pilot workload. Most FBW systems also have soft or hard limiting for maximum AOA, Vmo/Mmo over-speed and over-stress.
“Once you have the algorithms, it's not difficult for the control laws to accomplish such functions,” says David McLaughlin, Parker Aerospace's chief engineer for systems.
As noted, airframe manufacturers each add their own high-level FBW functions in order to differentiate their digital flight control systems from their competitors' designs.
Dassault Aviation'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 Rafale 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.