These components are known as electric backup hydrostatic actuators, EBHAs for short. They function as conventional EHSAs if hydraulic power is available. However, in the event of hydraulic system failure, the actuators revert to a backup mode that uses tiny electrically powered hydraulic pumps mounted on the actuators. The actuator electric pumps generate the fluid power required to move the control surface. The design eliminates the need for a full-time, electrically powered third hydraulic system, thereby saving considerable system weight.
In the event of failure of both primary flight control computers and both hydraulic systems, the backup flight control unit sends commands directly to the EBHAs.
And if both engine-driven generators were to fail, the G650's 15 KVA ram air turbine (RAT) can be extended to generate emergency electrical power to supply the FBW system. Below 180 KIAS, 24 VDC emergency batteries take over from the RAT to assure an uninterruptable power supply to all electrical components in the FBW system.
Following Boeing's lead on the 777 and 787, Gulfstream elected to retain a conventional control yoke wheel for the G650 instead of fitting the aircraft with sidestick controllers. The yokes add weight, cost and complexity, but they are mechanically interconnected so that the pilot not flying can see and feel the pilot flying's control inputs. Gulfstream and Boeing engineers assert that the design promotes crew situational awareness and enhances crew resource management.
The G650's high-level Normal Laws, hosted by the dual primary flight control computers, are quite similar to those of the Boeing 787. The basic pitch law is similar to the 787's C*U design. C* means that fore/aft yoke movement commands pitch rate by means of a simple Direct Law mode on the ground and vertical acceleration, or g rate, in the air using several inputs to the FCCs. U means that the aircraft is speed stable, so the pilot must trim nose up or down with speed change.
Three-axis stability augmentation makes the aircraft easy to fly because it compensates for trim changes caused by control surface or landing gear extension or retraction. Similar to the 787, the G650's FBW system has a maneuver load alleviation function that progressively deflects the ailerons and outboard spoilers at 1.5 g's and above to reduce the lift produced by the outboard wing sections and thus the wing bending moment. Other high-level control functions include automatic retraction of the speed brakes under certain conditions, dynamic rudder travel limiting to prevent overstress of the vertical fin, and elevator split limiting to prevent overstress of the empennage.
Similar to Boeing and Embraer FBW designs, the G650 has no hard limits on pitch or roll angles. However, the system does have hard limit maximum AOA and high-speed flight envelope protections in the Normal Law mode.
If air data or IRS information is insufficient or not available, the FCCs revert to Alternate Law mode. The autopilot becomes inoperative and flight envelope protections are degraded. If all four channels of the primary FCCs are unavailable, the FBW system reverts to Direct Law mode in which the control surfaces respond directly and proportionately to cockpit flight control inputs.
The Future of FBW
Bombardier has announced that its two new ultra-long-range, high-speed business jets, the Global 7000 and Global 8000, will be fitted with FBW controls. Many components will be supplied by Parker Aerospace.