There are four primary flight control modes, three of which are hosted by the dual FCCs. Normal mode, using inertial and air data inputs in addition to pilot commands, provides flight envelope protection, maneuver and dynamic rudder load alleviation, and stability augmentation, including a speed stability function. Stability augmentation includes automatic compensation for pitching moments induced by changes in flap, landing gear or speed brake configuration. Gust load alleviation, thrust asymmetry compensation and P-Beta yaw/roll coupling compensation are not built into the control laws.
Direct law, rather than more complex pitch rate command law, is used for takeoff and landing. Up and away, the primary high-level pitch control law is Nz, or vertical acceleration, command with “U” speed stability, meaning the pilot has to trim pitch for speed changes. The Boeing 777 and 787 and Embraer E-Jets also use similar FBW speed stability control laws.
Certain sensor or communications malfunctions cause the FCCs to revert to alternate law mode. Most high-level FCC control law functions are retained, but envelope protection and the autopilot are not available. In addition, stall warning stick shaker is activated at a lower angle of attack. And Vmo, Mmo and crosswind limits are reduced.
If so many sensors and/or malfunctions make alternate law unavailable, then the FCCs revert to direct law. All high-level control law functions are lost, including automatic actuation of the speed brakes and ground spoilers. Control inputs from the pilots to the yoke, rudder, speed brake handle and trim switches directly result in control surface movement.
Finally, if all functionality of both FCCs were to be lost, there is a backup flight control computer that provides complete control, but all high-level control law functions are lost.
The G650 is the first large-cabin Gulfstream to have a fully trimmable horizontal stabilizer. Inputs to the normal or backup pitch trim switches send signals to command movement of the horizontal stab. Pitch trim commands, though, may be sent to the elevator during certain phases of flight. The horizontal stabilizer then is automatically trimmed by the FCCs to relieve the load on the elevator.
The roll trim switch on the center console repositions the neutral point of the control wheel, commanding movement of the ailerons and spoilers. However, moving the rudder trim switch only commands movement of the control surface, not the rudder pedals. An “auto center” button in the console provides a convenient way to center the rudder, such as on short final.
The aircraft has simple, four-position, slotted, trailing edge Fowler flaps. Multifunction spoilers, with three panels on each wing, provide roll augmentation, air brake and ground spoiler functions.
The G650 retains the dual hydraulic system architecture of its predecessors, although it has FBW flight controls with no mechanical links between the cockpit and the control surfaces. Most aircraft with fully powered flight controls, be they traditional hydromechanical or FBW systems, require three separate hydraulic systems in order to provide the required redundancy.