Gulfstream G650 Fly-by-Wire
The ultralarge-cabin, ultralong-range G650 is the first production Gulfstream to use fly-by-wire technology to manipulate the flight control surfaces. That puts the G650 in the same company as many high-performance military aircraft (including the venerable General Dynamics F-16).
In keeping with its history of product and industry innovation, Gulfstream is paving the way to bring the extremely robust fly-by-wire system into the realm of business aviation. To help understand what makes this system such an exciting addition to the G650, I offer the following primer on fly-by-wire technology.
When pilots move the cockpit flight controls in conventional jets, that control input is transmitted through a mechanical linkage of cables and rods that operate a hydraulic actuator mounted near the flight control surfaces in the wings and tail. This mechanical system functions much like the typical power-steering unit in a car, where the steering wheel is physically linked to the front wheels, but a hydraulic system is there to amplify your efforts.
In fly-by-wire equipped aircraft such as the G650, the flight controls are a series of electronic devices not all that different from the mouse or trackball used to operate your computer. When the pilot moves the controls, an electronic signal is sent to a series of computers that interpret the control movement into the appropriate deflection of the flight control surface. Hydraulic actuators still perform the real work of moving the flight controls, but the actuators receive their commands electronically instead of mechanically.
Fly-by-wire systems have a number of advantages over conventional mechanical flight controls including weight savings, improved redundancy and greater efficiency that translates into higher speed.
For example, fly-by-wire control systems are programmed to keep the aircraft within the tested and certified operational envelope of the aircraft and prevent it from exceeding those critical limits even if a pilot inadvertently attempts such a maneuver. In the G650’s case, the fly-by-wire system will not allow the aircraft to fly into an aerodynamic stall even if the pilots somehow were to lose focus on maintaining the proper airspeed. And at the high-speed edge of the envelope, the computers will resist lowering the nose of the aircraft to an angle that would exceed the critical airspeed limits in a descent.
While not the case for Gulfstream, some jet makers have opted to give the fly-by-wire computers extensive authority over all portions of the flight envelope rather than trusting the pilot. These aircraft essentially fly as if the autopilot is engaged and flying at all times.
In contrast, Gulfstream engineers chose to design a fly-by-wire system that operates much like a conventional flight control system, giving the pilot control to fly the aircraft within specific parameters and still protecting the aircraft from exceeding design limits.
The G650 fly-by-wire system is, however, more capable than any human pilot can be in fine-tuning the controls to minimize drag, which in turn maximizes speed and range. For example, when the G650 stabilizes at cruise speed, the fly-by-wire computers make minute adjustments to the flight control surfaces so that everything is trimmed and streamlined to reduce aerodynamic drag.
The fly-by-wire system is also designed to manipulate the flight controls in such a way that it improves the smoothness of the ride quality for passengers.
From a reliability standpoint, the global standard for fly-by-wire reliability is that at least three independent computer and wiring systems be installed. A triple-redundant electronic system provides a level of safety and security difficult to produce in a mechanical system and provides a check and balance. When two of the systems agree and the third doesn’t, the majority rules and the defective system is overridden. If the control systems reach an ambiguous state, the pilots select the system that is operating properly, not the computer.
The G650 exceeds the global standard by providing a quadruple-redundant flight control computer. Each of the four channels has a command/monitor lane that must agree for the channel to operate. There is yet another dissimilar backup flight control unit providing three-axis control for continued safe flight and landing capability in the extremely remote event of loss of the primary flight control computer. The backup unit is not needed to meet certification integrity or availability requirements.
The dual flight control actuation system in the G650 uses energy from the aircraft hydraulic systems to articulate the flight controls. That is the norm. The third flight control operating system in the G650 is, however, totally independent of the hydraulic systems. These third-level backup actuators have their own electric hydraulic pumps and fluid reservoirs, with the actuators located near the flight control surfaces in the wings and tail. If, for some extremely unlikely reason, both normal hydraulic systems were to become inoperative, the electric actuators take over. The “all-electric” actuation is a dissimilar power source that improves safety and availability while eliminating the need for a third hydraulic system. The G650 once again sets the standard in advanced fly-by-wire technology in business jets.
And to prove the point, Gulfstream test pilots have flown entire mission profiles using only the electrically powered backup flight control actuators and found the performance to be the same as the normal systems. And thanks to redundant electrical systems, the G650 has several sources of electrical power available to the actuators.
Over the years Gulfstream has introduced a myriad of new technology—often being adopted as the industry norm by other jet makers. And Gulfstream has done the same with the fly-by-wire design of the G650. The system reaps the weight savings and efficiency gains possible without radically changing the way pilots operate the jet. And the electric flight control actuators set a new standard in redundancy, which is what we have come to expect in a Gulfstream.
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