FBW flight control systems
Fly-by-wire (FBW) is the flight control systems which use computers to process the flight control inputs made by the pilot or autopilot, and which send corresponding electrical signals to the flight control surface actuators.
FBW technology replaces the heavy pushrods, cables, and pulleys previously used to move control surfaces on an aircraft’s wings and tail. Compared to a mechanical control system, fly-by-wire is smaller, lighter, offers improved performance, and is more responsive to pilot inputs.
The system is easier to install than mechanical linkages, thus lowering manufacturing and maintenance costs. Space and weight once given to mechanical linkages can be used to carry more passengers and cargo, to increase fuel capacity, and give the aircraft greater range.
Designers also benefit from computerized flight controls which make dozens of subtle adjustments a second, keeping an airplane on its intended flight path, and allowing for designs that otherwise would be aerodynamically unstable.
AV-8B Harrier II: not a fly-by-wire aircraft
Given all FBW’s qualities, do all modern aircraft use “fly-by-wire” type flight control systems? If not, why?
Pete Bowen, former US Marine Corps (USMC) AV-8B Harrier II pilot, explains on Quora;
‘The AV-8B Harrier is not a fly-by-wire aircraft and there are good reasons for that.
‘The AV-8B has the ability to rotate the exhaust nozzles into any position from pointing directly aft, like a conventional jet, to pointing straight down, under the aircraft, so it can hover (picture). The pilot controls where the nozzles point by moving the nozzle lever, which sits just inboard from the throttle. Push the nozzle lever full forward and the nozzles point aft. Pull the nozzle lever all the way in, and the nozzles will point 20d ahead of the airplane.
‘In the Harrier, it is critical that all the engine nozzles point the same way. Having one point a different direction than the others could cause catastrophic loss of control of the aircraft.
‘When engineers were designing the AV-8B, they considered making the nozzle lever a fly-by-wire type lever where lever position sent an electronic signal to electric motors that pointed each nozzle. They rejected that approach. Instead, they designed a cable system that mechanically turned the nozzles using motorcycle chain. The motorcycle chain was simpler, more reliable, and more accurate—so they used it.’
Bowen concludes;
‘Electronics are not always the best answer.’
Photo credit: DCS and Sgt. Jamean Berry / U.S. Marine Corps
