Fly-by-wire (FBW) Control Systems (4) advantages
Improved handling. The many small deficiencies in handling of even modern airliners can be eliminated through the efficient use of computers which can make the aircraft responses to control inputs match exactly what the pilot would want them to be.
Fuel saving. A more compelling commercial reason for the implementation of FBW was the potential for fuel savings to be made if the aircraft could be flown at centre of gravity (CG) positions which would normally have made the aircraft unacceptably unstable. Although there is currently perceived to be a need to allow civil aircraft to be flown manually in the event of complete computer or electrics failure, and thus for the aircraft always to exhibit normal stability, it is likely that at some time in the future, when confidence in FBW systems is complete, flight with relaxed stability will be permitted. Such flight could only be safely achieved through the use of FBW and, once it has been allowed, significant fuel savings could be made.
Fly-by-wire (FBW) Control Systems (5)
Advanced FBW concepts
The introduction of FBW to an aircraft could simply provide a computer link between the pilot's controls and the control surfaces; in other words, a movement of the pilot's controls would cause a corresponding and proportionate movement of the control surfaces. Such a level of development would provide the weight savings promised by FBW but would do little to improve the handling of the aircraft, and would not advance the technology very far towards allowing aircraft with relaxed stability to be flown. In order to achieve either of these two goals the computer must be made to do a little more and, typically, this would be to cause the aircraft to respond in a certain manner to the pilot's inputs by driving the controls as appropriate. The pilot could be enabled, for example, to demand changes in the pitch rate or the flightpath of the aircraft rather than demand simple control surface movements. Such an FBW system is often called an 'Active Control' system because the control system itself is more than a passive conveyor of instructions.
Fly-by-wire (FBW) Control Systems (6)
Architecture
In order to provide some redundancy and to improve safety by allowing comparisons to be made of the output demands of more than one computation, it is normal for an active control system to comprise several different computers. Each one of these computers will have specific functions and no single computer will be permitted to exercise control without its commands being monitored by at least one other computer. Ideally the computers should be constructed separately and their programs written independently in order to avoid the possibility of a design fault or software error being common to them all.
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