Thank you for the correction. While we have got you here, would you please explain the Quadruplex fly by wire and in layman terms what it means. Thanks in advance.
A
Quad means 'four', obviously...
But what is not obvious is: Four of what ?
It means four channels of pitch, of roll, of yaw, and of air data.
It begs the question: Why four ?
Why not have only one set of wires from the cockpit to the FLCS computers to the flight control surfaces ?
In theory, that should work.
But, as given in post 3499 about safety, what if this one channel failed, either from combat or non-combat related reasons ?
So what General Dynamics did for the F-16, which is the pioneer for this quadruplex thingy, was that they physically replicated four channels of pitch, of roll, of yaw, and of air data, and placed them in physically distinct locations on the aircraft.
There is only one pilot control stick. But from that single source, four separate electrical signals for pitch and for roll are produced.
There is only one set of rudder pedals. But from that one set of rudder pedals, four separate electrical signals for yaw are produced.
So let us take
PITCH ONLY for now...
From the cockpit, four physically separated electrical signals, aka 'channels', are routed in different paths. This is so that if one channel is damaged in any way, most likely from combat related reasons, then there are three other channels for which the FLCS computer can work with.
Quad means 'four', obviously...But why four ? Why not three or even two ?
Any EE worth his diploma can immediately answer that the moment you introduce longer wiring, more connectors, etc...etc...there will be slight differences -- impedance for example -- between channels despite them sharing the same source. So if you know and have two slightly different signals, how do you know which one is the closest to source ?
Then why not have three channels and electrically vote ? The two channels that are closest to each other will average out and we will use that signal to command pitch.
But what if one channel is damaged or electrically degraded in comparison to the the other two ? Now we are down to only two channels, which lead us back to the original problem.
This is where the concept of quadruplex (four channels) came to be and is the standard for ALL aircrafts that want the advantages of FBW and safety.
The concept is called 'two fail operability' or 'two fail redundancy' system.
Operability - Wikipedia, the free encyclopedia
...is the ability to keep an equipment, a system or a whole industrial installation in a safe and reliable functioning condition, according to pre-defined operational requirements.
In a four channels system, three are used for electrical voting with the fourth in standby.
If one channel is degraded or lost for any reason, the standby channel would be in play and we would still have three channels for voting. This is 'one fail' operability.
If one more channel is degraded or lost for any reason, we now have only two channels in operation, the voting process is eliminated and one channel is selected for primary usage. This is 'two fail' operability.
If one more channel is degraded or lost for any reason, the entire FLCS fall back to a set of pre-set signals. At this point, if the jet is still airborne, the F-16 might as well declare an in-flight emergency (IFE) and limp home. All responses to commands will be much slower than normal.
In the F-16's history, there is no known 'two fail' operation.
Now...Just in case anyone thinks I am making this stuff up...
IEEE Xplore Abstract
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Redundancy Management of Shuttle Flight Control Rate Gyroscopes and Accelerometers
The Space Shuttle Flight Control System is required to be two-fault tolerant; i.e., the system must be capable of returning to a landing site even though two system failures occur. This is referred to as fail-operational/fail-safe performance. To achieve this fault tolerance, the avionics equipment is made redundant. A redundancy management (RM) system is required to detect failures and reconfigure the avionics accordingly. This paper describes the Flight Control System (FCS) rate gyro and accelerometer redundancy management design, analysis tools and analysis results.
IEEE is a respected source.
If the 'two fail operation' concept is good enough for the Space Shuttle, it is good enough for everyone else.
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