In the references regarding the flight control (for example in F-16) I see the word "wraparound". For example "Rudder coil wraparound", "IBU pitch wraparound", "IBU lateral wraparound"or "AMUX bus wraparound failure". What does this word mean in this context?
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Wraparound is a method used to ensure flight control computer (FLCC) commands are reaching the actuators as they were inteded.
To maintain a desired flight condition FLCC calculates necessary control surface movements. These calculations are translated into control signals that are sent to actuators. These signals are periodically monitored by wraparound loop to verify they match the command originally calculated by FLCC.
This is the quickest (and the only viable) way to verify system integrity, since there is no need to wait for aircraft sensors to verify the plane did what FLCC wanted. Should the wraparound signal not match what FLCC commanded, the control reverts to a backup system.
Without wraparound, faulty signals would lead to incorrect control surface movements, that would in turn lead to corrective action by FLCC, and should the fault in the signal delivery persist, the corrective action would of course be also wrong. Pilot action to supress these incorrect control outputs would be difficult at best, impossible at worst, since commands given by pilot are delivered through the faulty system. In the case of modern, unstable fighter jets, the worst possible outcome is the most likely one.
A diagram of wraparound arrangement can be seen here: NASA technical paper 2857, page 91
Wraparound is pretty well described in this patent application: Failsafe digital bus to analog protocol converter system
A failsafe avionics interface unit (10) serves as a protocol converter system for translating data for control purposes from a digital communication bus (65) to an analog control channel as an upgrade or interface to an existing analog control system. The avionics interface unit (10) includes digital interface bus logic (101), a digital data processor module (102), an analog interface, and a mode selection circuit (104). Data communicated over the digital interface bus (65) is periodically translated into equivalent analog control signals. For safety purposes, the avionics interface unit (10) ensures the integrity of the system by repetitively looping back or wrapping the digital equivalents of the analog output signals and comparing them to commanded digital control values. The protocol converter system additionally monitors communications over the digital data bus for data continuity. If the difference between a wrapped digital equivalent of an analog output signal and an expected value exceeds tolerable limits or if communication fails, the system disconnects its interface from control using redundant software and hardware and returns control to the analog control system (51).
