In Figure2, redundancy flight control software is the upper software layer which uses redundancy function simulation API to access the original hardware driver API to get input data, synchronize other channels, cross-translate data and output control commands. Based on this redundancy function simulation API, the fault injection client can inject the specific fault to the real hardware data according to the needs of system testing. For the application layer (redundancy flight control software), there is no way to distinguish the real hardware data or the injecting-fault data. So the redundancy management strategy and flight control software can be tested.