**2. Research Workflow**

The main objective of the "CleanSky2/Astib" consortium was the definition of an iron bird for a regional transport aircraft with fully electrical flight-control systems acting as a technological demonstrator for several new technologies, including PHM functionalities monitoring the health status of the flight-control actuators.

The iron bird followed the scheme provided in Figure 1 [8] and was roughly divided into 2 major sections, namely a "real" half-wing and a "simulated" half-wing. The electromechanical actuators developed during the program were seated within the "real" half-wing and coupled with a set of electrohydraulic actuators controlled in force. The "simulated" half-wing was instead realized through a real-time representation of the very same actuators present in the "real" half-wing (Actuators Simulation Module), while an Engineering Test Station allowed changing the parameters of the simulation and introducing faults or failures within the "simulated" actuators. Both the "real" and the "simulated" half-wings interacted with the Flight Control Computers (FCC), which received input from a real cockpit. A real-time simulation of the aircraft dynamics (Flight Mechanics Simulation Module) dialogued with the FCCs and computed the vehicle attitude considering the behavior of the flight-control system, of the simulated throttle, and of the simulated atmospheric conditions [13]. The choice to divide the FCS into "real" and "simulated" halves resided in the need to simulate the effects that ongoing faults or degradation may have at the component, system, and aircraft level without damaging the iron bird and while reducing the need for spare equipment. The Health Management System Module (HMSM) finally collected the data coming from both the "real" and the "simulated" actuators and ran the PHM routines. The PHM routines were developed following the scheme provided in Figure 2. The case study was first analyzed, assessing which were the constraints (only component-level signals available, system-level signals, and so forth), which were the sensors that could be employed to monitor the health status of the actuators, and which were the most critical failure modes. This information drove the definition of a first high-fidelity model of the system. Such a model was built to represent the component dynamics in detail and was exploited to assess which signals could be useful for PHM, and which features could be reasonably extracted to perform diagnostics/prognostics tasks with the help of an experimental setup [14]. In parallel, a streamlined version of the high-fidelity model, capable of representing the effects of ongoing degradations over the system dynamics and suitable for real-time deployment, was prepared. The goal of such a model was twofold. On the one hand, it was prepared to be deployed within the Actuators Simulation Module ("simulated" half-wing) of the iron bird. On the other hand, it could be used offline to quickly produce the datasets required to test the PHM routines before their installation on the iron bird, thus allowing the code to be debugged off-site and providing a preliminary performance assessment prior to the iron bird commissioning.

**Figure 1.** Iron-bird schematics.

**Figure 2.** Research program workflow.
