**1. Introduction**

Prognostics and Health Management (PHM) is a relatively new, multidisciplinary research field aimed at the definition of routines capable of predicting the time of failure (ToF) of a defective system or component based upon a set number of signals (or "features") extracted from the system itself. The capability to anticipate the failure occurrence and to estimate the remaining useful life (RUL) of a system or a component would provide a set number of valuable advantages. If completely realized, it would provide important strategic information pertaining to the opportunity to perform maintenance operations, the available time window to successfully replace the faulty component, and eventually to provide advice or an automatic reconfiguration of the defective system to compensate for the effects of the degradation or to extend the RUL [1]. Although application-agnostic in nature, PHM is of particular interest for aerospace applications, where the occurrence of unanticipated failures causes the disruption of aircraft availability, which is a costly and potentially dangerous situation in both commercial and military aviation. As such, the benefits of PHM are not limited to the optimization of the maintenance policy and a reduction of its costs but have significant ramifications over the maintenance logistics (e.g., spare parts, personnel, and dedicated facilities), business choices (spare aircraft number)

**Citation:** Bertolino, A.C.; De Martin, A.; Jacazio, G.; Sorli, M. Design and Preliminary Performance Assessment of a PHM System for Electromechanical Flight Control Actuators. *Aerospace* **2023**, *10*, 335. https://doi.org/10.3390/ aerospace10040335

Academic Editors: Spiros Pantelakis, Andreas Strohmayer and Jordi Pons-Prats

Received: 27 February 2023 Revised: 23 March 2023 Accepted: 24 March 2023 Published: 28 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and eventually strategic decisions (finish the mission or return to base). In this context, onboard actuation systems are one of the most critical aircraft systems and one of the major causes of disruption of aircraft availability. The vast majority of currently in-service aircraft are equipped with electrohydraulic or electrohydrostatic actuators. However, the growing push toward the design of "more-electric" aircraft has encouraged several research activities aimed at the design and certification of electromechanical solutions. Compared with hydraulic technology, Electromechanical Actuators (EMAs) completely avoid the environmental and cost issues associated with the use of aggressive hydraulic fluid, provide significant advantages in terms of reliability and system layout design, and offer a combination of weights competitive with the hydraulic counterpart, especially for low power requirements. Despite these advantages, EMAs are seldom used in flight-control systems and are mostly limited to UAVs or nonsafety critical controls due to their susceptibility to single points of failure, which can cause potentially catastrophic events like the jamming of the aerodynamic surface. Although these issues must be solved by design or through changes in the flight control architecture, the definition of a reliable PHM system would potentially help mitigate the probability of jamming, thus pushing the adoption of EMA technology. Although the literature on the definition of health monitoring schemes for electromechanical actuators and their most important components is fairly extensive [2–5], little can be found on the subject of the implementation and performance of such PHM logics on a real operating platform. There are two main reasons for this: first, the number of flying aircraft equipped with electromechanical actuation systems is still extremely low; and second, it is often impossible to gain access to and record the signals generated by the EMA sensors due to hardware constraints and data propriety restrictions. The lack of in-flight data represents a significant stopgap toward the definition of the PHM algorithm for flight-control systems since it becomes more difficult to fully represent the features uncertainty due to the widely varying operating conditions (temperature, loads, command pattern). To overcome these issues and reproduce as closely as possible the scenario in which the flight controls usually operate, researchers have resorted to highly complex simulation environments [6] and challenging experimental procedures, such as the in-flight test bench [7]. The aim of the ASTIB project (Development of Advanced Systems Technologies and hardware/software for the flight simulator and iron bird demonstrators for regional aircraft) is to bridge the gap between preliminary analysis and full-scale implementation through the design and realization of a technological demonstrator, or iron bird, for a new regional aircraft with fully electrical flight controls and PHM functionalities [8]. In the literature, very few iron birds have been presented, mainly aimed at studying electrohydraulic solutions. Li et al. [9] presented the development of an iron bird for an already in-service regional jet aircraft aimed at validating its troubleshooting plans, making some design improvements, and understanding the root of certain observed failures on the hydraulic nose landing gear system. A similar approach was followed by Spangenberg et al. [10], which developed a hardware-in-the-loop iron bird to perform certification tests during the early stage of aircraft design without constructing prototypes, focusing on electrohydraulic actuators for primary flight-control surfaces. Blasi et al. [11] analysed the control architecture of a modular iron bird oriented to the test of small/medium UAV's electromechanical actuators subjected to realistic in-flight load conditions obtained by a real-time flight simulator. In [12], an iron bird of an F-18 research aircraft was constructed to execute in-flight tests to perform system integration tests, verification, validation, and failure mode analyses for electromechanical aileron actuators. The novelty of the iron bird presented in this paper resides in its implementation: in fact, it has a hybrid layout in which a half-wing is composed of real equipment, while the other half is completely simulated with a hardware-in-the-loop scheme. Furthermore, it not only represents a commercial technological demonstrator but also embraces innovative research goals, being the first iron bird also aimed at the definition of PHM algorithms. This paper deals with the activities pertaining to the development of such PHM systems and their implementation within the iron bird structure and is organized as follows. First, the research objectives, perimeter, and

workflow are detailed, framing the research activities and highlighting the expected goals and known limitations of the proposed work. Then, the simulation activities necessary for the definition of the PHM framework are detailed and justified. Such activities include the definition of a high-fidelity dynamic model of the system under analysis needed to study the effects of the most common failure modes on the actuators signals and a real-time declination of such models to quickly generate data. The PHM system is then introduced and tested against several synthetic datasets to provide a preliminary assessment of its expected performances.
