Feasibility Study of a PHM System for Electro-hydraulic Servo-actuators for Primary Flight Controls

Abstract

Electro-Hydraulic Servo-Actuators (EHSA) are by far the mostly used type of actuators in aircraft primary flight control systems. Though electrical actuation is been considered since long as a possible replacement of hydraulic actuation for aircraft systems, EHSAs are still the technology of choice in the primary flight control systems of new commercial aircrafts. Considering that 10 or more EHSAs are typically used in an aircraft flight control system, the development of an effective PHM system for this equipment could provide large benefits and be of great interest for the OEMs and for the air fleet operators.
 This paper presents the results of a feasibility study making up the first part of an ongoing research activity focused on the development of a PHM system for EHSAs used in fly-by-wire primary flight control systems and takes as a use case the primary flight control actuator of a wide body commercial aircraft. One of the key features of the research is the implementation of a PHM system without the addition of new sensors, taking advantage of the available signals. This offers the possibility of implementation of the PHM system on the existing platforms and not only as a proposition for new aircrafts designed with a complement of additional sensors. The enabling technologies for this PHM system borrow from the area of Bayesian estimation theory and specifically particle filtering and the information acquired from EHSA in-flight and during pre-flight check is processed by appropriate algorithms in order to obtain relevant features, detect the degradation and estimate the Remaining Useful Life (RUL). The results are evaluated through appropriate metrics in order to assess the performance and effectiveness of the implemented PHM.
 This paper describes the methodology of the feasibility study, which shows how the novel PHM technologies proposed for a PHM system for the EHSAs of primary flight control actuators can allow the migration from unscheduled / on-condition maintenance to condition based maintenance targeting the perceived objectives of the OEM and of the aircraft operator.

A Case Study on the Detection and Prognosis of Internal Leakages in Electro-Hydraulic Flight Control Actuators

“…The approach followed in this paper follows the template for preliminary PHM studies provided in [9] and is a direct follow-up of the feasibility analysis described in [10]. To better introduce the work presented in this paper, the main conclusions of the feasibility analysis are hereby reported.…”

“…According to [13], the definition of a PHM system for a given use-case must always start from a dedicated failure mode effect and criticality analysis (FMECA), which main purpose is to evaluate which failure modes are expected to be most frequent and critical, and at the same time to provide an early estimate on their observability. According to early investigations, the occurrence of excessive leakage within the actuator is between the most common cause of return from the field for EHSAs currently employed in primary flight control systems [9]. Extensive simulation activities, supported by experimental tests allowed to establish the feasibility of a PHM system for EHSAs through a combination of in-flight and pre-flight analysis, providing some preliminary conclusion on the feature selection process.…”

“…The set-up also allows to impose variations over the supply pressure sent to the actuator by manually acting on the pressure reducer valve interposed between the test bed and the remote hydraulic power generator. The system was at first tested with no additional modifications with an array of sinusoidal, step and ramp commands mimicking the on-ground dedicated sequence defined in [9], while the load actuator was kept at tank pressure.…”

“…Although extremely promising, this methodology was not tested against data sets representative of the operating conditions typically encountered by real flight-control actuators (wide temperature variance, unpredictable external loads, significant variations of the inlet and return channel pressures). Recently, the authors of this paper have investigated the effects of several faults mode on electro-hydraulic servo actuators (EHSA) through theoretical [9] and experimental [10] activities, addressing the feasibility of the development of a dedicated PHM system for a fixed-wing aircraft without resorting to additional sensors, thus using only the signals already available on-board. The investigated fault modes included degradations affecting both the servo-valve (backlash and crack progression within the feedback spring, degradation of the torque motor magnets, mechanical strain of the jet pipe) and the actuator (occlusion of the by-pass channel, wear of the sealing elements, backlash in the rod-end).…”

“…This paper provides the results of an advancement towards the definition of an operative prognostic tool focused on the detection, identification, and prognosis of the internal leakage of an EHSA employed as a primary flight control actuator in a wide-body aircraft currently in service. The paper is an autonomous continuation of the work presented in [9,10] and is organized as follows; at first, the case study under analysis is presented, highlighting its peculiarities and operational modes. The adopted approach is then presented, and both the simulation model and the experimental set-up introduced.…”