Flight Cycle Research Articles

Efficient Numerical Integration Algorithm of Probabilistic Risk Assessment for Aero-Engine Rotors Considering In-Service Inspection Uncertainties

Numerical integration methods have the characteristics of high efficiency and precision, making them attractive for aero-engine probabilistic risk assessment and design optimization of an inspection plan. One factor that makes the numerical integration method a suitable approach to in-service inspection uncertainties is the explicit derivation of the integration formula and integration domains. This explicit derivation ensures accurate characterization of a multivariable system’s failure risk evolution mechanism. This study develops an efficient numerical integration algorithm for probabilistic risk assessment considering in-service inspection uncertainties. The principle of probability conservation is applied to the transformation of the integration domain from the current flight cycle to the initial (N = 0) computational space. Consequently, the integration formula of failure probability is deduced, and a detailed mathematical demonstration of the proposed method is provided. An actual compressor disk is evaluated using the efficient numerical integration algorithm and the Monte Carlo simulation to validate the accuracy and efficiency of the proposed method. Results show that the time cost of the proposed algorithm is dozens of times lower than that of the Monte Carlo simulation, with a maximum relative error of 5%. Thus, the efficient numerical integration algorithm can be applied to failure analysis in the airworthiness design of commercial aero-engine components.

SELECTION OF A SIGNIFICANT INDICATOR OF OPERATOR-PILOT TENSION ON THE BASIS OF OBJECTIVE INFORMATION ABOUT HIS CONDITION DURING FIGHT TASKS ON THE SEMI-NATURAL MODELLING COMPLEX

As a significant indicator for assessing the intensity of activity that affects performance, a local assessment was chosen in the form of an index of tension according to R. Baevsky, accumulating the effects of the influence of temporal, statistical and spectral indicators of the heart rate, its variability, followed by modification of the assessment into an integral indicator R as a coefficient body reserve. The peculiarity of the local assessment is that it is a dynamic parameter that characterizes the state of the body at a given point in time, and to obtain it, the Holter technology was used with the formation of digital files of heart rate variability (HRV) indicators recorded by a monitoring system with a set of four clusters each, spaced in time and containing data of implementations of flight tasks of the initial, middle and final phases of the flight. Necessary and sufficient to display the dynamics of HRV was the time duration of the clusters of 300 seconds, which made it possible to obtain the necessary digital content of the HRV parameters in 40…60 minutes of flight time. The informational significance of the data obtained is determined by the content of the implemented flight tasks for various purposes. The capabilities of the semi-natural modeling complex made it possible to create complex and special flight situations, that negatively affect the psycho-physiological state of the operator. Local estimates of the tension index were obtained, which generally fell within the range of acceptable values, although in special cases of flight they went into the range characteristic of the states of effective adaptation of the organism. Levels were also recorded, signaling the exhaustion of the body’s capabilities, fatigue and a reduced level of performance. A generalized integral indicator of tension is proposed – the reserve coefficient of the body R as a linear combination of local estimates of tension: the maximum local intensity index achieved in a given flight cycle, “passport” indicator of the operator’s tension, fixed before the flight task at the stage of pre-flight preparation, and an indicator of the current tension at a particular point in time in a particular situation. At the same time, the smaller the value of R, the smaller the reserve of the organism. The obtained experimental integral estimates of the body reserve in the process of operator activity are not only of research interest, but also of interest in practical applications in the tasks of ergonomic expertise of aviation equipment and the design of the operator-pilot activity.

Perhitungan Jumlah Maksimum Flight Cycle Tanpa Refuelling pada Kondisi Full Tank dan Full Payload pada Jaringan Rute Commuter Pesawat ATR 72

Transportation is the process of moving or moving people and goods from one place to another with a specific purpose, ATR aircraft are very suitable for regional flights, so that fuel utilization must be calculated to make the transportation cost is minimum. Indonesian airlines have very big demands in world aviation competition. One of Indonesia's domestic route type is commuter, ATR aircraft is very suitable for regional flights with commuter route, flights with short range with small aircraft for carrying passengers between airports with low passenger capacity. One of commuter advantage is the aircraft didn't have to refueling in the transit airport. The purpose of this research is to determine maximum flight cycle without refueling based on weight variation using FCOM data and fuel consumption calculation using analytical methods based on ATR aircraft specification. Using an analytic method for calculating fuel consumption from FCOM data with full tank strategy, full payload and refueling strategy in every airport for Nusa Tenggara and Sumatera route. This research calculation has resulted for Nusa Tenggara route total fuel is 4750,57 kg and Sumatera 4474,88 kg strategy full tank. Calculation has result for Nusa Tenggara 2789,69 kg and Sumatera 2658,98 kg strategy full payload.Then obtained full tank strategy not as much as 12 flights cycle, full payload as much as 7 flights cycle in Nusa Tenggara route and full tank strategy as much as 10 flight cycle, full payload obtained 6 flight cycle in Sumatera route On the Sumatra route.

Response analysis of solid missile grain in UAV-borne environment

Solid rocket motor of UAV-borne missile experiences the effect of cyclic temperature. For the single chamber double thrust rocket motor, the finite element model of the solid motor grain is established. Temperature field and stress-strain field under a mounted flight cycle are studied. The research results can provide reference for the structural integrity evaluation of the single chamber double thrust rocket motor and UAV flight system.

A dynamic performance diagnostic method applied to hydrogen powered aero engines operating under transient conditions

At present, aero engine fault diagnosis is mainly based on the steady-state condition at the cruise phase, and the gas path parameters in the entire flight process are not effectively used. At the same time, high quality steady-state monitoring measurements are not always available and as a result the accuracy of diagnosis might be affected. There is a recognized need for real-time performance diagnosis of aero engines operating under transient conditions, which can improve their condition-based maintenance. Recent studies have demonstrated the capability of the sequential model-based diagnostic method to predict accurately and efficiently the degradation of industrial gas turbines under steady-state conditions. Nevertheless, incorporating real-time data for fault detection of aero engines that operate in dynamic conditions is a more challenging task. The primary objective of this study is to investigate the performance of the sequential diagnostic method when it is applied to aero engines that operate under transient conditions while there is a variation in the bypass ratio and the heat soakage effects are taken into consideration. This study provides a novel approach for quantifying component degradation, such as fouling and erosion, by using an adapted version of the sequential diagnostic method. The research presented here confirms that the proposed method could be applied to aero engine fault diagnosis under both steady-state and dynamic conditions in real-time. In addition, the economic impact of engine degradation on fuel cost and payload revenue is evaluated when the engine under investigation is using hydrogen. The proposed method demonstrated promising diagnostic results where the maximum prediction errors for steady state and transient conditions are less than 0.006% and 0.016%, respectively. The comparison of the proposed method to a benchmark diagnostic method revealed a 15% improvement in accuracy which can have great benefit when considering that the cost attributed to degradation can reach up to $702,585 for 6000 flight cycles of a hydrogen powered aircraft fleet. This study provides an opportunity to improve our understanding of aero engine fault diagnosis in order to improve engine reliability, availability, and efficiency by online health monitoring.

Validation of the safety requirements of the landing gear using fault tree analysis

We analyze the functionality of the landing system of a regional aircraft in the extension and cruise flight modes and validate safety requirements through the fault tree analysis. The main landing gear system is captured in the electromechanical–fluidic domain and system behavior is abstracted in an elementary hydraulic circuit. The functional representation is then constructed into a fault tree which allows analysis of the failure propagation originating at different branch terminals, for instance, at the main landing gear actuator which extends the gear and holds it retracted during the cruise, door actuator, door uplocks, and hydraulic power supply. Each component is assigned a failure probability. Each failure mode is abstracted as a top-level event having a probability of failure and through Boolean combinations of component failures in the lower branches. Two reliability aspects considered are the availability to fully lower the landing gear and the integrity of inadvertent gear or door extension while cruising. Architectural changes through undercarriage system reconfiguration and component redundancy have been exploited to improve system failure rates. The analysis determines the overall system failure rate against the flight cycles. The process is agile to accommodate design changes with the evolution of architecture during the systems engineering lifecycle.

Micromechanical properties of vapour-exposed SiCf/BN/SiC ceramic-matrix composites

SiCf/BN/SiC Ceramic-Matrix Composites are candidate materials for aero-engines, but their interphase stability after potential low-temperature water-vapour exposures during flight cycles is not well known. The examination of these composites exposed for 50, 250 and 500 h at low temperature (65 °C) and 95% relative humidity was therefore performed, in order to understand if resulting oxidation products affected the mechanical properties at the micro-scale. The composites were subject to fibre push-out tests to compare pristine from degraded composites. It was found that whilst the sample exposed for 50 h had no significant change from pristine, the samples exposed for 250 and 500 h had a clear decrease in interfacial shear strengths measured. Parallel studies also revealed that whilst damage was strongly localised, the diffusion of water within the composite was not fully complete at exposure times between 50 and 250 h. The permeability of the CMC was shown to be affected at longer exposure times where differences in mechanical performances even between tows and within tows were noted.

Damage tolerance of a hybrid lower wing bonded stiffened panel with a Fiber Metal Laminate skin

Fiber Metal Laminates (FML) represent a highly damage tolerant option for lower wing skins that can be included in an advanced high performance metallic wing box. The purpose of the present study is to assess the damage tolerance behavior of a panel made of FML with 0.8 mm 2024 T3 sheet and bonded stringers.A 5-stringer test panel, 800 mm wide, is tested at R=0.1 at a maximum stress of 135 MPa. This stress conservatively represents a spectrum with a sustained flight stress (1 g stress) of 90 MPa corresponding to a 20% increase versus a conventional 2024 T351 lower wing allowable stress.Crack growth from an initial defect is monitored using Thermal Surface Analysis and visual inspection. The propagation life between the chosen detection event and the residual strength test is 137580 cycles. This detection event is a broken stringer; it occurs after the crack in the FML skin has reached a 39 mm length. The residual strength exceeds the required limit load. The total life from crack initiation at the notch to the crack length reached before the residual strength test is 220000 cycles.Combining the stress allowable increment and density reduction, the test justifies a 25% weight saving potential versus a lower wing in 2024 T351, with 45000 flight cycles inspection intervals and 90000 flight cycles Design Service Goal.

Calculation of fatigue fracture in structural elements using a multimode damage model

The problem of nucleation and development for the fatigue fracture process in element of aircraft structure is solved using previously developed cyclic damage model. The model contains the evolution equation for the damage function depending on stress state and amplitudes of the cyclic loads. On this basis, calculations of the initiation and development of crack-like fracture zones are carried out for an aircraft structure element of a disk with a blade in a flight loading cycle (HCF mode). The damage function was calculated based on the elastic solution for the element of structure loaded by centrifugal forces in disk and blade and aerodynamical pressure on blade. An alternative cyclic loading regime associated with high-frequency vibrations of the blades was investigated. Fatigue fracture in this process corresponds to the VHCF mode. Simulation of the combined cyclic loading mode was also carried out. In all these modes, the real time of the nucleation and development of quasi-cracks was estimated up to reaching the observed disk surface.

Damage-failure transition in VT6 titanium alloy under consecutive dynamic and VHCF loads

The lifetime of VT6 (Ti-6Al-4V) titanium alloy under consecutive dynamic and very-high cycle fatigue loads is studied. The relevance of the problem is determined by the applications to the prediction of the endurance fatigue limit for materials and structural elements of aircraft gas turbine engines experiencing random dynamic effects under flight cycle conditions. The titanium alloy specimens were used to perform High Cycle Fatigue (HCF) and Very High Cycle Fatigue (VHCF) tests on the ultrasonic testing machine, which allows fatigue loading for 108-1010 cycles with amplitude up to several tens of micrometers and frequency of 20 kHz. It is shown that VT6 alloy specimens in the 108 cycle regime subject to dynamic preloading reduces the fatigue strength by 70%. Structural study of the fracture surface for the specimens after consecutive loading was conducted using the profilometry data to identify the roughness scale invariants induced by defects for corresponding areas responsible for the staging of fatigue damage-failure transition. The scale invariants and corresponding lengths were used to formulate the generalized Paris law governing the crack growth in damaged materials.

Thermocyclic fatigue and destruction of high pressure turbine blades in their critical sections

In the article the critical sections and points at the leading edges of the airfoils of high-pressure turbines were reasonably defined for experimental studies of thermocyclic durability. Experimental studies of thermocyclic durability were performed for the samples of heatproof alloys for the cyclic change parameters in extreme temperatures and thermomechanical stresses that exert at the starting and running down of aircraft gas turbine engines and ground gas turbine power plants. The influence of protective coatings on thermocyclic durability of alloys and parts is mentioned in the article. The article describes the methods of determination the thermal stress in the blades. It is emphasized that extreme thermal stresses of compression in the edges of nozzle vanes and stretch on the pressure side of the airfoil occur on the operating mode "starting", and the cracks are revealed mainly on the mode "shutdown". In the intermediate stages of the flight cycle the do not exceed of the yield strength of the blade material and do not significantly affect there lative thermocycle durability of heatproof alloys. The destruction of the blades of high-pressure turbines in their cross sections along the airfoil shroud platform, in the zone of maximum temperature sand in the root sections explained from the position of the three– and multi component approach.

Self-enhancing model-based control for active transient protection and thrust response improvement of gas turbine aero-engines

A self-enhancing active transient protection (SeATP) control approach using model-based strategies is proposed for gas turbine aero-engines, which aims at pro-actively handling surge margin limit and turbine entry temperature limit over the life cycle. The feature of SeATP is a bank of self-enhancing loops with periodically updated controller parameters for different flight cycles. This is realized by an off-line gain tuning via global optimization approach. Additionally, a sensor-based baseline controller and a model-based active transient protection (ATP) controller (with fixed gains) are developed as comparison bases. Numerical simulations for the examined controllers are carried on a validated aero-thermal turbofan engine model for idle to full-power acceleration tests in Matlab/Simulink environment. Simulation results demonstrate that ATP controller owns a considerable thrust response improvement for both the new engine and a severely degraded engine, compared with the baseline controller. Moreover, the proposed SeATP controller ensures a 65.77% recovery rate of thrust response deviation caused by the ATP controller for the degraded engine. Particularly, a low transient surge margin trajectory and a high turbine entry temperature route are fulfilled by the SeATP controller. SeATP controller also shows better robustness performance for degradation variation than ATP controller. Hence, the SeATP control performance is confirmed.

The synergistic effect of Re and W on the evolution of TCP phases in Nickel-based superalloys

In this work, the synergistic effect of Re and W on the evolution of the topological close-packed phases (TCP) in Nickel-based superalloys were studied by means of Thermo-Calculation (TC), solidifying slowly, Differential Thermal Analysis (DTA), and solution-aging treatment. The specimens were analyzed by means of SEM-EDS, TEM-SADP, and EPMA. The calculated results indicated that the P phase precipitated prior to the μ phase with the temperature decrease, and both phases μ and P coexisted in a certain temperature range when the content of Re was higher ( Re: 5–7 wt%) and the content of W was lower ( W:4–6 wt%); Only phase μ precipitated from the alloy when Re content was less than 4 wt% and W content was higher than 7 wt%. The experimental results showed that the μ phases were slats-like and distributed in parallel in different orientations; and the μ/P intergrowth phases with coherent relationship were short rod-like. In addition, the gathered μ/P intergrowth phases around the big blocky-like γ' phases were found in alloys solidified slowly, which was prone to cause holes. Based on phase-transition temperature obtained by combining calculation and DTA, the solution-aging process of the alloys were determined. After solution-aging treatment, the long strip shape μ phases and short rod-like μ/P intergrowth phases distributed uniformly in the matrix, and the big blocky-like γ' phases and holes disappeared. The experimental results showed that the total amount of TCP phase decreased and the P phase gradually disappeared when the content of Re decreased and the content of W increased. The research results had a guiding role in optimizing the composition of alloys and predicting the safety performance of aerocraft during the flight cycle.

Remaining Useful Life Prediction of Aircraft Engines with Variable Length Input Sequences

Remaining useful life (RUL) is the expected remaining operating life of an asset until it can no longer perform its intended function. The 2021 PHM Society Data Challenge posed the problem of estimating the RUL of aircraft engines with various competing failure modes and underlying degradation trajectories. In this work, we describe the approach and solution to the challenge where we map from the multivariate time series sensor readings to the remaining useful life, measured in the remaining number of flight cycles until failure. The proposed solution utilizes a deep convolutional neural network that can take inputs of variable length. Furthermore, we preprocess the data according to a normalization procedure that help reveal the degradation trend that is obfuscated by the continuously varying flight conditions. The normalization procedure involves training a feedforward neural network on a non-degraded subset of the data that maps from the flight conditions to the sensor outputs. The difference between the expected sensor readings and the actual observations is then interpreted as the extent of deviation from normal, i.e., degradation. Finally, we describe the sampling techniques which is designed to reduce the number of non-informative samples fed to the neural network.

On the Potential of Extending Aircraft Service Time Using Load Monitoring

Aircraft structures experience various kinds of loads over their entire lifetime, typically leading to fatigue, and ultimately structural failure. To avoid structural failure during operation, the maximum number of flight cycles and flight hours is regulated by laws ensuring continued airworthiness. However, because every flight impacts the aircraft differently, not all airframes are equally stressed at the time of decommissioning. This paper proposes a new retirement criterion based on the so-called fatigue damage index (FDI). The criterion takes into account that aircraft are differently operated and thus enables an individual decommissioning of aircraft without compromising its safety. Based on aircraft sample data covering 95% of the Airbus A320 fleet over two years, the enhanced decommissioning criterion is estimated to significantly extend the average aircraft service life. The impact varies within the fleet, depending on the experienced seat load factors, cruise altitudes, and taxi times considered for the individual aircraft during operation. Whereas seat load factors and flight altitudes significantly affect the defined FDI, the influence of taxi times is only minor. Based on the estimated increase in aircraft service life, the paper argues that for service life extensions, the FDI shall be considered as the limit of validity in the regulatory framework governing the decommissioning of aircraft.

Impact of Load Retention on Aircraft Engine Parts Under Real Flight Cycle Conditions in Service Life Monitoring

Abstract One of the major problems in the development of algorithms for monitoring the life of aircraft gas turbine engines is that the character of loading in real flight cycles is crucially different from the character of the static and dynamic loading during the testing of samples. This paper proposes a method for taking into account the effect of retentions at maximum stresses and cycle temperatures on the low-cycle fatigue (LCF) of the heat-resistant alloys used in engine parts. Regularities in repeated-static loading (RSL) are used in combination with the method of linear accumulation of damage due to the LCF and RSL, with retentions of a variable length. A non-linear equation is derived for the summation of these damages, the solution of which determines the durability (life) of the part while taking into account the retention duration. The theoretical results were verified by using the experimental characteristics of the GS-6K and EI-437B nickel-based alloys, previously reported by other researchers.

Теоретичні та експериментальні методи визначення характеристик міцності лопаток турбін при термомеханічному навантаженні

The subject of this article is the methods of research and evaluation of the properties of turbine blades of a cooled structure under thermomechanical loading. The purpose of the article is to review the world achievements of leading enterprises and research institutions in the issue of fatigue tests of turbine blades under complex loading (cyclic temperature exposure, dynamic and static loading), as well as an overview of the state of this topic at SE "Ivchenko-Progress" and suggestions for its further studying. As a result of the analysis of publications and scientific articles, it can be concluded that specialized research institutes and leading aircraft engine-building enterprises from the end of the twentieth century are studying the properties of turbine blades in the conditions of their operation as part of an engine. In world practice, there are calculated and experimental methods for thermomechanical testing of turbine blades. These tests are aimed at determining the most damaging loads, establishing the flight cycle modes at which these loads are recorded. As a result, it was found that the greatest threat to the strength of the turbine blades is carried by transient modes of engine operation, which are short in time (measured in seconds), but at which there is a change in the parameters of the temperature field, loads from axial and centrifugal forces. And it is the cycling of these parameters that leads to a decrease in the cyclic durability of the turbine blades, especially of the cooled structure (the presence of perforations, internal cooling channels, and other structural elements leads to a complication of the volumetric stress state of the blades). The article analyzes various crystallographic structures of blades and their relationship with the volumetric stress state; examples of studies that were carried out at SE "Ivchenko-Progress" and their results are given, which emphasize the need for further experiments in the field of assessing strength characteristics under complex cyclic loading. An example of an installation for testing blade joints and samples of gears is considered, which can be adapted for testing blades with three-component loading (temperature, dynamic loads, and imitation of the effect of centrifugal forces). It is concluded that when using exclusively computational methods, it is impossible to reliably estimate the level of stresses and their distribution since the calculations are limited by the boundary conditions, which are set according to the capabilities of a particular computational model. Summing up, it can be noted that it is advisable to start assessing the strength of blades under thermomechanical loading with several series of tests of samples of blade material to study the effect of temperature and power cycles of loads, the effect of the orientation of the load vector concerning the crystallographic orientation of the blade. It is noted that tests of full-scale blades under thermomechanical loading are also important since the features of the volumetric stress state of the material during real operation of the blades as part of an engine are not reproduced during testing of samples. The above entails the development of methods and specialized installations for thermomechanical testing.

Developing a non-routine maintenance load forecasting procedure in maintenance, repair and overhaul (MRO) XYZ company: A case study of B737NG aircraft

Turn Around Time (TAT) is one of the most important performance indicators in a Maintenance, Repair, and Overhaul (MRO) company. MRO companies need a high percentage of on-time TAT to compete in the industry. In 2019, there is a 29% difference between the planned and the actual TAT in MRO XYZ. Based on observations in MRO XYZ, there is no planning to perform non-routine maintenance. 54% of the total maintenance loads is a non-routine maintenance, therefore, it needs to be planned. The first step of the research is to identify routine maintenance tasks that dominate the non-routine maintenance loads based on ATA Chapter and CAMP Number using Pareto Analysis. The next step is to develop a procedure which determines the variables and the mathematical model to estimate the Non-Routine Ratio (NRR) workloads. The last step is to implement the procedure to obtain the NRR estimation model, which is also as the validation of the developed procedure. Several routine maintenance tasks dominate Non-routine maintenance loads are categorized by ATA Chapter, CAMP Number, and task types. Dominant ATA Chapters are ATA 53, 25, and 57. Dominant CAMP numbers are 53-140-00, 53-800-00, and 53-866-00. Dominant task categories are DVI for the system, internal GVI for structure, and external inspection for zonal. The NRR forecasting model for B737NG C-Check is composed of C-Check number, aircraft’s age, the ratio of ATA 53, the ratio of ATA 25, and the ratio of ATA 57. The NRR forecast model can be improved by adding some variables, such as Flight Hours (FH) and Flight Cycles (FC).

Development of Monitoring and Mating Disruption against the Chilean Leafroller Proeulia auraria (Lepidoptera: Tortricidae) in Orchards.

Simple SummaryProeulia auraria is a native and growing pest insect in fruit orchards in Chile, which calls for environmentally friendly management methods. Using synthetic pheromone compounds, we conducted field trials to optimize the septa load for monitoring adult moths. Using the optimized blend we studied the phenology of males in vineyards, apples, and blueberries, finding two large flight cycles lasting from September to May. Afterward, based on field trials, we concluded that 250 point sources loaded with a total of 78 g/ha of the pheromone blend, provided high disruption of male-female encounters for mating in all crops tested for at least 5 months. We concluded that mating disruption is feasible for P. auraria, needing now the development of a commercial product and of protocols to control this pest.The leafroller Proeulia auraria (Clarke) (Lepidoptera: Tortricidae) is a native, polyphagous, and growing pest of several fruit crops in Chile; it also has quarantine importance to several markets, thus tools for management are needed. Using synthetic pheromone compounds, we conducted field trials to optimize the blend for monitoring, and to determine the activity period of rubber septa aged under field conditions. We concluded that septa loaded with 200 μg of E11-14:OAc + 60 μg E11-14:OH allowed for efficient trap captures for up to 10 weeks. Using this blend, we studied the phenology of adult males in vineyards, apple, and blueberry orchards, identifying two long flight cycles per season, lasting from September to May and suggesting 2–3 generations during the season. No or low adult activity was observed during January and between late May and late August. Furthermore, mating disruption (MD) field trials showed that application of 250 pheromone point sources using the dispenser wax matrix SPLAT (Specialized Pheromone and Lure Application Technology, 10.5% pheromone) with a total of 78 g/ha of the blend described above resulted in trap shutdown immediately after application, and mating disruption >99% in all orchards for at least 5 months. We concluded that MD is feasible for P. auraria, needing now the development of a commercial product and the strategy (and protocols) necessary to control this pest in conventional and organic orchards in Chile. As far as we know, this is the first report on MD development against a South American tortricid pest.

Model-based Remaining-Useful-Life prognostics for aircraft Cooling Units

Prognostics for the Remaining-Useful-Life (RUL) of aircraft components are crucial to support efficient aircraft maintenance planning and, in particular, to limit unscheduled maintenance due to unexpected component failures. As such, predictive methods for the RUL of aircraft components are increasingly a priority for aircraft Maintenance, Repair and Operations (MROs). In this paper we develop model-based RUL prognostics for aircraft Cooling Units using operational data recorded during the flights of several wide-body aircraft. A Cooling Unit is a vapor cycle refrigeration unit consisting of a condenser, a flash tank, an evaporator and a compressor. After some time of usage, the filter of these Cooling Units is clogged with burned oil, moist and sludge from the compressor. This accelerates the wear of the components. Long time utilization of these components in these conditions leads to a failure. To model the degradation of the Cooling Units, we use an exponential functional form for the degradation. Together with sequential Monte Carlo methods, we estimate the probability distribution of the RUL of these components. The exponential functional form of the degradation is based on the fact that the cumulative damage in the components has an effect on the degradation rate. It has been shown that an exponential model is a good approximation for non-linear degradation processes like corrosion, bearing degradation, or deterioration of LED lighting. In fact, the Cooling Units can also be seen as subject to corrosion and accelerated wear. We evaluate our RUL prognostics for various prediction horizons, i.e., at 30, 20 and 10 flight cycles before failure. The results show that our proposed methodology is able to estimate the RUL of the Cooling Units well, and that the uncertainty associated with the prognostics decreases as the prediction horizon decreases, i.e., as the components approach failure. The choice of the prediction horizon is relevant from the point of view of MROs, which re-evaluate periodically their aircraft maintenance schedules. In practice, regular maintenance checks are scheduled every two weeks. Having accurate RUL prognostics over such time horizons enables the maintenance planners to better plan tasks, limiting unscheduled failures. In addition, the fact that we estimate the uncertainty associated with the RUL prognostics enables the maintenance planners to prioritize the maintenance of the components. Overall, our results provide support for maintenance planners to make informed and efficient maintenance schedules.