Aging Aircraft Research Articles

Enhancing aircraft crack repair efficiency through novel optimization of piezoelectric actuator parameters: A design of experiments and adaptive neuro-fuzzy inference system approach

This study addressed the critical problem of repairing cracks in aging aircraft structures, a safety concern of paramount importance given the extended service life of modern fleets. Utilized a finite element (FE) method enhanced by the design of experiments (DOE) and adaptive neuro-fuzzy inference system (ANFIS) approaches to analyze the efficacy of piezoelectric actuators in mitigating stress intensity factors (SIF) at crack tips—a novel integration in structural repair strategies. Through simulations, we examined the impact of various factors on the repair process, including the plate, actuator, and adhesive bond size and characteristics. In this work, initially, the SIF estimation used the FE approach at crack tips in aluminum 2024-T3 plate under the uniform uniaxial tensile load. Next, numerous simulations have been performed by changing the parameters and their levels to collect the data information for the analysis of the DOE and ANFIS approach. The FE simulation results have shown that changing the parameters and their levels will result in changing of SIF. Several DOE and ANFIS optimization cases have been performed for the depth analysis of parameters. The current results indicated that optimal placement, size, and voltage applied to the piezoelectric actuators are crucial for maximizing crack repair efficiency, with the ability to significantly reduce the SIF by a quantified percentage under specific conditions. This research surpasses previous efforts by providing a comprehensive parameter optimization of piezoelectric actuator application, offering a methodologically advanced and practically relevant pathway to enhance aircraft structural integrity and maintenance practices. The study innovation lies in its methodological fusion, which holistically examines the parameters influencing SIF reduction in aircraft crack repair, marking a significant leap in applying intelligent materials in aerospace engineering.

Identification of damage and corrosion effect in aging aircraft data transmission lines

Corrosion in aircraft structures and systems persists to pose significant problem in the aircraft industry. It affects both the aircraft structures and all the wiring systems including data transmission lines at the points of mechanical damage (missing or damaged insulation and jacket materials) and connections. Reliability of data transmission through aircraft data transmission lines (ATDL) is essential for maintaining performance capability and safety of flights. Unfortunately, there is no existing method for detecting corrosion and predicting its effect for wiring systems including ADTL. Detecting corrosion and predicting its effect on time would prevent unexpected system failures and catastrophes. This paper presents novel experimental method that has been employed, for the first time, to detect and study the effect of localized corrosion process on reflected and transmitted power signals passing through locally corroded ADTL with mechanically induced damage. ADTL with three types of induced damage were corroded in a Q-Fog cyclic accelerated corrosion chamber for over 14 weeks (2352 h) using ASTM G85-A5 Prohesion test that resembles the environment of aircraft operation. Cable specimens were taken out every week and investigated by measuring transmitted and reflected power signals passing through the corroded ADTL using Vector Network Analyzer (VNA), where signals from 0 to 6 GHz were swept into the ADTL. Reflected and transmitted power signals (S11 and S21 respectively) were acquired in time domain and processed in MATLAB. Analysis of results show that corrosion effect is not arbitrary and can be tracked over time and predicted. Reflected signal peak width changed over time of corrosion exposure and is proportional to corrosion propagation width inside the ADTL. It is also observed that corrosion induced extra distortion of signals passing through the corroded damage region. The results are very promising for detecting corrosion and predicting its effect in ADTL.

STUDY ON THE IMPACT OF AIRCRAFT AGE ON FLIGHT SAFETY FOR A MEDIUM COURIER TURBOPROP AIRPLANE

This paper introduces the connection between old aircraft and flight safety as well as the present state and potential futures for managing old aircraft. Age can be managed in two ways: retiring the aircraft and buying a newer one, or properly sustaining aging aircraft through additional and targeted maintenance. To do this, manufacturers, maintainers, operators, and owners must work together with regulators. A good way to guarantee proper maintenance is through continuing airworthiness programs and supplemental inspection programs. While the effects of aging on an aircraft can be detrimental to safety, they can be reduced with proper maintenance. If the operators follow the maintenance programs, current and future maintenance programs will operate as a preventative strategy to lessen the safety risk associated with aging aircraft.

Senior Aviator Medical Qualification Status.

Aviation personnel are expensive to train and challenging for the services to retain because of lucrative opportunities in the civilian aviation sector and a desire among pilots for agency. The military services' retention efforts have typically utilized a combination of high continuation pays and longer service commitments of up to 10 years following initial training. One area that the services have overlooked in their attempts to retain senior aviators is quantifying and reducing medical disqualifications. Much as aging aircraft require increasing maintenance to retain full operational capability, so may pilots and other aircrew members. This article reports on a prospectively collected cross-sectional research study evaluating the medical status of senior aviation personnel who reported being considered or selected for command. The study was deemed exempt from human subjects research by the Institutional Review Board and a waiver of Health Insurance Portability and Accountability Act was granted. The study, which collected data at the Pentagon Flight Medical Clinic over 1 year, used a chart review of routine medical encounters and flight physicals to collect descriptive data. The goals of the study were to establish the prevalence of medically disqualifying conditions, assess the association between disqualifying conditions and age, and generate hypotheses for further research. Logistic regression of waiver need was performed for variables including prior waivers, waiver count, service, platform, age, and gender. ANOVA of readiness percentages vs. DoD targets were assessed between the services individually and in aggregate. The study demonstrated medical readiness rates among command-eligible senior aviators ranging from 74% for the Air Force to 40% for the Army with the Navy and Marine Corps in the middle. The sample was insufficiently powered to demonstrate differences in readiness between the services; however, the population as a whole was significantly below the DoD's readiness target of >90% (P = .000). None of the services met the DoD minimum readiness target of 90%. Markedly higher readiness was observed in the Air Force, the only service with a medical screening incorporated into its command selection process, but this difference was not statistically significant. Waivers increased with age and musculoskeletal concerns were common. A larger prospective cohort study should be considered to further elucidate and confirm the findings of this study. If further research confirms these findings, consideration should be made of screening command applicants for medical readiness.

Structure health inspection for aging transport aircraft

Purpose This study aims to present a diagnosis method to inspect the structure health for aging transport aircraft based on the postflight data in severe clear-air turbulence at transonic flight. The purpose of this method development is to assist certificate holder of aircraft maintenance factory as a complementary tool for the structural maintenance program to ensure that the transport aircraft fits airworthiness standards. Design/methodology/approach In this study, the numerical approach to analyze the characteristics of flight dynamic and static aeroelasticity for two four-jet transport aircraft will be presented. One of these two four-jet transport aircraft is an aging one. Another one is used to demonstrate the order of magnitude of the static aeroelastic behaviors. The nonlinear unsteady aerodynamic models are established through flight data mining and the fuzzy-logic modeling technique based on postflight data. The first and second derivatives of flight dynamic and static aeroelastic behaviors, respectively, are then estimated by using these aerodynamic models. Findings Although the highest dynamic pressure of aging aircraft is lower, the highest absolute value of static aeroelastic effects response to the wing of aging aircraft is about 3.05 times larger than normal one; the magnitude variations of angles of attack are similar for both aircrafts; the highest absolute value of the static aeroelastic effects response to the empennage of aging aircraft is about 29.67 times larger than normal one in severe clear-air turbulence. The stabilizer of aging aircraft has irregular deviations with obvious jackscrew assembly problems, as found in this study. Research limitations/implications A lack of the measurement data of vertical wind speed sensor on board to verify the estimated values of damping term is one of the research limitations of this study. This research involved potential problem monitoring of structure health for transport aircraft in different weights, different sizes and different service years. In the future research, one can consider more structural integrity issues for other types of aircraft. Practical implications It can be realized from this study that the structure of aging transport aircraft may have potential safety threat. Therefore, when the airline managed aging transport aircraft, it ought to be conducted comprehensive and in-depth inspections to reduce such safety risks and establish a complete set of safety early warning measures to deal with the potential problem of aircraft aging. Social implications It can be realized that the structure of aging transport aircraft has potential safety threat. The airline managed aging transport aircraft; it should conduct comprehensive and in-depth inspections to reduce safety risks and establish a complete set of safety early warning measures. Originality/value This method can be used to assist airlines to monitor aging transport aircraft as a complementary tool of structural maintenance program to improve aviation safety, operation and operational efficiency.

Study on Health Monitoring and Fatigue Life Prediction of Aircraft Structures

In this paper, some progress and achievements in aircraft integrity requirements, structural health monitoring, load spectrum measurement and life assessment research were presented. Several concepts of structural health monitoring were analyzed and compared, and the basic flow chart for health monitoring and life prediction of an aircraft structure was given. The selection of control points, construction of load/strain equations and stress calculation of control points were also described. Reliable IAT (Individual Aircraft Tracking) and life monitoring methods and software for IAT were developed for a certain type of aircraft, and fatigue life prediction of an aging aircraft was conducted based on actual measurement of load spectrum. The main features such as damage calculation, life evaluation and result output were discussed, and the future research focuses relating to intelligent structural health monitoring were finally explored.

Maintenance Cost Growth in Aging Aircraft: Analysis of a New DHS Dataset

This article finds that maintenance cost per flight hour increases by 8.0% as the average age of a fleet of aircraft increases by 1 year. Cost growth appears to decrease slightly as aircraft age, with an older group of aircraft showing maintenance cost per flight hour increasing by 7.6%. This is based on analysis of a new database from U.S. Customs and Border Protection Air and Marine Operations (CBP AMO). The article employs a Fixed Effect (FE) Least-Squares Dummy Variable (LSDV) regression model based on panel data from Fiscal Year 2015 (FY2015) to FY2020. The conclusion describes possible areas of additional research. This project’s findings may be interesting to the U.S. Coast Guard (USCG) and the Department of Defense (DoD). USCG aircraft conduct many similar missions to AMO’s aircraft (both organizations are part of the Department of Homeland Security [DHS]). DoD may find the analysis interesting for three reasons. First, while many of AMO’s approximately 200 aircraft are based on commercial designs, it includes military aircraft such as the UH-60, P-3, and MQ-9. Second, many of DHS’s acquisition and maintenance processes mirror DoD’s. Third, AMO’s data are a new dataset for examining aircraft maintenance costs already analyzed using DoD databases such as the Air Force Total Ownership Cost (AFTOC) and Naval Visibility and Management of Operating and Support Costs (VAMOSC) databases.

Uncertainty analysis of MSD crack propagation based on polynomial chaos expansion

Widespread fatigue damage (WFD) is one of the main damages in aging aircraft structures, and the prediction of multi-site damage (MSD) crack propagation plays an important role in the safety assessment of aging aircraft. However, the existing multi-crack propagation stochastic models need to calculate multiple integrals, and the calculation efficiency is low. Based on the Polynomial Chaos Expansion (PCE) method, this paper proposes a new uncertainty analysis method to effectively solve the problem of multi-crack mutual interference. The rivet area of aircraft skin mostly appears randomly in the form of short cracks. Therefore, 30 groups of multi-crack fatigue propagation experiments are carried out to obtain the data of multi-cracks at different stages. On the basis of the existing multi-crack mutual interference model, a new semi-analytical multi-crack propagation model is designed by considering the uncertainty of multi-crack initiation and the interaction factors between cracks. The random variables are substituted into the improved multi-crack propagation model, and the Hermite polynomials are introduced and expanded into orthogonal polynomial series, and the time-varying polynomial coefficients, moments and probability densities are solved by stochastic regression method. The accuracy and effectiveness of the method are verified by comparing with the experimental and Monte Carlo methods.

Uncertainty Analysis of multi-site damage Crack Propagation Based on the Monte Carlo Method

Abstract Widespread fatigue damage is one of the main damages of aging aircraft structures, and the problem of multi-site damage has always been a research hotspot in engineering analysis. The traditional probabilistic model for calculating the growth rate of multiple cracks takes into account the multiple uncertain parameters that affect each other’s cracks. However, when solving the multi-site damage (MSD) crack growth problem, the traditional random model needs to solve multiple nonlinear equations. This method becomes no longer applicable. Based on the statistical point of view, this study gives the mutual influence factors of the crack growth process in the form of distribution, and then calculates the probability density function of the crack length under different cycles by sampling with the Monte Carlo (MC) method. Finally, compared with the experimental data, the results show that the calculation results of this method are more accurate and are in good agreement with the experimental data. This method is feasible and accurate for multi-crack propagation analysis of random parameter structures.

Simulation Investigation on Environment-Induced Pitting Corrosion on the Metal Surface

The prediction of corrosion damage is one of effective research methods in the safety inspection of aging aircraft structures. A mathematical model for quantifying corrosion damage is used in this paper to predict the onset of corrosion on structural surfaces exposed to aggressive environments. Based on the finite difference technique, the evolution process of local pitting corrosion on the surface of aluminum alloy in the medium is simulated, which can consider the sudden onset and the randomness of pitting corrosion. The effect of local ion concentration and oxide film damage on subsequent pitting nucleation was analyzed. Based on the efficient calculation program, the effectiveness of the mathematical model is verified by the comparison between the corrosion damage morphology and the experimental data in the literature. The results show a more widespread distribution of subsequent pits because of stronger aggressive ions are released during the life cycle of active pits and the higher diffusion coefficient of the aggressive ions. The three dimensional morphology is generated by image processing method based on the gray value of the two dimensional image of pits.

Heteroepitaxy of flexible piezoelectric Pb(Zr0.53Ti0.47)O3 sensor on inorganic mica substrate for lamb wave-based structural health monitoring

Active Lamb wave detection is an effective and simple monitoring method for structural health monitoring (SHM). Piezoelectric lead zirconate titanate (PZT) ceramics are most commonly used as transducers to excite and receive Lamb wave. However, due to their hardness, thickness and brittleness, PZT ceramics are severely limited in detecting damage on curved surface structures. Herein, we report that flexible PZT film sensor is an excellent candidate for receiving Lamb wave used in Lamb wave-based SHM of aircraft. An improved piezoelectric coefficient d33 (~130 pm V−1) is obtained in epitaxial PZT films grown on flexible inorganic mica substrates via van der Waals heteroepitaxy. Superior stable ferroelectricity and piezoelectricity retain in flexible PZT film after bending 104 cycles under a radius of 8 mm. As proven by experiment and simulation, flexible PZT film is demonstrated as an ultrasonic sensor with high sensitivity to be bonded on the curved aluminum plate for Lamb wave-based damage monitoring. Our work demonstrates that flexible PZT films have enormous potential for real-time light-weight and high-sensitivity receiver sensors for the SHM of aging aircraft.

Residual Strength Prediction of Aluminum Panels with Multiple Site Damage Using Artificial Neural Networks.

Multiple site damage (MSD) cracks are small fatigue cracks that may accumulate at the sides of highly loaded holes in aging aircraft structures. The presence of MSD cracks can drastically reduce the residual strength of fuselage panels. In this paper, artificial neural networks (ANN) modeling is used for predicting the residual strength of aluminum panels with MSD cracks. Experimental data that include 147 unique configurations of aluminum panels with MSD cracks are used. The experimental dataset includes three different aluminum alloys (2024-T3, 2524-T3, and 7075-T6), four different test panel configurations (unstiffened, stiffened, stiffened with a broken middle stiffener, and bolted lap-joints), many different panel widths and thicknesses, and the sizes of the lead and MSD cracks. The results presented in this paper demonstrate that a single ANN model can predict the residual strength for all materials and configurations with high accuracy. Specifically, the overall mean absolute error for the ANN model predictions is 3.82%. Furthermore, the ANN model residual strength predictions are compared to those obtained using the most accurate semi-analytical and computational approaches from the literature. The ANN model predictions are found to be at the same accuracy level of these approaches, and they even outperform the other approaches for many configurations.

Calculation of Stress Intensity Factor on Typical MSD Structure of Aircraft Wing

The research object of this paper is Multiple site damage (MSD) structures which are common in the service of aging aircraft. A complex function modified method is proposed to calculate the stress intensity factor of MSD cracks. On the basis of Muskhelishvili complex function method and its approximate superposition method, the concept of equivalent crack is introduced to solve the accurate stress function of MSD cracks. The stress intensity factor of MSD hole edge crack of infinite plate under far end uniform load is calculated and compared with the finite element method. The method is applied to the typical MSD structure of aircraft wing, and the stress intensity factors of cracks in three modes are compared and analyzed. This method can be used to discuss the damage tolerance of MSD structure, it has certain application value in the reliability evaluation of aircraft.

Further Studies into Crack Growth in Additively Manufactured Materials.

Understanding and characterizing crack growth is central to meeting the damage tolerance and durability requirements delineated in USAF Structures Bulletin EZ-SB-19-01 for the utilization of additive manufacturing (AM) in the sustainment of aging aircraft. In this context, the present paper discusses the effect of different AM processes, different build directions, and the variability in the crack growth rates related to AM Ti-6Al-4V, AM Inconel 625, and AM 17-4 PH stainless steel. This study reveals that crack growth in these three AM materials can be captured using the Hartman–Schijve crack growth equation and that the variability in the various da/dN versus ΔK curves can be modeled by allowing the terms ΔKthr and A to vary. It is also shown that for the AM Ti-6AL-4V processes considered, the variability in the cyclic fracture toughness appears to be greatest for specimens manufactured using selective layer melting (SLM).

Structural Life Management in a Combat Aircraft

The paper briefly describes some of the underlying concepts, issues and strategies for ensuring efficient management of structural integrity of an airframe throughout the useful life of an aircraft—especially that of a military combat aircraft. Much of the discussion in the paper is based on the experience of the authors during the design and development phase of the Indian light combat aircraft, namely, LCA Mk-I Tejas, and the information available about the LCA in the open literature or public domain. Even though not an ageing aircraft, LCA is taken as an example to facilitate bringing out various issues and strategies to resolve them, particularly with relevance to the Indian aeronautical scenario. The use of both metallic and composite materials in modern combat aircraft structures necessitates different design strategies to be adopted for their structural integrity throughout their lifetime. In this respect too, the LCA serves as a good example. Within the limited scope of the paper, the description and discussions are primarily limited to some of the important aspects, such as the fatigue degradation in metallic materials and impact damage in composites—which do form the bulk of the effort of life management of airframe structures. Other issues such as corrosion in metals and environmental degradation in composites are also briefly mentioned. A brief description of the aircraft and its structure is given initially. Major steps in designing for structural life management, such as, estimation of static loads, derivation of fatigue spectra for the intended usage, degradation through fatigue or impact damage, lifing philosophies, estimation of life and monitoring of health and usage of individual aircraft are described. A brief mention is also made about structural health monitoring (SHM) as a future direction in life management. It is expected that the exposition in the paper will help understand issues and strategies that would eventually be required as a combat aircraft gets aged in service.

The Study of Aircraft Accidents Causes by Computer Simulations

Defects in an aircraft can be caused by design flaw, manufacturer flaw or wear and tear from use. Although inspections are performed on the airplane before and after flights, accidents still result from faulty equipment and malfunctioning components. Determining the causes of an aircraft accident is an outcome of a very laborious and often very long investigation process. According to the statistics, currently the human factor has the biggest share within the causal groups. Along with the development of aviation technology came a decline in the number of accidents caused by failures or malfunctions, though such still happen, especially considering aging aircraft. Discovering causes and factors behind an aircraft accident is of crucial significance from the perspective of improving aircraft operational safety. Effective prevention is the basic measure of raising the aircraft reliability level, and the safety-related guidelines must be developed based on verified facts, reliable analysis and logical conclusions. This article presents simulation tests carried out by finite element method and constitutive laboratory tests leading to the explanation of the direct cause of a military aircraft accident. Computer-based simulation methods are particularly useful when it comes to analysing the kinematics of mechanisms and potential stress concentration points. Using computer models enables analysing an individual element failure process, identifying their sequence and locating their primary failure source.

Effect of staggered holes with multi-site damage on fatigue performance based on tests, DIC technique and numerical calculations

Abstract The fatigue performance was investigated numerically and experimentally of 2024-T3 Al-clad sheet with staggered hole arrangement and multi-sites damage (MSD), which is mainly used in aging aircraft fuselage skin. Firstly, the influences of row spacing, column spacing and inter-crack direction on fatigue life were studied. Then, the strain nephogram and crack path were analyzed with the assistance of digital image correlation (DIC) technique. Finally, a numerical solution for the specimens with staggered holes and MSD was developed based on the combination of ABAQUS and Paris formula. The results show that with the increase of row spacing, the fatigue life of MSD specimens with fixed column spacing increases first and then decreases. In the case of fixed row spacing, the fatigue life of specimens increases with the increase of column spacing. The arrangement parameter α is introduced to reveal the crack propagation form. Meanwhile, the common form of fracture is through the upper row of hole. In addition, the distribution of rivet holes has an effect on the initiation life of fatigue cracks in the structure with MSD, especially the row spacing. The numerical solution could provide a conservative prediction of fatigue lives, and the predicted error is less than 5% for cases of HH-2g24.

Corrosion issues in structural health monitoring of aircraft

Last couple of decades have seen significant progress in the paradigm of structural health monitoring (SHM) which is expected to provide enhancements in the levels of safety and mission readiness of an aircraft or a fleet of aircraft at an affordable cost. These developments comprise, broadly, (1) a diagnosis part which deals with detection of structural damage through various sensing technologies, (2) a prognosis part which attempts to forecast the performance with the current damage state under anticipated future loading environments and (3) a damage mitigation strategy for dealing with the identified damage. Much of the research and developmental effort on these three fronts has focused on mechanical damage such as cracks and similar flaws in metals and impact damage and delaminations in composites and on their growth under operational loads through processes such as fatigue. Relatively less attention has been devoted to problems of metal corrosion, which has been also a bothersome factor in ensuring structural integrity of airframe structures. This paper makes an attempt to highlight some of the issues that are being faced or may be faced while bringing in corrosion into the SHM framework for aircraft. Towards this objective, a review of the current state of the art on various aspects of monitoring of corrosion damage is presented. Beginning with the phenomenon of corrosion (especially in Al alloys) to identify parameters that may be helpful for its monitoring, the review covers important aspects of corrosion damage, sensing concepts and technologies. Five categories of techniques are described: (1) direct monitoring of corrosion damage, (2) monitoring the state of corrosion through coupons and surrogates through physical means, (3) electrochemical techniques, (4) monitoring the corrosivity of the environment as an indirect assessment of corrosion and (5) monitoring the degradation in corrosion prevention coatings as a means to infer underlying corrosion. A special subsection is devoted to use of optical fibres in corrosion sensing. Instances of actually using all such sensors in aircraft are briefly mentioned. An overall discussion of the current state of the art is provided. It is emphasised that corrosion sensing and monitoring are increasingly using developments in “smart technology”, such as smart coatings, thin-film technology, smart materials such as magneto-resistive and magneto-optical, smart polymers and microdevices fabrication. It is expected that the issues highlighted here will be of relevance not only for ageing aircraft but also for new aircraft under design or in operation.

The Application of DIC Technique to Evaluate Residual Tensile Strength of Aluminum Alloy Plates with Multi-Site Damage of Collinear and Non-Collinear Cracks

This paper introduces an aviation industrial application of digital image correlation (DIC) technique on the measurement of residual tensile strength (RTS). In order to investigate multi-site damage (MSD) that is common in the fuselage of aging aircraft, RTS of 2024-T4 aluminum alloy sheet with MSD was evaluated using DIC technique. Firstly, the four-factor and three-level orthogonal experiment was designed to optimize the DIC method to control the strain calculation error by considering subset size, interpolation tap, calibration score and step size. Secondly, RTS and strain fields were generated to analyze the path of crack propagation. The results show the optimal factor combination is 0.018 of calibration score, 23 pixels of subset size, step size is 1/4 of subset size and the filter size of interpolation calibration is 8 pixels. With the increase of spacing between adjacent holes, the RTS increases and the collinear cracked specimen becomes more perilous than that of non-collinear cracked while the hole spacing is 25 mm from the statistical analysis. Based on the Net section yield criterion, the RTS was calculated, which can give a conservative prediction of RTS.

Validation of the SmartBasing aircraft rotation and retirement strategy

SmartBasing provides fleet managers tools with which to manage their end-of-life aircraft fleets. The principles of SmartBasing include reassigning aircraft to different bases and assigning aircraft to a new mix of mission types to actively manage the remaining useful lifetime of each aircraft in a fleet. This paper employs a single case study aircraft to validate the SmartBasing approach for a dynamic strategy for aircraft retirement. The United States Air Force’s A-10 Thunderbolt II was used for validation, because it is an aging aircraft fleet that experienced a partial fleet retirement in 2013. The efficacy of the SmartBasing principles was tested using the aircraft retired in 2013 by altering usage patterns and basing locations in the years leading to the 2013 retirement. It was shown that SmartBasing would have been a valid technique for managing the A-10 fleet prior to its partial retirement. Better aircraft utilization planning could have expended more residual aircraft lifetime prior to retirement, resulting in savings of more than 1.88 full aircraft lifetimes or over 83 million USD in aircraft acquisition costs.