1. Introduction
Engine condition monitoring is considered a key technology for lowering maintenance, repair and overhaul expenses while improving the safety and availability of aircraft [
1]. Estimating the current health state of the aircraft engine gained from engine condition monitoring systems by analyzing in-flight measurements provides the foundation for effective maintenance planning. Besides tracking and trending long-term deterioration, engine condition monitoring applications detect, isolate and identify single faults [
2].
Current state-of-the-art engine condition monitoring systems i.e., [
3,
4,
5,
6] are based on analyzing a minimum of one steady-state snapshot per flight. The sparsity of available data negatively impacts fault detection as there are difficulties distinguishing between random scatter and an underlying fault. Depending on the fault type and severity, it can take several flights until fault detection [
5,
7,
8]. The resulting latency in fault detection increases the risk of secondary damage. Recently, with the increased adoption of non-mandatory data acquisition equipment, continuously sampled datasets are available covering whole flights. These continuously sampled datasets are also referred to as full-flight data. Full-flight data provide sufficient data points to detect engine faults based on a statistically relevant sample size within a single flight, enabling faster response times. Despite the advantages of full-flight data, analyzing the corresponding datasets heavily increases the amount of data to be processed [
9]. For timely analysis of the increased number of incoming data, novel algorithms are required.
One approach to improve analysis performance is to reduce data by focusing on representative data points within steady-state operating regimes. The utilization of a linearized state-space model for fault detection in combination with a Kalman Filter for the isolation and identification of the fault is described in [
10,
11]. An alternative approach combining a steady-state data filter with a thermodynamic engine model and a Once-Class Support Vector Machine for fault detection is proposed in [
12]. These methods work well for flights with extended cruise segments where many steady-state operating regimes can be identified. For short-haul flights without extended cruise segments, on the other hand, the total number of identified steady-state data points might be insufficient for fault detection. The results of the steady-state data filter presented in [
12] applied to two example flights are visualized in
Figure 1 to emphasize this issue. In order to perform fault detection for short-haul flights, alternative approaches are required that analyze the entire flight, including transient operating regimes.
According to [
13] information redundancy is required for fault detection and diagnosis. In current engine condition monitoring applications, this redundancy is typically established by utilizing thermodynamic engine models for computing reference values representing the nominal performance of the aircraft engine. Fault detection performs a comparison between these reference values and in-flight measurements. Significant deviations between the measurements and model predictions indicate an underlying fault. In general, fast execution times are required to analyze the large number of data points provided by full-flight data. Thermodynamic engine models are generally slow since the solution is determined iteratively. On the other hand, state-of-the-art machine learning approaches are well suited for analyzing full-flight data providing fast execution times omitting the slow iterative computation of thermodynamic engine models. Depending on the configuration of the data acquisition, full-flight data often include discrete features resembling the position of valves, e.g., for anti-icing and customer bleed extraction. Building a physically sound thermodynamic engine model without profound system information is difficult as a meaningful relationship between discrete parameter setting and mass flow extraction has to be derived. On the other hand, data-driven models can infer the effect of such discrete parameters. The sometimes limited system information, in combination with the requirement for timely data analysis, makes data-driven model building a good alternative for processing full-flight data.
Different data-driven methods such as artificial neural networks [
14,
15,
16,
17,
18,
19], Generalized Additive Models [
17,
19,
20] or Support Vector Regression [
21] have already been successfully applied to model the performance of gas turbines. However, one major drawback of data-driven approaches is their black-box characteristic making it difficult to substantiate the results. Especially the widespread utilization of artificial neural networks also covering safety-critical applications, e.g., self-driving cars [
22], or medical diagnosis [
23] lead to increased research in uncertainty quantification, improving the reliability and robustness of their results.
In general, two types of uncertainty are differentiated in model building: aleatoric uncertainty and epistemic uncertainty [
24]. Aleatoric uncertainty defines the inherently probabilistic variability of a dataset caused by measurement uncertainty. On the other hand, epistemic uncertainty defines the uncertainty caused by the insufficient coverage of the relevant value range by the available data. For example, when using artificial neural networks for approximating the input-output characteristic of a technical system, they basically define a high-dimensional curve fit. However, the output of the artificial neural network is essentially only trustworthy in operating regimes for which sufficient data have been available for training. Otherwise, the extrapolation error becomes dominant [
25,
26]. While the epistemic uncertainty can be minimized by taking additional data points of different operating regimes into account, the aleatoric uncertainty is more or less fixed. Dedicated algorithms handle the approximation of the aleatoric and epistemic uncertainty. The epistemic uncertainty can be approximated, for example utilizing Ensemble Models [
27], Out-of-Distribution Detection [
28], Dropout [
29] or Bayesian Neural Networks [
30]. The aleatoric uncertainty can be evaluated by approximating the probability density functions of individual measurements with artificial neural networks [
31]. Despite an existing concept for approximating the aleatoric uncertainty for full-flight engine data [
32], there is no method taking both the aleatoric and epistemic uncertainty into account.
In the following, artificial neural networks are chosen for approximating the performance of aircraft engines. Correctly assessing the temporal correlations in full-flight data is a prerequisite for approximating the engine performance [
33] and is more difficult to achieve with other data-driven modeling methods. Amongst artificial neural networks, there are specific architectures to process time series, such as Long-Sort Term Memory (LSTM) [
34], Gated Recurrent Units (GRU) [
35], or Dilated Convolutional Neural Networks [
36]. Apart from the proven capability of the above listed artificial neural networks to model the steady-state and transient performance of gas turbines, there is additionally existing research in uncertainty quantification for neural networks. One existing method for approximating the aleatoric uncertainty in [
32] is extended by an Out-of-Distribution Detection for additionally taking the epistemic uncertainty into account. The proposed approach is then tested utilizing full-flight data of a commercially operated regional jet. A comprehensive investigation of the detection rates underlying different fault cases is provided. With the results obtained, it can be shown that the additional uncertainty quantification leads to higher detection rates with faster response times.
4. Discussion
This paper presents a novel approach for estimating the aleatoric and epistemic uncertainty in data-driven engine fault detection. The algorithm can detect arbitrary faults requiring only datasets representing nominal engine performance. All tests conducted were based on in-flight data of a commercially operated regional jet, ensuring real changes in environmental conditions and controller settings. Compared to alternative approaches only accounting for the aleatoric uncertainty, the presented approach results in improved detection rates and faster response times. Especially if low false positive detection rates are required, methods based on only the aleatoric uncertainty lead to too low true positive detection rates unsuitable for operational application. Various fault cases could be detected within a single flight removing the latency of current state-of-the-art fault detection based on steady-state snapshots. For the tests, only minimal instrumentation was provided. Fault detection can potentially be further enhanced by providing additional sensors to improve the observability of the engine.
In the presented use case, the engine model was trained based on datasets of an individual engine to avoid the impact of production scatter and account for engine degradation. To ensure fast coverage of an engine within condition monitoring, the dataset used for training the model covers only a short period of time, limiting the diversity of training data and increasing the epistemic uncertainty.