The Auxiliary Power Unit (APU) is an integral part of an aircraft, providing electrical and pneumatic power to various on-board sub-systems. APU failure results in delay or cancellation of a flight, accompanied by the imposition of hefty fines from the regional authorities. Such inadvertent situations can be avoided by continuously monitoring the health of the system and reporting any incipient fault to the MRO (Maintenance Repair and Overhaul) organization. Generally, enablers for such health monitoring techniques are embedded during a product's design. However, a situation may arise where only the critical components are regularly monitored, and their status presented to the operator. In such cases, efforts can be made during service to incorporate additional health monitoring features using the already installed sensing mechanisms supplemented by maintenance data or by instrumenting the system with appropriate sensors. Due to the inherently critical nature of aircraft systems, it is necessary that instrumentation does not interfere with a system's performance and does not pose any safety concerns. One such method is to install non-intrusive vibroacoustic sensors such that the system integrity is maintained while maximizing system fault diagnostic knowledge. To start such an approach, an in-depth literature survey is necessary as this has not been previously reported in a consolidated manner. Therefore, this paper concentrates on auxiliary power units, their failure modes, maintenance strategies, fault diagnostic methodologies, and their acoustic signature. The recent trend in APU design and requirements, and the need for innovative fault diagnostics techniques and acoustic measurements for future aircraft, have also been summarized. Finally, the paper will highlight the shortcomings found during the survey, the challenges, and prospects, of utilizing sound as a source of diagnostics for aircraft auxiliary power units.

辅助动力装置（APU）是飞机的组成部分，可为各种机载子系统提供电力和气压。APU的故障会导致航班延误或取消，并伴有区域当局的巨额罚款。可以通过连续监视系统的运行状况并将任何初期故障报告给MRO（维护修理和大修）组织来避免这种无意的情况。通常，在产品设计过程中会嵌入用于此类健康监控技术的促成因素。但是，可能会出现这样的情况，其中仅定期监视关键组件，并将其状态显示给操作员。在这种情况下，在维修过程中，可以通过使用已安装的传感机制（包括维护数据或通过使用适当的传感器对系统进行检测）来补充其他健康监控功能。由于飞机系统具有内在的关键特性，因此仪器必须不干扰系统的性能并且不构成任何安全隐患。一种这样的方法是安装非侵入式振动声传感器，以在保持系统完整性的同时最大化系统故障诊断知识。要开始这种方法，有必要进行深入的文献调查，因为以前没有以统一的方式进行报告。因此，本文主要关注辅助动力装置，它们的故障模式，维护策略，故障诊断方法，及其声学特征。还总结了APU设计和要求的最新趋势，以及对未来飞机的创新故障诊断技术和声学测量的需求。最后，本文将重点介绍在调查中发现的利用声音作为飞机辅助动力装置诊断来源的缺点，挑战和前景。