Experimental Investigation and Simulation of a Boeing 747 Auxiliary Power Unit

Auxiliary power units (APUs) are a major driver of maintenance on civil aircraft. However, experimental data and performance simulations are rarely seen in public domain literature. While there is recourse to aircraft engine experience, this does not address the loading and the failure modes of an APU. This work aims to add to the literature, by experimentally investigating a Boeing 747 APU, collecting data under various power settings and ambient conditions, and using these data to calibrate a simple simulation model. This simulation model will subsequently be used to explore failure modes in the APU and hence what sensors may be needed for health monitoring purposes in future work. In this paper, a Boeing 747 APU rig development process and the testing strategy are presented. The rig is validated through a process that includes uncertainty analysis, repeatability tests, consistency tests, and comparison of the collected data with the calibrated simulation model. The results from the rig's validation indicate that the data collected from the APU is independent of its running time or the order of loading cycles imposed on it, i.e., the results are path independent. Changes in pneumatic and electrical power result in small changes in the rotational speed despite the fact that the rotational speed should remain constant. The rotational speed shows a slightly increasing trend when the extracted power rises, and this affects the APU thermodynamic characteristics. This work has resulted in a calibrated simulation model that will be further used in examining fault mode scenarios, as injecting these directly into the rig is seen as high risk.