Compressor surge imposes a limit on aero-engine operability and can compromise integrity because of significant aerodynamic loads imparted on the engine components. The aim of this article is to use 3D unsteady computational fluid dynamics (CFD) to predict the surge loadings on a compression system of a modern three-spool engine. The compression system is matched at a high power condition and computations are performed using a whole-assembly approach. In this study, the effect of two types of surge initiation on the maximum loading recorded during surge are studied: throttling of the high pressure compressor (HPC) and turning of the intermediate pressure compressor (IPC) variable stator vanes (VSV). An explanation of the main physical phenomena that contribute to those loadings is offered. It was found that in an aero-engine surge event, the maximum overpressure is driven by a combined effect of the surge shock wave passing and high pressure gas blown toward the front of the engine during depressurization. The amplitude of maximum overpressure is dictated by the compression system exit pressure at the moment of surge inception. The surge initiation via HPC throttling produces larger overpressure and therefore should be considered for design intent.