Aero-engine oil systems need to pump and de-aerate air-oil in a two-phase flow. The oil lubrication systems combine three important functions of the Main Oil Pump (MOP) for lubrication and scavenging: the de-aeration and de-oiling of the air-oil mixture generated in the bearing and gearbox sumps, and pumping the oil towards the tank. These are critical functions for the engine. An engine lubrication system and an integrated pump and separation of gas-liquid mixture has been developed and characterized experimentally to increase UTAS Engine and Control Systems research and development productivity, as well as engine reliability and system performance. This pump and separator system is specially designed to handle air-oil mixtures generated in aero-engine lubrication systems. To address this need, a Computational Fluid Dynamic (CFD) analysis of the pump and separation system that allows in-flight performance prediction is presented in this paper. This CFD model applies different flight conditions under different engine rotational speed that change during each flight phase, bleed air flow rate, and gearbox rotational speed leading to the variables that compromise the design of subsystems.