An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. The primary purpose of an aircraft APU is to provide power to start the main engines. Turbine engines must be accelerated to a high rotational speed to provide sufficient air compression for self-sustaining operation. This work provides a good understanding of spur gear design and analysis from basics, considering bending, pitting and scoring criteria. A detailed discussion of various correction factors and overview of material selection for gears is covered. It also focuses on optimization and reliability. A solved example is provided for better understanding of the concepts. In this paper, we dealt with the whole procedure for design and analysis of spur gears in a nutshell. We assumed fundamental parameters and arrived at whole geometry. Based on the given data, assumed fundamental parameters like diametrical pitch, pressure angle, etc. and using tooth proportions we arrived at the geometry of tooth. Calculating loads and stresses for force analysis of individual gears in gear train and for calculating the loads and the stresses. We used stress equations suggested by AGMA. We Estimated strength or life of gears and for that, we need to assume a standard material for gears and estimate the strength of gears using strength equations suggested by AGMA. Later we evaluated Scoring resistance. Finally, we optimized and arrived at final design by performing several iterations for optimum design.In this way, the different tasks were carried out and we arrived at a conclusion.