Fuel scheduling of gas turbine aero engines at sub-idle mode with the aim of achieving a fast and reliable starting process is a challenging area. To investigate factors that determine the metering performance of the fuel control unit at start-up regime, this research comes into being. This paper establishes mathematical models revealing the effect of these determinants, which are positive overlap and leakage in the system valves. A comprehensive test stand is then designed and manufactured in order to validate the mathematical modeling and simulation results. The fuel control unit has then been practically tested in the developed assessment stand and compared and evaluated with the simulation results. Finally, the results show that the positive overlap of the pressure compensating valve has a strong effect on the fuel scheduling in the starting area. Furthermore, the effect of downstream and upstream conditions on the metered fuel flow has been investigated. The results show that changes in upstream and downstream conditions have very little effect on the metered fuel flow and the pressure compensating valve reduces their effects on the fuel control unit. Using the dynamic model presented for the FCU of the turbojet engine, a suitable platform is going to be provided to investigate and control the simultaneous behavior of the gas turbine aero engines and the fuel control unit at the start-up regime.