Using Computational Fluid Dynamics simulations, we investigate whether the oxygen concentration in the ullage of an aircraft’s fuel tank increases during ascent, subsequently increasing the risk of ignition. The problem was studied for two sets of initial conditions: a non-inert ullage (air with mass fractions 23 wt% oxygen and 77 wt% nitrogen) and an inert ullage (air with mass fractions 14 wt% oxygen and 86 wt% nitrogen). Our results show that as the pressure of the tank decreases with altitude, oxygen and nitrogen initially dissolved in the fuel are released into the fuel tank’s ullage. However, the ratio of oxygen and nitrogen in the released gases is constant throughout the aircraft’s ascent. In turn, only a minor increase in the average oxygen concentration is observed inside the ullage. Despite this small increase in the average concentration, our results show localized oxygen spikes, “hot-spots”, next to the pressure valve of the initially inerted ullage. We attribute this to atmospheric air occasionally flowing into the fuel tank. Since atmospheric air has a higher percentage of oxygen than the air in an inert ullage, it momentarily increases the ullage’s oxygen concentration. However, we believe that this is a result of the fuel bay being modelled in direct contact with the atmosphere and will not occur on a real aircraft.