TDMA is largely adopted by many networks for providing to real-time applications with determinism. However, this determinism could be jeopardized under the transient environmental perturbations, causing application failures. In this paper, we investigate the impact of the transient perturbations (especially due to Electromagnetic Interferences) on the application dependability. We give a contribution on the method for evaluating the application failure probability in function of the TDMA cycle error probability within an EMI zone. This method extends the existing one on the "consecutive-k-out-of-n:F" systems to including variable probability. Numerical applications have shown its efficiency for computing the application failure probability within a given EMI area.