Performability modeling and evaluation methods are applied to the SIFT computer in the computational environment of an air transport mission. User-visible performance of the "total system" (SIFT plus its environment) is modeled as a random variable taking values in a set of "accomplishment levels." These levels are defined in terms of four attributes of total system behavior: safety, no change in mission profile, no operational penalties, and no economic penalties. The "base model" of the total system is a stochastic process whose states describe the internal structure of SIFT as well as relevant conditions of its environment. Base model state trajectories are related to accomplishment levels via a "capability function" which is formulated in terms of a three-level model hierarchy. Solution methods are then applied to determine the performability of the total system for various choices of computer and environment parameter values.