Adaptive flight control systems have the potential to be more resilient to extreme changes in airplane behavior. Extreme changes could be a result of a system failure or of damage to the airplane. A direct adaptive neural-network-based flight control system was developed for the National Aeronautics and Space Administration NF-15B Intelligent Flight Control System airplane and subjected to an inflight simulation of a failed (frozen) (unmovable) stabilator. Formation flight handling qualities evaluations were performed with and without neural network adaptation. The results of these flight tests are presented. Comparison with simulation predictions and analysis of the performance of the adaptation system are discussed. The performance of the adaptation system is assessed in terms of its ability to decouple the roll and pitch response and reestablish good onboard model tracking. Flight evaluation with the simulated stabilator failure and adaptation engaged showed that there was generally improvement in the pitch response; however, a tendency for roll pilot-induced oscillation was experienced. A detailed discussion of the cause of the mixed results is presented. Bosworth, John T. and Williams-Hayes, Peggy S. Armstrong Flight Research Center NASA/TM-2007-214629, H-2751, AIAA-2007-2818 ADAPTIVE CONTROL; FLIGHT CONTROL; NEURAL NETS; FORMATION FLYING; AIRCRAFT STABILITY; STABILITY TESTS; ADAPTATION; ROLL; SYSTEM FAILURES; FLIGHT SIMULATION; STABILIZERS; FLIGHT TEST VEHICLES