Verification and Validation (V&V) of gain-scheduled flight control systems relies on analysis of gain and phase margins across the flight envelope. Because similar tools are not available for nonlinear systems, V&V of such systems requires numerousMonte Carlo simulations, the cost of which grows with the increasing complexity of the system. The recently developed ℒ1 adaptive control methodology addresses this issue by providing a systematic framework for the design of nonlinear adaptive control laws. It extends the classical notions of gain and phase margins to a nonlinear adaptive control scheme, and enables the design of a nonlinear closedloop control system with guaranteed, analytically provable, bounded away from zero, time-delay margins. In this paper, we apply the ℒ1 adaptive control methodology to an X-48B aircraft model at different flight conditions and explicitly verify performance bounds using the ℒ1 framework. We demonstrate that the resulting ℒ1 adaptive controller does not require tuning or redesign from one flight condition to another. Furthermore, once designed according to the theoretical guidelines, the ℒ1 adaptive controller ensures uniform transient and steady-state performance across the entire flight envelope in the presence of significant cross-coupling and control surface failures.