NASA's Generic Transport Model (GTM) is a remote-controlled, 5.5 percent scale commercial aircraft. An L 1 adaptive controller was recently designed and flight tested on the GTM. Oscillations in the elevator command were observed at 1.4 to 2 Hertz during the first flight test in September 2009. In most flight conditions the L 1 adaptive controller can be approximated by a linear time-invariant system. Thus linear analysis tools can be used to assess the performance and robustness of the feedback system with the L 1 controller. The initial L 1 design met the NASA requirement for 60 msec of time delay margin. However, the linear analysis indicates that the margin requirements were insufficient due to inaccurate models that were available at the time of the first flight test. A revised L 1 controller has significantly larger margins and demonstrated good performance during subsequent flight tests.