The objective of the NASA Aviation Safety Program is to improve the safety of current and future aircraft operating in the National Airspace System. Research under this program has focused on vehicle design, construction, operation and maintenance. Reducing aircraft loss of control accidents is critical to increasing aviation safety as it is the largest and most fatal aircraft accident category. Loss of control accidents result in aircraft operation outside the normal flight envelope in regions where aerodynamic data is either poorly characterized or unavailable. Hence it is important to monitor, in real-time, aircraft states and environmental conditions to assess the current state of the aircraft flight envelope. This paper describes the development of algorithms for dynamic flight envelope assessment using reachable set and nonlinear region of attraction techniques and their application to the NASA Generic Transport Model (GTM). The ability to estimate a safe envelope around various operating trim points is demonstrated.