Current Flight Management Systems (FMS) can autonomously fly an aircraft from takeoff through landing but may not provide robust operation to anomalous events. We present an adaptive trajectory planner capable of dynamically adjusting its world model and re-computing feasible flight trajectories in response to changes in aircraft performance characteristics. To demonstrate our approach, we consider the class of situations in which an emergency landing at a nearby airport is desired (or required) for safety considerations. Our system incorporates a constraint-based search engine to select and prioritize emergency landing sites, then it synthesizes a waypointbased trajectory to the best airport based on post-anomaly flight dynamics. We present an engine failure/fuel starvation case study and illustrate the utility of our approach during a simulated thrusting power failure for a B-747 over the Bay Area.