  Fixed-wing aerial vehicles provide an efficient way to navigate long
distances or cover large areas for environmental monitoring applications. By
design, they also require large open spaces due to limited maneuverability.
However, strict regulatory and safety altitude limits constrain the available
space. Especially in complex, confined, or steep terrain, ensuring the vehicle
does not enter an inevitable collision state(ICS) can be challenging. In this
work, we propose a strategy to find safe paths that do not enter an ICS while
navigating within tight altitude constraints. The method uses periodic paths to
efficiently classify ICSs. A sampling-based planner creates collision-free and
kinematically feasible paths that begin and end in safe periodic (circular)
paths. We show that, in realistic terrain, using circular periodic paths can
simplify the goal selection process by making it yaw agnostic and constraining
yaw. We demonstrate our approach by dynamically planning safe paths in
real-time while navigating steep terrain on a flight test in complex alpine
terrain.
