  The National Airspace System (NAS) is a complex and evolving system that
enables safe and efficient aviation. Advanced air mobility concepts and new
airspace entrants, such as unmanned aircraft, must integrate into the NAS
without degrading overall safety or efficiency. For instance, regulations,
standards, and systems are required to mitigate the risk of a midair collision
between aircraft. Monte Carlo simulations have been a foundational capability
for decades to develop, assess, and certify aircraft conflict avoidance
systems. These are often validated through human-in-the-loop experiments and
flight testing.
  For many aviation safety studies, manned aircraft behavior is represented
using dynamic Bayesian networks. The original statistical models were developed
from 2008-2013 to support safety simulations for altitudes above 500 feet Above
Ground Level (AGL). However, these models were not sufficient to assess the
safety of smaller UAS operations below 500 feet AGL. In response, newer models
with altitude floors below 500 feet AGL have been in development since 2018.
Many of the models assume that aircraft behavior is uncorrelated and not
dependent on air traffic services or nearby aircraft.
  Our research objective was to compare the various uncorrelated models of
conventional aircraft and identify how the models differ. Particularly if
models of rotorcraft were sufficiently different than models of fixed-wing
aircraft to require type specific models. The primary contribution is guidance
on which uncorrelated models to leverage when evaluating the performance of a
collision avoidance system designed for low altitude operations. We also
address which models can be surrogates for noncooperative aircraft without
transponders.
