  Aircraft collision avoidance systems have long been a key factor in keeping
our airspace safe. Over the past decade, the FAA has supported the development
of a new family of collision avoidance systems called the Airborne Collision
Avoidance System X (ACAS X), which model the collision avoidance problem as a
Markov decision process (MDP). Variants of ACAS X have been created for both
manned (ACAS Xa) and unmanned aircraft (ACAS Xu and ACAS sXu). The variants
primarily differ in the types of collision avoidance maneuvers they issue. For
example, ACAS Xa issues vertical collision avoidance advisories, while ACAS Xu
and ACAS sXu allow for horizontal advisories due to reduced aircraft
performance capabilities. Currently, a new variant of ACAS X, called ACAS Xr,
is being developed to provide collision avoidance capability to rotorcraft and
Advanced Air Mobility (AAM) vehicles. Due to the desire to minimize deviation
from the prescribed flight path of these aircraft, speed adjustments have been
proposed as a potential collision avoidance maneuver for aircraft using ACAS
Xr. In this work, we investigate the effect of speed change advisories on the
safety and operational efficiency of collision avoidance systems. We develop an
MDP-based collision avoidance logic that issues speed advisories and compare
its performance to that of horizontal and vertical logics through Monte Carlo
simulation on existing airspace encounter models. Our results show that while
speed advisories are able to reduce collision risk, they are neither as safe
nor as efficient as their horizontal and vertical counterparts.
