  Urban air mobility (UAM) has attracted the attention of aircraft
manufacturers, air navigation service providers and governments in recent
years. Preventing the conflict among urban aircraft is crucial to UAM traffic
safety, which is a key in enabling large scale UAM operation. Pre-flight
conflict-free path planning can provide a strategic layer in the maintenance of
safety performance, thus becomes an important element in UAM. This paper aims
at tackling conflict-free path planning problem for UAM operation with a
consideration of four-dimensional airspace management. In the first place, we
introduced and extended a four-dimensional airspace management concept,
AirMatrix. On the basis of AirMatrix, we formulated the shortest flight time
path planning problem considering resolution of conflicts with both static and
dynamic obstacles. A Conflict-Free A-Star algorithm was developed for planning
four-dimensional paths based on first-come-first-served scheme. The algorithm
contains a novel design of heuristic function as well as a conflict detection
and resolution strategy. Numerical experiment was carried out in Jurong East
area in Singapore, and the results show that the algorithm can generate paths
resolving a significant number of potential conflicts in airspace utilization,
with acceptable computational time and flight delay. The contributions of this
study provide references for stakeholders to support the development of UAM.
