Analysis of Near Mid-Air Collision Reports for the
Identification of Locations of High Mid-Air Collision Risk
Fabrice Kunzi
Massachusetts Institute of Technology, Cambridge, MA, 02139
In the United States alone, there has been an average of 12.4 mid-air collisions of General Aviation (GA) aircraft
per year.1 One way of reducing the risk of mid-air collisions is by providing traffic alerts to the pilot by means of
aircraft avionics. The work presented here identifies where the risk for a mid-air collision is highest and thus where
a traffic alerting system would be most beneficial. This in turn can be used to define the operational requirements on
such systems.
Aviation Safety Information Analysis And Sharing (ASIAS) reports categorized as near mid-air collisions were
visualized. Two databases in the ASIAS system were of interest: ASRS and NMACS. The databases were searched
for every event classified as a near mid-air collision (NMAC) from 2000 to 2010.2
The ASRS and NMACS databases were first evaluated based on the flight phases of the reporting and target
aircraft. Reports that included a field left as “unknown” are not shown. Figure 1 and Figure 2 show the near mid-air
collision reports for both databases with flight phases on the X and Y axes. The Z axis is the percentage of a given
interaction. The flight phases on both axes are aligned such that the diagonal represents the encounters between two
aircraft on the same flight phase. In the ASRS as well as the NMACS data, the flight phase interactions most often
observed are those of two aircraft on “Initial Approach” (24% and 14% respectively). A review of the ASRS
narratives showed that reports with flight phases categorized as “Initial Approach” were most often in the pattern.
Overall, oth figures show that the area surrounding an airport is the location where most encounters are reported
(ASRS 64% and NMACS 47%).
1
AOPA Air Safety Foundation. 1997-2008 Nall Reports. http://www.aopa.org/asf/publications/nall.html
Filing an ASRS report gives the reporter certain protections against possible charges and as such creates a reporting bias toward events where
the pilot violated a regulation. Also, because of the subjectivity of the reports, the reports “…represent what the reporter believes he/she saw or
experienced.” Thus, care needs to be given when interpreting the results of this analysis.
2
1
ASRS Database: Flight Phase
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Figure 1: Near mid-air collisions reported in the ASRS database by respective flight phase. Encounters along
the diagonal are between aircraft in the same flight phase.
NMACs Database: Flight Phase
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Figure 2: Near mid-air collisions reported in the NMACs database by respective flight phase. Encounters
along the diagonal are between aircraft in the same flight phase.
2
One interaction that is of particular interest is that between GA and Part 121 (commercial) aircraft. The flight
phases of the GA/Part 121 encounters were analyzed in more detail and are visualized for both databases in Figure 3
and Figure 4. The data indicates that the encounters are most likely when the GA aircraft is in cruise and the Part
121 aircraft is in any other flight phase, specifically climbing or descending. This is likely due to Part 121 aircraft
often transitioning through altitude layers where GA aircraft would be cruising. Additionally, the altitude
distribution of the NMACS reports shows a distinct second peak around 10,000ft MSL as seen in Figure 4. Upon
reviewing the narratives, the low level peak is mostly from VFR traffic while the mid-altitude peak is from cruising
IFR traffic as well as sailplanes. Additionally, the second peak may be a result of increased aircraft velocities due to
the airspeed restriction of 250kts below 10,000ft.
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In summary, the airport environment is the location where most near mid-air collisions occurred (ASRS 64% and
NMACS 47%). As a result, a system that is to provide traffic alerting would have to be operational in the airport
environment. One major challenge in designing such systems is that the airport environment is a high-density
environment with aircraft performing frequent and abrupt maneuvers. Unfortunately, most currently available
systems such as TAS or TCAS are of limited usefulness in the airport vicinity because of their high false alarm rate
3
in high-density environments. With the introduction of ADS-B as the primary aircraft surveillance source, however,
it may be possible to improve alerting systems to operate in the airport environment.
4