NTSB Mid-Air Collision Report Analysis 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 resulting in an average of 19.1 fatalities per year from 1991-2008.1 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 operational requirements on such systems.
The narratives of NTSB mid-air collision accident reports from January 2000 until June 2010 were analyzed (total of 112). All accidents were separated into three categories based on their proximity to the airport (Figure ). As can be seen, the area surrounding an airport is where mid-air collisions most often occurred (59%). As a single category, the airport pattern was the location with the most accidents (45%).
Figure : Percentage of Mid-Air Collisions by Location
The geometry for each collision was analyzed. The description of aircraft heading differed between reports; thus, to allow for the comparison of the horizontal encounter geometries, all accidents were grouped into bins of 45° based on flight track intersection angle. The 5 groups were centered on the 5 cardinal directions of one half of a compass rose (see Figure ). The own-ship aircraft is in the center and the intruder aircraft is on the perimeter.
Figure : Track intersect angle summarized for all NTSB mid-air collision reports
To gain a better understanding of the characteristics of encounters based on their location, each of the three environments identified in Figure was analyzed individually. The following paragraphs summarize the accidents for each location.
Out of the 112 reported cases, 50 occurred in the airport pattern. Over 80% of those mid-air collisions occurred on final, short final or on the runway. The narratives of these reports paint a common picture for most of these accidents: two aircraft slowly settling into each other as they approach the same runway.
Figure : Location Distribution and Geometry of Mid-Air Collisions in the Airport Pattern
Figure visualized the 16 accidents that occurred in the airport vicinity. 9 of those were between aircraft that had identical flight phases, i. e. both aircraft were departing or arriving at the airport. Just as with the pattern encounters, the aircraft settled into each other, although at faster relative velocities.
 Figure : Geometry of Mid-Air Collisions in the Airport Vicinity
A total of 46 accidents occurred away from the airport. The accidents included aircraft that were in cruise as well as aircraft engaging in flight training, surveying, firefighting, EMS transport, aerial application or news reporting (all referred to as “Maneuvering” in Figure ).
 Figure : Frequency of Flight Phase for Mid-Air Collisions away from the Airport
Of the 46 accidents, 13 (28%) involved at least one aircraft conducting maneuvers such as surveying, firefighting or flight instruction. The intersect angle most often observed is that of two aircraft with perpendicular tracks (29%). This may be due to blind spots resulting from wings and/or window frames out the side of the aircraft.
Figure : Track intersection angle for mid-air collisions away from the airport with and without formation flights
In summary, the airport environment is the location where most mid-air collisions occurred (59%). 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 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.
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