Incident Overview
On June 27, 2011, at 2058 Mountain standard time, a reported near midair collision (NMAC) occurred between Allegiant Airlines flight 169 (AAY169), a Boeing/McDonnell Douglas MD-83, and N1421K, a Cessna 182T. The aircraft passed within 600 feet vertically and 0.36 nautical miles laterally at approximately 6,300 feet over Apache Junction, Arizona. There was no reported damage to either aircraft and no reported injuries to passengers or crew.
Sequence of Events
AAY169 was operating as a scheduled 14 CFR part 121 passenger flight from Quad City International Airport, Moline, Illinois, to Phoenix-Mesa Gateway Airport (IWA), Mesa, Arizona. N1421K was operating as an unscheduled 14 CFR part 91 personal flight, landing at Falcon Field (FFZ), Mesa, Arizona.
At 2053:10, the pilot of AAY169 contacted the Phoenix Terminal Radar Approach Control (TRACON) Santan sector controller, reporting descent to 10,000 feet and requesting a visual approach to IWA. The controller cleared AAY169 for the visual approach at 2053:21.
At 2057:34, the controller transmitted to AAY169: "traffic at eleven to twelve o'clock, six miles northbound, a Skyhawk descending VFR out of 7,700." The pilot acknowledged. The controller then advised the Cessna pilot of AAY169's position; the Cessna pilot responded "Traffic in sight 21 Kilo, we are descending."
At 2058:14, the controller updated AAY169: "the Cessna now about eleven thirty, 3 miles, 7000 feet." Twelve seconds later, AAY169 reported responding to a TCAS resolution advisory and descending. The controller again pointed out AAY169's position to the Cessna pilot, who reported the MD-83 in sight. The controller instructed the Cessna pilot to "maintain separation." The Cessna continued descending, and the aircraft passed within 600 feet and 0.36 nautical miles laterally.
At 2059:38, AAY169 advised that they had received a resolution advisory (RA) to descend followed by an RA to climb. At 2100:58, AAY169 canceled IFR and asked about the Cessna's cleared altitude. The controller responded that the Cessna was descending under VFR into Falcon Field, outside Class Bravo airspace. AAY169 landed at IWA without further incident.
Air Traffic Control Actions
Traffic advisories were provided to both pilots in accordance with FAA Order 7110.65, paragraphs 2-1-21 "Traffic Advisories" and 5-1-8 "Merging Target Procedures." The first traffic call occurred at 2057:34 when the aircraft were 6 miles apart; the second at 2058:14 when they were 3 miles apart. Both pilots reported the opposing traffic in sight.
When the initial traffic call was made, the Cessna was descending through 7,700 feet and AAY169 was leaving 7,200 feet. At the second call, the Cessna was leaving 7,100 feet and AAY169 was leaving 6,700 feet. Twelve seconds later, AAY169 received a descend RA because the Cessna was above. Radar data indicated the Cessna pilot increased descent rate, quickly descending below AAY169's altitude, causing the RA to reverse to a climb command.
Background and Mitigation Efforts
The area where the NMAC occurred is a heavily traveled north-south VFR flyway that crosses the flight path of IFR traffic landing at IWA. According to information from Phoenix TRACON and review of TCAS reports, conflicts between VFR and IFR traffic in the area have been the source of multiple reported TCAS RA events. Phoenix TRACON and the Phoenix Airspace Users Workgroup (PAUWG) have recognized these conflicts and developed several mitigation efforts as required by FAA Order 7400.2G.
The first mitigation effort was a voluntary decision by area flight schools to route transiting VFR aircraft operating between Falcon Field and the south practice areas farther east of IWA and at a lower altitude. Routes were developed by plotting locations of reported TCAS RAs on sectional maps and de-conflicting routes and altitudes. The routes were moved east, increasing vertical separation between IWA arrivals and VFR traffic.
The second mitigation effort was development of a Standard Terminal Arrival Route (STAR) into IWA by Phoenix TRACON. The PAUWG determined that publishing an arrival procedure would allow VFR users to better anticipate IFR traffic. The STAR was scheduled for publication during 2012. The Phoenix Terminal Area Chart was also to be modified to depict suggested VFR routes through the area to assist pilots in minimizing conflicts with IWA arrival traffic.