What happened
On August 9, 2003, at approximately 12:45 JST, a Japan Air Commuter NAMC YS-11A, registration JA8781, and a privately owned Cessna A185F, registration N185GW, passed each other in close proximity over the sea approximately 4 nautical miles south of Okinoerabu Island.
The NAMC YS-11A was operating a scheduled passenger flight from Kagoshima to Yoron under Instrument Flight Rules (IFR). At the time of the incident, the aircraft was descending through 4,30 and was carrying 70 people (65 passengers and 5 crew). The Cessna A185F was in level flight at 4,500 feet, operating under Visual Flight Rules (VFR) on a route from Naha to Fukuoka with 2 persons on board.
As the aircraft approached each other on nearly reciprocal tracks, the Traffic and Collision Alert Device (TCAD) on the NAMC YS-11A issued an aural warning. The crew of the NAMC YS-11A eventually sighted the Cessna A185F at a 11 o'clock position. At the point of closest approach, the horizontal separation was approximately 200 meters and the vertical separation was approximately 200 feet. Neither crew took any avoidance maneuvers.
The investigation
Investigators examined digital flight data recorder (DFDR) recordings from the NAMC YS-11A, air traffic control (ATC) radar data, and radio communications. The investigation also included interviews with the flight crews and an analysis of the aircraft's equipment.
Key elements of the investigation included:
- The performance of the TCAD on the NAMC YS-11A, which detected the intruder approximately 20 seconds before the closest approach.
- The radar coverage at Naha Control, which displayed the NAMC YS-11A but failed to show the Cessna A185F.
- The altitude regulations for VFR flights in the region, specifically regarding appropriate cruising altitudes for northeast-bound traffic.
- The possibility that an auxiliary fuel tank on the Cessna A185F may have obstructed the transponder signal from the surveillance radar.
Findings
- The investigation determined that the aircraft were not on a collision course because the NAMC YS-11A was descending and the aircraft were on nearly parallel tracks with a horizontal offset.
- Naha Control was unable to provide traffic information because the Cessna A185F was not visible on their radar screen, potentially due to signal blockage by the aircraft's auxiliary fuel tank.
- The Cessna A185F was flying at an altitude appropriate for westbound VFR flights rather than the altitude required for eastbound flights.
- The crew of the NAMC YS-11A did not take avoidance action because they perceived the other aircraft was not a hazard due to the descending flight path and vertical separation.