What happened
Two pilots, operating a Piper and a Cessna, were both inbound to the same airport under visual meteorological conditions. The pilot of the Cessna reported being one mile south of a published reporting point located 13.5 miles northeast of the airport. Following instructions from the local controller, the pilot flew a southerly heading for four miles before turning to a southwesterly heading.
Shortly after, the pilot of the Piper contacted the local controller, reporting a position 10 miles north of the airport. The controller directed the Piper to enter a left downwind for runway one niner. During this sequence, the controller informed the Cessna pilot of possible traffic at a half-mile westbound at 1,000 feet. The Cessna pilot acknowledged the advisory. Approximately 41 seconds later, the pilot of the Piper reported a collision and declared an emergency.
The investigation
The local controller stated during an interview with the National Transportation Safety Board that while scanning runways, the traffic pattern, final approach course, and the remote automated radar display, two VFR targets were observed close to one another northeast of the field. The controller believed the target on a southwesterly heading was a VFR helicopter passing north of the airport. Consequently, the controller issued a traffic advisory to the southbound aircraft, believing it was the Cessna pilot when the advisory was actually directed toward the Piper pilot.