Date: 201202
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: Sabreliner 65
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Flight phase: Final Approach; Initial Approach
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Second aircraft: Sundowner 23
Reporter organization: Government
Reporter function: Approach; Departure
Reporter qualification: Air Traffic Control Developmental
Human factors: Communication Breakdown; Confusion; Situational Awareness
Anomaly: ATC Issue All Types; Conflict Airborne Conflict
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued New Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: A Sabreliner 65 checked in northwest of the airport at 110 without an approach request. I attempted to clear the Sabreliner 65 direct to the airport; and descend to 40 for a Visual Approach. I believe I transmitted three times without response. This would not have been an issue had I not lost communication capabilities previously that day. I attempted contact with the only other aircraft on frequency; a BE23. He also did not respond. I believe I attempted twice. I went back and forth between the two while switching handsets and jacks to trouble shoot the problem; assuming it was a frequency issue. At some point it was established that both aircraft could communicate with me clearly. By that time; the Sabreliner 65 was north of the airport and requesting vectors for an ILS approach to Runway 32R. I descended the Sabreliner 65 to 40; and chose to put him on a right downwind rather than a left like I would have done had I had his request earlier. I issued the Sabreliner 65 a heading of 120. He read back 110 which was fine with me; so I left it. The BE23 was also on a right downwind for an ILS approach to Runway 32R; heading 140. My plan was to keep the Sabreliner 65 at 40 and base the BE23 to follow when there was appropriate spacing. Both aircraft had additional approach requests following the first. I told the Sabreliner 65'on departure; turn right heading 100; climb and maintain 30.' He read back 'heading 100; maintain 30.' I did not notice the fact that he read it back as if it were current; rather than a future instruction. The Sabreliner 65 continued his descent. Since I had not applied merging target procedures; the Sabreliner 65 did not know that the BE23 was in front of him on the downwind; nor vice versa. He keyed up twice (to turn; and stop the descent of the Sabreliner 65) without transmitting because my CIC noticed the situation and wanted me to climb the Citation. I told him to turn right heading 220 and maintain 35; his present altitude. I transmitted again to turn immediately and climb to 40. I know I had less than 3 miles; but I do not know how much separation was lost. Use opposite downwinds. Apply merging target procedures. Wait until the aircraft is in a position (i.e.; on final) that leaves less doubt to whether instructions are to be followed immediately or in the future. Listen more carefully.




Synopsis: Approach Controller in training described a loss of separation event when failing to insure a clearance was understood and read back correctly.