Date: 201808
Local time of day: 1201-1800
Location: A90.TRACON, NH
Flight conditions: Mixed
Weather elements / visibility: 6
Aircraft: Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B BOS
ATC / advisory: TRACON A90
Second aircraft: Large Transport
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Developmental
Human factors: Communication Breakdown; Workload
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Airspace Structure; Human Factors
Primary problem: Human Factors

Narrative: Weather: 8 SM; ceilings bkn around 3;000. [I] was briefed aircraft were not seeing the runway at 8sm; but they could at 6 SM. While assuming the position I had to immediately have a small report visual behind a large; as the spacing did not look to be adequate to ensure wake turbulence. At the same time Aircraft Z had been cleared onto the localizer; with traffic behind. The traffic behind was through the localizer for the spacing; and I had two large aircraft on a right downwind; with Aircraft Y from the south aimed north. I decided to take Aircraft Y across the localizer for the right downwind; as he was going to fly out of my airspace if he joined the left downwind. I got the Aircraft Z cleared; as well as the large following behind. I was concerned about the spacing with the large behind the Aircraft Z; when I told Aircraft Y to fly heading 330; vector across the localizer. I also stated maintain 3;000 feet; while in my head I was thinking maintain 5;000. I had Aircraft X south at 4;000 and Aircraft A on a right downwind at 4;000. My plan was to fly close behind Aircraft X and then turn Aircraft Y onto the right downwind to follow the Aircraft A. Aircraft A read back heading 330 and maintain 3;000. I missed his read back; as I diverted my attention back to the Aircraft A and the spacing behind him with the large. I observed Aircraft Y at 4;800; and I immediately issued the altimeter and issued 'maintain 5;000 feet' Aircraft Y stated I had issued him a descent and he would climb and maintain 5;000. I observed him at 4;700; and called the Aircraft X flight at his ten o'clock and 4;000. Aircraft Y stated he had traffic in sight; and I instructed Aircraft Y to maintain visual. Aircraft Y said he would maintain visual and was level at 5;000. I then re-sequenced the aircraft and he proceeded inbound to BOS. As for recommendations; I made a mistake with issuing a bad altitude as well as missing the read back. In my head I was thinking 5;000 feet; but I think with issuing the heading of three-three-zero' the '3' for three-thousand just came out of my mouth. I know I need to slow down and REALLY listen to the read back; and yet my brain did not do that. I will take this experience and adjust how I handle the situation; should it occur again.




Synopsis: A90 Controller reported issuing an incorrect altitude which resulted in a NMAC situation.