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.

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Original NASA ASRS Text

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

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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.