Narrative:

This was a near midair collision between aircraft X and aircraft Y at the denver centennial (kapa).aircraft X was making right closed traffic for runway 17R; doing touch and goes. The pilot in command was a student pilot on their first solo flight. Aircraft Y was a ce-182 departing off of runway 17L. I was the pilot in command of aircraft Z; also working in the pattern doing full stop landings. Apa tower cleared aircraft Y for takeoff on runway 17L; and instructed the aircraft to fly straight out. Apa tower instructed aircraft Z to 'line up and wait' on runway 17RAPA tower then cleared aircraft Z for takeoff on runway 17R instructing us to 'follow the cessna'.apa tower pointed out traffic to aircraft X; and the pilot said he had the aircraft in sight.apa tower then instructed aircraft Y to turn right and fly westbound to I25; then northwest on course.aircraft Y incorrectly reads back 'turn left'; fly to the interstate; then on courseapa tower corrects aircraft Y and says 'gonna be a right turn; right turn; west of I25 right turn'aircraft Y doesn't answer initially and eventually asks if the last transmission was for them. Apa tower says; 'affirmitive; in order to go to the west you need to make a right turn'aircraft Y then reads back; 'roger; right turn...inside of the interstate'; which is incorrect according to the clearance. Tower missed the end piece of the readback; but I will say that I was unable to make out what was said until reviewing the recording this evening.aircraft Y asks tower to verify that I25 is the highway running north to south; obviously still confused.apa tower acknowledges the aircraft; and says 'affirmative; I need you on the west side.'at this point; aircraft Y; was quickly approaching aircraft X who was now on downwind and still climbing to pattern altitude. Aircraft Y was on a collision course for aircraft X. The radios were silent indicating that tower was unaware of the conflict. I saw the quickly closing conflict; and verifying that the two aircraft were getting dangerously close using both visually references; and ads-B tis display; I keyed my ptt switch and broadcasted 'stop your climb; stop your climb aircraft X'. The pilot acknowledged that he was stopping the climb; and had the traffic in sight. The conflict was averted; but only by a small margin. After talking to the pilot of aircraft X after the flight; we estimate that the two aircraft were within only 100 feet or less of one another. This near accident is the result of failure of the pilot of aircraft Y to adequately follow ATC clearance; and failure of the tower controller to verify the incorrect read back of aircraft Y. Additionally; the tower controller failed to realize that aircraft Y was on a dangerous collision course with aircraft X.a radar monitor position; with a no transgression zone; and a specific controller in charge of issuing breakout instructions during operations on the close parallel runways at denver's centennial airport; could have helped prevent this near accident from ever happening. Additionally; use of better control instructions such as 'fly heading 270'; rather than 'fly west to the interstate' could help unfamiliar pilot better understand the air traffic procedures at centennial.

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Original NASA ASRS Text

Title: A General Aviation Pilot reported seeing two aircraft on a collision course so he alerted them using his radio and averted a collision.

Narrative: This was a near midair collision between Aircraft X and Aircraft Y at the Denver Centennial (KAPA).Aircraft X was making right closed traffic for runway 17R; doing touch and goes. The pilot in command was a student pilot on their first solo flight. Aircraft Y was a CE-182 departing off of runway 17L. I was the pilot in command of Aircraft Z; also working in the pattern doing full stop landings. APA Tower cleared Aircraft Y for takeoff on runway 17L; and instructed the aircraft to fly straight out. APA Tower instructed Aircraft Z to 'line up and wait' on runway 17RAPA Tower then cleared Aircraft Z for takeoff on runway 17R instructing us to 'follow the Cessna'.APA Tower pointed out traffic to Aircraft X; and the pilot said he had the aircraft in sight.APA Tower then instructed Aircraft Y to turn right and fly westbound to I25; then northwest on course.Aircraft Y incorrectly reads back 'turn left'; fly to the interstate; then on courseAPA Tower corrects Aircraft Y and says 'gonna be a right turn; right turn; west of I25 Right turn'Aircraft Y doesn't answer initially and eventually asks if the last transmission was for them. APA Tower says; 'Affirmitive; in order to go to the west you need to make a right turn'Aircraft Y then reads back; 'Roger; right turn...inside of the interstate'; which is incorrect according to the clearance. Tower missed the end piece of the readback; but I will say that I was unable to make out what was said until reviewing the recording this evening.Aircraft Y asks tower to verify that I25 is the highway running north to south; obviously still confused.APA Tower acknowledges the aircraft; and says 'affirmative; I need you on the west side.'At this point; Aircraft Y; was quickly approaching Aircraft X who was now on downwind and still climbing to pattern altitude. Aircraft Y was on a collision course for Aircraft X. The radios were silent indicating that tower was unaware of the conflict. I saw the quickly closing conflict; and verifying that the two aircraft were getting dangerously close using both visually references; and ADS-B TIS display; I keyed my PTT switch and broadcasted 'Stop your climb; stop your climb Aircraft X'. The pilot acknowledged that he was stopping the climb; and had the traffic in sight. The conflict was averted; but only by a small margin. After talking to the pilot of Aircraft X after the flight; we estimate that the two aircraft were within only 100 feet or less of one another. This near accident is the result of failure of the pilot of Aircraft Y to adequately follow ATC Clearance; and failure of the tower controller to verify the incorrect read back of Aircraft Y. Additionally; the tower controller failed to realize that Aircraft Y was on a dangerous collision course with Aircraft X.A radar monitor position; with a no transgression zone; and a specific controller in charge of issuing breakout instructions during operations on the close parallel runways at Denver's Centennial airport; could have helped prevent this near accident from ever happening. Additionally; use of better control instructions such as 'fly heading 270'; rather than 'fly west to the interstate' could help unfamiliar pilot better understand the air traffic procedures at Centennial.

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.