Date: 201702
Local time of day: 0601-1200
Location: APA.Airport, CO
Flight conditions: Marginal
Weather elements / visibility: Snow; Rain; 6
Light: Daylight
Aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Operating under: Part 91
Flight plan: VFR
Mission: Training
Flight phase: Initial Climb
Airspace: Class D APA
ATC / advisory: Tower APA
Second aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Reporter organization: Personal
Reporter function: Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument
Human factors: Communication Breakdown
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft TA
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Human Factors; Airspace Structure
Primary problem: Human Factors

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.




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