Tower Controller and Developmental reported an aircraft in the traffic pattern turned in front of another aircraft; causing an unsafe situation. Controller issued a traffic alert and the second aircraft performed a go-around.
Synopsis
Tower Controller and Developmental reported an aircraft in the traffic pattern turned in front of another aircraft; causing an unsafe situation. Controller issued a traffic alert and the second aircraft performed a go-around.
Narrative
I was training on Local Control (LC) moderate traffic session. I was sequencing mixed VFR traffic to RWY XX/YY. Aircraft X was inbound from the east told to make left traffic for RWY XX. Aircraft Y was in right closed pattern for one lap. Once Aircraft X reported mid field left downwind I instructed him he was number 2 to follow Aircraft Y wings up right base. Aircraft X read back number two looking. Aircraft Y was cleared number one and was intercepting approximately a 1 1/2 mile final straight in at this time. I told Aircraft X number two follow Aircraft Y traffic that just joined 1 1/2 mile final. Aircraft X read back left base cleared to land. I did not get a good read back from Aircraft X and the aircraft never reported their traffic or interval in sight. After doing a RWY scan and clearing the RWY I turned back and that's when Aircraft X was in the base low and from my perspective it looked as if Aircraft X turned close behind Aircraft Y not in front. Instructor keyed up and instructed Aircraft X to re join the down wind to which he did not respond and Aircraft Y was now low on a straight in. I keyed up asking Aircraft X if he had Aircraft Y traffic ahead in sight he responded no instructor keyed up again and told Aircraft Y to make a right turn. Aircraft Y keyed up asking whos the aircraft in front of them. At this point I realized Aircraft X had cut off Aircraft Y I issued a traffic alert to Aircraft Y she instructed she was going around and she had been cut off. The aircraft were in unsafe positions and Aircraft Y had sight of the conflict and was well above Aircraft X est 400-500ft. I should have corrected Aircraft X on the bad read back and not have cleared Aircraft X until he had Aircraft Y in sight.I should have sent Aircraft Y around. Recommendation - Better awareness on read backs. Safety alerts with a resolution action. Better radar or allowing the use of fused to get more accurate and faster information.
Second reporter narrative
I was the OJTI in the situation on Local Control. Aircraft X was in the left downwind; midfield to Runway XX; sequenced to follow a Cirrus [Aircraft Y] on right base; to which he replied 'number two; looking for the Cirrus'. Once the Cirrus at had turned final; the trainee resequenced Aircraft X number two; follow the Cirrus now on a mile and a half final; Runway XX cleared to land. Aircraft X then began his base. I advised Aircraft X to make a right turn and rejoin the downwind; to which he didn't respond. I then told Aircraft Y at to also make a right turn to which she responded she was going around. Aircraft Y appeared to be behind and higher than Aircraft Y at all times. Recommendation - Pilot to remain in contact with tower and not turn base until they have traffic to follow insight; or advise tower they do not; and for tower to call their base turn.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.