A tower local controller did not notice that an aircraft that was supposed to follow another aircraf...

Attributes
ACN	1424314
Time
Date	201702
Local Time Of Day	0001-0600
Place
Locale Reference	HIO.Tower
State Reference	OR
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	PA-44 Seminole/Turbo Seminole
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Other VFR Pattern
Flight Plan	VFR
Aircraft 2
Make Model Name	PA-44 Seminole/Turbo Seminole
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Other VFR Pattern
Flight Plan	VFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Experience	Air Traffic Control Time Certified In Pos 1 (yrs) 0.5
Events
Anomaly	Conflict Airborne Conflict Deviation - Procedural Clearance Deviation - Track / Heading All Types

Narrative:

I was working local 1. Aircraft X went around for runway 31L. I instructed him to follow the second [aircraft] on downwind (aircraft Y) and make left closed traffic. Several minutes later I had to extend aircraft Y in the downwind. I told aircraft X to continue following the aircraft Y and extend downwind. Aircraft X read back 'extend downwind.' several minutes later I missed aircraft X state on frequency 'turning base.' he turned base on his own and managed to cut off the aircraft Y he had been following. Aircraft Y had to make an evasive left turn to avoid aircraft X; who cut him off on final. I was able to resequencing everyone; and they landed without incident.I was busy and I missed aircraft X turning base on his own. In the future I will continue to exercise vigilance to ensure that the aircraft are following my instructions.

Original NASA ASRS Text

Title: A Tower Local Controller did not notice that an aircraft that was supposed to follow another aircraft turned in front of the aircraft it was supposed to follow.

Narrative: I was working Local 1. Aircraft X went around for runway 31L. I instructed him to follow the second [Aircraft] on downwind (Aircraft Y) and make left closed traffic. Several minutes later I had to extend Aircraft Y in the downwind. I told Aircraft X to continue following the Aircraft Y and extend downwind. Aircraft X read back 'extend downwind.' Several minutes later I missed Aircraft X state on frequency 'turning base.' He turned base on his own and managed to cut off the Aircraft Y he had been following. Aircraft Y had to make an evasive left turn to avoid Aircraft X; who cut him off on final. I was able to resequencing everyone; and they landed without incident.I was busy and I missed Aircraft X turning base on his own. In the future I will continue to exercise vigilance to ensure that the aircraft are following my instructions.

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.

A tower local controller did not notice that an aircraft that was supposed to follow another aircraft turned in front of the aircraft it was supposed to follow.