Date: 201510
Local time of day: 1801-2400
Location: BJC.Airport, CO
Flight conditions: VMC
Light: Dusk
Aircraft: Light Transport; High Wing; 2 Turboprop Eng
Operator: Personal
Flight plan: VFR
Flight phase: Final Approach
Airspace: Class D BJC
ATC / advisory: Tower BJC
Second aircraft: Small Aircraft; High Wing; 1 Eng; Fixed Gear
Reporter organization: Government
Reporter function: Local
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Communication Breakdown; Workload; Situational Awareness; Physiological - Other; Confusion
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Airspace Structure; Environment - Non Weather Related; Human Factors
Primary problem: Human Factors

Narrative: I was working Local Control and recall working a trickier session than normal (perhaps it had been awhile since I had worked a busier session on this configuration). I had issued Aircraft Y a clearance to land on 12L and he was entering the left base. Aircraft X was cleared touch and go on 12R and was on a right base entry. In my scan; I suddenly noticed Aircraft X making a steep turn out the window; as if they were taking evasive action. I realized I had not exchanged traffic between the two aircraft; and simultaneously Aircraft Y was asking about the intentions of Aircraft X off his right; noting it appeared he was headed for him. I confirmed that the other aircraft was inbound on the parallel runway; and Aircraft X advised they overshot the final and were making a turn back to it. Each aircraft continued without expressing further concern.I can recall there was some complexity with local pattern traffic leading up to the event; where I was having difficulty tracking a helicopter in the local pattern that was flying patterns that were higher than normal. Aircraft Y also had an issued with their transponder and this created additional workload while they were inbound. I found it challenging at times to scan some of my traffic; as I did realize upon assuming the position that some of the shades in the cab were not in a 'normal' configuration and thus some areas were darker/lighter than normal. It was obvious when traffic was lighter later and I could make adjustments - but during a busier; complex period - I could not initially understand the lighting conditions affected by the shades were making it harder to track traffic in my pattern. The temperature also seemed warmer than usual and was distracting until it was adjusted. These environmental factors are not an excuse for overlooking traffic in this scenario (and it is my responsibility to make adjustments to either correct those factors or work through them) - but they may have played a role by making it challenging to work through a higher complexity traffic situation by putting me behind issuing a traffic advisory that I normally would have covered. Exchanging traffic sooner can help the pilots perform their duties in a VFR pattern and avoid these situations. While I usually try to be proactive and issue traffic as timely as I can; I overlooked it in this instance. I believe traffic complexity and frequency congestion - in conjunction with some environmental items mentioned above - were factors that made it more challenging to keep up and contributed to this oversight. I am reminded it is important to make environmental adjustments during periods of slower activity or prior to assuming a control position (equipment settings; temperature; shades; etc.) and have other tower team members assist with these when appropriate; in order to help prevent environmental factors adding to complexity when traffic builds.




Synopsis: A small aircraft on a left base to final for the left parallel runway with a light twin engine on a right base to final for the parallel runway. The light twin flew across their final towards the left runway arrival but corrected. ATC had not issued a traffic advisory to either aircraft. The small aircraft saw the light twin and asked the Tower what that aircrafts intentions were. ATC should have issued traffic to both aircraft.