Tower controller described a near midair collision between a departing aircraft and another near by ...

Narrative:

A baron [was] inbound for runway 12R on a right downwind. A diamond departed the crossing runway 23 for a right turn out and a northbound flight along the river. When I anticipated the baron would be past the departure end of runway 23; I cleared the diamond for takeoff. The diamond turned right early instead of continuing runway heading for a half mile this put him directly in the baron's flight path instead of behind him. I did not notice the diamond's early turn because I was dealing with opposite direction traffic over the river both north and southbound there were 3 northbound and 2 southbound on visual sight seeing tours going opposite direction. I was pointing out traffic to the sight seeing tours that were passing on the river; looked to see that the separation existed with the diamond and cleared him for take off looked back to the arch to point out traffic for the sight seeing tours and the baron called near midair collision as the diamond turned early in his face. Instead of anticipating the diamond would continue 1/2 mile before turning as is standard; I should have clarified that he needed to continue straight out for 1/2 mile before starting his westerly turn.

Original NASA ASRS Text

Title: Tower Controller described a NMAC between a departing aircraft and another near by aircraft. The reporter noted that more specific instructions should have been issued to the departure.

Narrative: A Baron [was] inbound for Runway 12R on a right downwind. A Diamond departed the crossing Runway 23 for a right turn out and a northbound flight along the river. When I anticipated the Baron would be past the departure end of Runway 23; I cleared the Diamond for takeoff. The Diamond turned right early instead of continuing runway heading for a half mile this put him directly in the Baron's flight path instead of behind him. I did not notice the Diamond's early turn because I was dealing with opposite direction traffic over the river both north and southbound there were 3 northbound and 2 southbound on visual sight seeing tours going opposite direction. I was pointing out traffic to the sight seeing tours that were passing on the river; looked to see that the separation existed with the Diamond and cleared him for take off looked back to the arch to point out traffic for the sight seeing tours and the Baron called NMAC as the Diamond turned early in his face. Instead of anticipating the Diamond would continue 1/2 mile before turning as is standard; I should have clarified that he needed to continue straight out for 1/2 mile before starting his westerly turn.

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1078373
Time
Date	201304
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Aircraft 1
Make Model Name	Baron 58/58TC
Flight Phase	Final Approach
Route In Use	Visual Approach
Flight Plan	IFR
Aircraft 2
Make Model Name	DA20 Undifferentiated
Operating Under FAR Part	Part 91
Flight Phase	Initial Climb
Flight Plan	VFR
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Conflict NMAC

Tower controller described a near midair collision between a departing aircraft and another near by aircraft. The reporter noted that more specific instructions should have been issued to the departure.