Tower controller described an near midair collision between an aircraft on the go turning an early d...

Narrative:

Aircraft X was in right closed traffic pattern. Bonanza was on the 45 entry for right traffic. After a touch and go; aircraft X turned crosswind earlier than I anticipated and flew directly underneath bonanza. I was first aware of this when the faint target of aircraft X appeared slightly southeast of bonanza's mode aircraft beacon target; followed directly thereafter with aircraft X's advising that they wanted to file a near midair collision. The supervisor was in the cab on controller in charge at the time. After contacting the [bonanza] pilot; he said he saw the cessna the whole time; and he estimated the cessna was 300-400 ft below him at their closest point. The time on position in an extremely complex traffic session for el monte was the only contributing factor that could be directly addressed by the facility. I felt that I could have used a break about the 1 hour mark; but didn't say anything to the flm at the time. An additional contributing factor is the extremely poor radar coverage at emt at and below traffic pattern altitudes. Traffic almost never tags up on the scope before 1;000-1;500 ft; making it worthless as a tool for separating traffic in the pattern.

Original NASA ASRS Text

Title: Tower Controller described an NMAC between an aircraft on the go turning an early downwind and an aircraft entering downwind; the reporter listing fatigue and poor RADAR coverage as casual factors.

Narrative: Aircraft X was in right closed traffic pattern. Bonanza was on the 45 entry for right traffic. After a touch and go; Aircraft X turned crosswind earlier than I anticipated and flew directly underneath Bonanza. I was first aware of this when the faint target of Aircraft X appeared slightly Southeast of Bonanza's Mode aircraft beacon target; followed directly thereafter with Aircraft X's advising that they wanted to file a NMAC. The supervisor was in the cab on CIC at the time. After contacting the [Bonanza] pilot; he said he saw the Cessna the whole time; and he estimated the Cessna was 300-400 FT below him at their closest point. The time on position in an extremely complex traffic session for El Monte was the only contributing factor that could be directly addressed by the facility. I felt that I could have used a break about the 1 hour mark; but didn't say anything to the FLM at the time. An additional contributing factor is the extremely poor RADAR coverage at EMT at and below traffic pattern altitudes. Traffic almost never tags up on the scope before 1;000-1;500 FT; making it worthless as a tool for separating traffic in the pattern.

Data retrieved from NASA's ASRS site as of April 2012 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	977982
Time
Date	201111
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.Tower
State Reference	US
Aircraft 1
Make Model Name	Skyhawk 172/Cutlass 172
Flight Phase	Initial Approach
Route In Use	None
Aircraft 2
Make Model Name	Bonanza 35
Flight Phase	Descent
Route In Use	None
Person 1
Function	Local
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	Conflict NMAC

Tower controller described an near midair collision between an aircraft on the go turning an early downwind and an aircraft entering downwind; the reporter listing fatigue and poor radar coverage as casual factors.