Miscellaneous -- delayed late conflict alert warning between cga X and cpr Y.

Attributes
ACN	287666
Time
Date	199411
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : zzz
State Reference	US
Altitude	msl bound lower : 1200 msl bound upper : 1400
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tower : zzz tower : lga
Operator	general aviation : corporate
Make Model Name	Helicopter
Operating Under FAR Part	Part 135
Flight Phase	cruise other descent : approach
Flight Plan	VFR
Aircraft 2
Operator	general aviation : corporate
Make Model Name	Cessna 340/340A
Operating Under FAR Part	Part 91
Flight Phase	descent : approach descent other
Flight Plan	VFR
Person 1
Affiliation	government : faa
Function	controller : local
Qualification	controller : radar
Experience	controller military : 4 controller non radar : 24 controller radar : 23 controller supervisory : 2
ASRS Report	287666
Person 2
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial
Events
Anomaly	other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	Other
Supplementary
Primary Problem	Navigational Facility
Air Traffic Incident	other

Narrative:

I was working the local control position in the tower and I had an EMS helicopter inbound to the airport via the lake shoreline. The EMS helicopter was cga X on an established inbound route to their landing site on the west side of the airport. Cpr Y, a cessna 340, was inbound from the north on a visual approach to runway 24 via a right base leg. Cga X was at 1200 ft MSL and cpr Y was descending from 2800 ft MSL. I exchanged traffic to both aircraft. I had both aircraft in sight and was providing visual separation. The helicopter sighted the C340 at 2 NM, the C340 was at 1400 ft MSL descending. The helicopter crossed the C340's 12 O'clock position at less than 1 mi distance. The aural alarm of the ARTS iia conflict alert never sounded. When the aircraft were at a distance of more than 1 mi on diverging headings, the aural alarm sounded. The aircraft were moving away from one another before the alarm sounded. This unfortunate incident occurs on a regular basis. The conflict alert feature is supposed to aid the controller in preventing conflicts collisions: however, does not function in this manner.

Original NASA ASRS Text

Title: MISC -- DELAYED LATE CONFLICT ALERT WARNING BTWN CGA X AND CPR Y.

Narrative: I WAS WORKING THE LCL CTL POS IN THE TWR AND I HAD AN EMS HELI INBOUND TO THE ARPT VIA THE LAKE SHORELINE. THE EMS HELI WAS CGA X ON AN ESTABLISHED INBOUND RTE TO THEIR LNDG SITE ON THE W SIDE OF THE ARPT. CPR Y, A CESSNA 340, WAS INBOUND FROM THE N ON A VISUAL APCH TO RWY 24 VIA A R BASE LEG. CGA X WAS AT 1200 FT MSL AND CPR Y WAS DSNDING FROM 2800 FT MSL. I EXCHANGED TFC TO BOTH ACFT. I HAD BOTH ACFT IN SIGHT AND WAS PROVIDING VISUAL SEPARATION. THE HELI SIGHTED THE C340 AT 2 NM, THE C340 WAS AT 1400 FT MSL DSNDING. THE HELI CROSSED THE C340'S 12 O'CLOCK POS AT LESS THAN 1 MI DISTANCE. THE AURAL ALARM OF THE ARTS IIA CONFLICT ALERT NEVER SOUNDED. WHEN THE ACFT WERE AT A DISTANCE OF MORE THAN 1 MI ON DIVERGING HDGS, THE AURAL ALARM SOUNDED. THE ACFT WERE MOVING AWAY FROM ONE ANOTHER BEFORE THE ALARM SOUNDED. THIS UNFORTUNATE INCIDENT OCCURS ON A REGULAR BASIS. THE CONFLICT ALERT FEATURE IS SUPPOSED TO AID THE CTLR IN PREVENTING CONFLICTS COLLISIONS: HOWEVER, DOES NOT FUNCTION IN THIS MANNER.

Data retrieved from NASA's ASRS site as of July 2007 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.