Date: 201303
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: 20
Light: Daylight
Aircraft: MD-500MG
Operator: Government
Operating under: Part 91
Flight plan: None
Mission: Utility / Infrastructure
Flight phase: Cruise
Airspace: Class D ZZZ
ATC / advisory: Tower ZZZ
Second aircraft: Eurocopter AS 350/355/EC130 - Astar/Twinstar/Ecureuil
Reporter organization: Government
Reporter function: Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Flight Instructor; Flight Crew Commercial
Human factors: Communication Breakdown; Situational Awareness; Workload; Confusion
Anomaly: Conflict NMAC
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Contributing factors: Procedure; Environment - Non Weather Related
Primary problem: Ambiguous

Narrative: I was PIC in a police helicopter when I took evasive action to avoid a near collision with an EMS helicopter while in the Class Delta airspace. I requested permission to enter the Class Delta airspace from Tower and was granted permission. I was at 1;900 FT MSL and on an approximate heading of 055 degrees and 80 KIAS. Tower advised me of an EMS helicopter in my vicinity to the east and westbound through the airspace. I advised the Tower I would be looking for the traffic. I heard Tower advise the other aircraft that I would be transitioning in the airspace also northeast bound. I hear the EMS pilot acknowledge the Tower and state he would be looking for me as well. We received no further warnings or updates from the Tower Controller. I began scanning the horizon for the westbound helicopter and periodically checking the Garmin 530 display for any traffic targets in that area. The Garmin showed no targets each time I checked it. After approximately 45 seconds to one minute of scanning without seeing the converging aircraft; my Observer pointed out the aircraft at my 11 o'clock position. The yellow AS350 helicopter was westbound and approximately 400 FT horizontal distance and nearly the same altitude if only slightly higher. I took evasive action by turning right approximately 10 degrees quickly and kept my altitude of 1;900 FT MSL. It did not appear that the yellow EMS helicopter deviated in any way on its course. The EMS pilot did not report having me in sight to the Tower and subsequently left the Class Delta airspace to the west. I continued my northeast bound course without incident.[In summary] two helicopters on opposite course at high rate of closure. Neither pilot seeing the other until the last moment. Traffic avoidance equipment in one helicopter [was] not displaying targets. Pilot of aircraft X assumed other aircraft was further away than it actually was based on ATC's description of where aircraft was at time of advisory warning.No visual identification made by either pilot. No follow-up from ATC as aircraft converged at nearly same altitude. Traffic advisory equipment/Garmin 530 [was] not working at time of incident.Ask for further update from ATC or advise 'No joy' on other aircraft as time passes without seeing the other aircraft. Ask crewmember to assist in looking for other aircraft.




Synopsis: Two helicopters; a MD500 and an AS350; had a near-miss in Class D Airspace after the Tower Controller initially pointed out mutual traffic but provided no updates and the MD500 TCAS did not display traffic.