Date: 201811
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Weather elements / visibility: 10
Light: Daylight
Aircraft: Skylane 182/RG Turbo Skylane/RG
Operator: Personal
Operating under: Part 91
Flight plan: VFR
Mission: Personal
Flight phase: Descent
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: Helicopter
Reporter organization: Personal
Reporter function: Single Pilot
Reporter qualification: Flight Crew Instrument; Flight Crew Private
Human factors: Situational Awareness; Time Pressure; Communication Breakdown
Anomaly: ATC Issue All Types; Conflict NMAC; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Procedure; Human Factors
Primary problem: Ambiguous

Narrative: Returning to ZZZ from the south at 4500 feet (low to avoid the strong north wind) with flight following; I advised Approach I was beginning descent to ZZZ. I have an iPad with ForeFlight on my control yoke and this displays the ADS-B information. At 2200 feet in descent; just south of the ZZZ1 area; the controller told me to hold at 2000 feet. I didn't know why until I saw a target westbound from ZZZ1; as if on takeoff/departure. It appeared it would intersect with my path; but it didn't climb above about 1400 feet. I expected to be cleared to descend again after the other aircraft crossed my path. But the aircraft turned north just before my path and went north; paralleling my path. I figured he must be making a right-traffic return to ZZZ1. But he didn't turn; he kept heading north. I turned about 20 degrees left (to about 310 degrees) to diverge from the other aircraft and coincidentally; to be directly aligned with the ZZZ runway. And because I was too high; getting close to ZZZ; I slowed to approach speed and dropped flaps in preparation for a quick descent. This also allowed me to drop back behind the other aircraft so I could descend behind him. ATC cleared me to descend and I immediately started down. The other aircraft then turned left; to the west; and presented a collision hazard. I turned about 30 degrees right to go behind the aircraft. He turned south; headed directly at me. I turned east in a steep turn and either leveled off or climbed a bit. The other aircraft went under my plane and seemed to be only 200 to 300 feet below me. It was a law-enforcement helicopter and it apparently had decided to circle there. I made a minor; not strong-enough complaint to ATC and he told me to change to local frequency (He was very busy). I was panicking and I don't remember how I got away from the jerk; but I remember making a steep; full-flap descent and normal landing at ZZZ. There are two possible ways to interpret this. One is that the helicopter pilot knew where I was all the time and was having fun bullying me. The other; more likely; is that this guy didn't look at all for other aircraft and was focused on events on the ground. I can't believe that someone who flies all the time in busy airspace like [here] would not be using ADS-B to be aware of other traffic; but if this guy had it; he didn't look at it any more than he looked around for traffic visually. Helicopters operating regularly in busy areas should be required to have ADS-B In and they should be trained to look for other aircraft. Or they should be in a patrol car instead of doing dangerous things in the air over populated areas.




Synopsis: Cessna 182 pilot reported an NMAC with a law enforcement helicopter.