Cessna 210 Centurion / Turbo Centurion 210C; 210D
Flight Crew Private; Flight Crew Instrument
Flight Crew Last 90 Days 9; Flight Crew Total 556; Flight Crew Type 214
Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Conflict NMAC; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew FLC complied w / Automation / Advisory; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft; Software and Automation
I was cruising VFR at 7;500 ft.; without flight following; but monitoring Center. My ADS-B (Automatic Dependent Surveillance - Broadcast) showed traffic 500 ft. below me; 12 miles out at my 1:30 position. This caught my attention because this condition indicates we will pass laterally very close. I heard Center alert the other aircraft about the traffic (my aircraft); but he was unable to get a visual. Center indicated my altitude was 7;200 ft. I turned on my taxi and landing lights to make my aircraft more visible to the other aircraft. We passed with 500 ft. vertical and about 150 ft. lateral separation. The other pilot called me on the phone later that day because he was concerned about a near miss (although he did get a visual on my aircraft just before we passed). His ADS-B indicated 150 ft. separation. My flights showed my ADS-B altitude 350 ft. lower than what it actually was; which was the cause for the other pilot's concern. I am reporting this to my avionics shop for repair; and in the future will pick up flight following earlier in the flight for better communication.
GA pilot reported a NMAC and later discovered that separation from the other aircraft was less than originally thought due to the reporter's ADS-B equipment not indicating the correctly.