Cardinal 177/177RG
Flight Crew Flight Engineer; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 130; Flight Crew Total 13500; Flight Crew Type 610
Situational Awareness; Troubleshooting; Communication Breakdown
ATC Issue All Types; Conflict NMAC
Flight Crew Took Evasive Action
Human Factors; Procedure
While in cruise at 5500 ft. MSL and under VFR flight following from Approach Control; I noticed an airplane on my ADS-B converging on me at the same altitude. I began to try and acquire the other airplane visually; but was unable. Frequency congestion prevented me from contacting Approach Control to inquire about the other airplane. When the other airplane was within 1.5 miles of me; still not in sight; and with no traffic calls from Approach Control; I initiated a rapid climb to avoid a potential collision. The other airplane passed directly underneath me with only 300 ft. of vertical separation. I did not observe the other airplane make heading or altitude changes. After I was clear of the traffic; I descended back to 5500 ft. MSL and asked Approach Control: 'Approach Control; Aircraft X; did you see that traffic that just went under me?' to which Approach Control responded 'Contact ZZZ1 Approach on xxx.xx'. Because the frequency was busy; I did not re-engage Approach Control and changed frequency as instructed.I understand VFR flight following is provided on a 'workload permitting' basis. However; my handoff into the airspace was accepted by the controlling agency and my ATC flight following was never terminated by the controlling agency due to workload. In this scenario; I had an expectation that traffic separation services would be provided. Had Approach Control terminated my flight following; I would have climbed sooner and had more/safer separation from the converging traffic.
General Aviation pilot reported an NMAC with another light aircraft in congested airspace.