Narrative:

I was flying eastbound at 9500 MSL; ceiling unlimited; visibility unlimited; when a white cessna citation was identified flying opposite heading approximately 2000 ft below. Visually monitored cessna citation turning south heading and attempted to track for maintaining aircraft separation. Lost visual contact; and continued searching for cessna citation until located immediately below (less than 100 feet) and slightly behind the aircraft. Unknown cessna citation was all white with only markings on rear fuselage. Markings were difficult to see/identify as the aircraft was directly positioned below the aircraft tail. Markings were black and appeared as two text words. Attempted to contact unknown aircraft via 121.5 and local unicom 122.9 received no response. Attempted to contact flight service and ATC; but was unable to establish communications at current altitude. Rocked wings and began clearing turns to communicate with unknown aircraft and establish separation. However; the unknown aircraft could not be further located. At no time did the unknown aircraft attempt radio or visual communications; nor fly in any position to make its presence obvious. Performed unplanned descent and landing to a nearby airport to contact ATC and flight service but was not able to obtain mobile phone coverage. Took off from airport and climbed to sufficient altitude to contact great falls flight service and salt lake center. Flight service was unaware of any special aircraft activity; operations or requirements in the area. Salt lake center made additional inquiries upon request and was unable to identify any special aircraft activity; operations or requirements; nor aircraft interceptions in the area. The close operation of both aircraft could be considered a collision hazard and unplanned/unarranged/unapproved formation flight.

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Original NASA ASRS Text

Title: C180 pilot had apparently purposeful same direction technical NMAC with a Cessna Citation which appeared to be flying in close proximity for a possible law enforcement mission.

Narrative: I was flying eastbound at 9500 MSL; ceiling unlimited; visibility unlimited; when a white Cessna Citation was identified flying opposite heading approximately 2000 FT below. Visually monitored Cessna Citation turning south heading and attempted to track for maintaining aircraft separation. Lost visual contact; and continued searching for Cessna Citation until located immediately below (less than 100 feet) and slightly behind the aircraft. Unknown Cessna Citation was all white with only markings on rear fuselage. Markings were difficult to see/identify as the aircraft was directly positioned below the aircraft tail. Markings were black and appeared as two text words. Attempted to contact unknown aircraft via 121.5 and local Unicom 122.9 received no response. Attempted to contact Flight Service and ATC; but was unable to establish communications at current altitude. Rocked wings and began clearing turns to communicate with unknown aircraft and establish separation. However; the unknown aircraft could not be further located. At no time did the unknown aircraft attempt radio or visual communications; nor fly in any position to make its presence obvious. Performed unplanned descent and landing to a nearby airport to contact ATC and Flight Service but was not able to obtain mobile phone coverage. Took off from airport and climbed to sufficient altitude to contact Great Falls Flight Service and Salt Lake Center. Flight Service was unaware of any special aircraft activity; operations or requirements in the area. Salt Lake Center made additional inquiries upon request and was unable to identify any special aircraft activity; operations or requirements; nor aircraft interceptions in the area. The close operation of both aircraft could be considered a collision hazard and unplanned/unarranged/unapproved formation flight.

Data retrieved from NASA's ASRS site as of April 2012 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.