Narrative:

[The M20] had been enroute to somewhere on the east coast when the aircraft experienced electronic issues with its FMS. The pilot decided to return to the departure airport to have the maintenance issue looked into. When I received the aircraft it was on a heading of 280 direct when able and reported level at 150 with no mode C. I had a departing erj-145 climbing to 140 converging with the M20. I was anticipating using visual separation to keep the erj-145 climbing so I turned him 10 degrees right to prevent the targets from merging if I used visual. I called traffic to the M20; he advised IMC. I called traffic to the erj-145 who also advised IMC; but I left him on the heading in order to achieve lateral separation a bit quicker. As the aircraft crossed the M20's mode C suddenly turned on and it showed 145 conflicting with the erj-145. I asked the M20 pilot to verify level at 150 he responded he was not and was having difficulty in the clouds. I immediately turned the erj-145 away and they advised responding to a TCAS alert and climbing. I tried turning the M20 but got no response and witnessed the aircraft make a radical right turn of greater than 180 degrees and a rapid descent to below 140. The erj-145 cleared the traffic and descended back to 140. I eventually regained contact with the M20 pilot who had made a radical left turn back westbound and rapid climb to 155; pilot garbled a response about autopilot issues and the aircraft making sudden changes. I got him back on course and level at 150 and the erj-145 back on course and climbing to requested altitude.

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Original NASA ASRS Text

Title: M20 pilot reports loosing his G500 PFD/MFD; autopilot; and flight director in cruise at 15;000 feet and electing to return to the departure airport for maintenance. Upon entering a small cumulus a strong down draft is encountered resulting in significant altitude loss and a large heading change; narrowly missing a CRJ at 14;000 feet. The report from the Controller involved is also included.

Narrative: [The M20] had been enroute to somewhere on the east coast when the aircraft experienced electronic issues with its FMS. The pilot decided to return to the departure airport to have the maintenance issue looked into. When I received the aircraft it was on a heading of 280 direct when able and reported level at 150 with no Mode C. I had a departing ERJ-145 climbing to 140 converging with the M20. I was anticipating using visual separation to keep the ERJ-145 climbing so I turned him 10 degrees right to prevent the targets from merging if I used visual. I called traffic to the M20; he advised IMC. I called traffic to the ERJ-145 who also advised IMC; but I left him on the heading in order to achieve lateral separation a bit quicker. As the aircraft crossed the M20's Mode C suddenly turned on and it showed 145 conflicting with the ERJ-145. I asked the M20 pilot to verify level at 150 he responded he was not and was having difficulty in the clouds. I immediately turned the ERJ-145 away and they advised responding to a TCAS alert and climbing. I tried turning the M20 but got no response and witnessed the aircraft make a radical right turn of greater than 180 degrees and a rapid descent to below 140. The ERJ-145 cleared the traffic and descended back to 140. I eventually regained contact with the M20 pilot who had made a radical left turn back westbound and rapid climb to 155; pilot garbled a response about autopilot issues and the aircraft making sudden changes. I got him back on course and level at 150 and the ERJ-145 back on course and climbing to requested altitude.

Data retrieved from NASA's ASRS site as of July 2013 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.