M-20 K (231) / Encore
Aircraft Equipment Problem Less Severe; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Aircraft Automation Overrode Flight Crew; Flight Crew Returned To Clearance; General Maintenance Action
Aircraft; Human Factors; Weather
[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.
Enroute approximately one hour after departure at 15;000 feet MSL I lost instrument data on G500 PFD/MFD; autopilot and flight director. Called Center to return for avionics repair. Assigned heading 280 at 15;000 feet. Hand flying in smooth air with Garmin 530W/430W stack providing direction; electric AI backup providing attitude; and conventional altimeter providing altitude. Entered small cumulus which I had passed through earlier heading east. Center advised a regional jet 500 feet below; I replied 'in IMC;' heard the RJ reply the same. Suddenly in sharp descent with wings not level. Recovered and returned to 15;000. Reported 'problem' during event to Center. Continued to destination; when descending through 5;000 feet for landing all instruments appeared to resume functioning; but I did not bother to read them and used a visual approach and landing. I left the airplane with the Avionics shop; asking them to see if they could find the cause of the descent and the problem with the instruments. They began to look at the airplane the next day and I have not heard yet what they have found.ATC personnel called me Thursday to inquire about event and ask about some numbers they had received from Mode C. They told me that my aircraft appeared to have come close to the regional jet; which had taken evasive action (climbing). They reported that they had begun receiving my Mode C just before the sudden descent; and that it had read an extremely high rate of climb during my recovery (the reported rate of climb was several thousand feet per minute; far in excess of the Mooney's capability).The Mode C coming alive just before the descent makes me think that the event was caused by a malfunction with altitude hold function of my aged King KFC200 autopilot. The computer in the autopilot seemed to be declining; first indicated by the loss of the flight director (the wings simply stayed in one place); and over the past few weeks by the malfunction of the altitude hold; which would at first occasionally and then more frequently initiate a steep descent when activated after I had trimmed the plane for straight and level flight at the assigned altitude. (I had already spoken to the Avionics shop about getting a new autopilot; which they said they would do.) The altitude hold function had been performing properly during the east-bound phase of my flight. When the G500 and autopilot became inoperable I relied on the other instruments and did not pull the CB's for the G500 and the autopilot; which left the Flight Director; Heading; and Altitude Hold lights still illuminated on the flight director panel even though they were not actually working. I think that when the Mode C came back on just before the descent; perhaps the Altitude Hold function also resumed functioning; and reacted just as though I had activated it afresh; with the steep descent that had been occurring before.The FAA called me about this event to say that FSDO would be conducting an investigation.
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.