Narrative:

The avionics system in our aircraft was recently retrofitted to include a garmin G600 mfd/pfd combination and dual garmin GTN750 GPS nav/comms by an authorized service center. During cruise at FL260; on autopilot (ap) with altitude and gpss mode engaged; the flight director (FD) began to command a series of very small up and down pitch oscillations - the autopilot followed the FD; resulting in a small; but noticeable; 'porpoising' effect. Altitude varied by no more than 20 feet. However; after about 10-15 seconds of the small oscillations; the FD commanded a sudden and rapid 15 degree pitch up command without any input from the pilot. The autopilot immediately followed the erroneous FD indications; pressing my four passengers and me into our seats. I quickly disconnected the autopilot; and initiated a manual correction in an attempt to avoid excessive altitude gain. Simultaneously with pushing the ap disconnect button; we experienced an attitude and heading failure on the pfd. Unfortunately; with the extreme pitch causing an excessive rate of climb; I was through FL264 before I could arrest the ascent and bring the airplane back down to level; and I still lifted my passengers out of their seats to accomplish level that 'quickly'. Upon holding wings level; the attitude heading reference system (ahrs) eventually corrected and attitude/hdg returned; but the attitude indicated on the pfd did not match the standby attitude indicator (sdi). Another attitude failure occurred; ultimately corrected with wings level; then the system appeared to behave normally for the rest of the flight while hand flying the aircraft. ATC never mentioned the altitude change; and it appears that there was no immediate safety threat from loss of separation in this instance. However; circumstances could very easily have been otherwise; causing a serious traffic conflict. Since the altitude excursion was never questioned; and the systems ultimately came back online; I didn't mention the brief failure to ATC; but in hindsight I definitely should have reported the system failures as soon as adequate aircraft control was regained.after the incident; the aircraft was returned to the service center to investigate the cause of the erroneous FD indications. We have learned from the service center that at least 10 or 11 socata tbm aircraft that have been retrofitted with a G600 package have reported the exact same uncommanded pitch excursion issue described in this incident - likely a fairly large portion of the fleet that has completed the upgrade. Since the uncommanded pitch change problem is apparently random; unpredictable; and recurring; the FD in this aircraft cannot be trusted to control the autopilot appropriately. I will only handfly the aircraft until garmin; daher-socata; and the service centers determine a solution; and I strongly urge any other tbm operators who have had a G600 retrofit to do the same.

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Original NASA ASRS Text

Title: Pilot described why he will only 'hand fly' their company Socata TBM 700B aircraft after being retrofitted with the Garmin G600 MFD/PFD combination and dual Garmin GTN750 GPS Nav/Comms. Sudden; uncommanded pitch up change problems in climb and cruise have been random; unpredictable and recurring when the Autopilot and Flight Director are engaged. Supplemental Type Certificate (STC) had been approved for the avionics retrofit.

Narrative: The Avionics system in our aircraft was recently retrofitted to include a Garmin G600 MFD/PFD combination and dual Garmin GTN750 GPS Nav/Comms by an authorized Service Center. During cruise at FL260; on Autopilot (AP) with ALT and GPSS mode engaged; the Flight Director (FD) began to command a series of very small up and down pitch oscillations - the autopilot followed the FD; resulting in a small; but noticeable; 'porpoising' effect. Altitude varied by no more than 20 feet. However; after about 10-15 seconds of the small oscillations; the FD commanded a sudden and rapid 15 degree pitch up command without any input from the pilot. The autopilot immediately followed the erroneous FD indications; pressing my four passengers and me into our seats. I quickly disconnected the autopilot; and initiated a manual correction in an attempt to avoid excessive altitude gain. Simultaneously with pushing the AP Disconnect button; we experienced an Attitude and Heading failure on the PFD. Unfortunately; with the extreme pitch causing an excessive rate of climb; I was through FL264 before I could arrest the ascent and bring the airplane back down to level; and I still lifted my passengers out of their seats to accomplish level that 'quickly'. Upon holding wings level; the Attitude Heading Reference System (AHRS) eventually corrected and Attitude/Hdg returned; but the attitude indicated on the PFD did not match the Standby Attitude Indicator (SDI). Another attitude failure occurred; ultimately corrected with wings level; then the system appeared to behave normally for the rest of the flight while hand flying the aircraft. ATC never mentioned the altitude change; and it appears that there was no immediate safety threat from loss of separation in this instance. However; circumstances could very easily have been otherwise; causing a serious traffic conflict. Since the altitude excursion was never questioned; and the systems ultimately came back online; I didn't mention the brief failure to ATC; but in hindsight I definitely should have reported the system failures as soon as adequate aircraft control was regained.After the incident; the aircraft was returned to the Service Center to investigate the cause of the erroneous FD indications. We have learned from the Service Center that at least 10 or 11 Socata TBM aircraft that have been retrofitted with a G600 package have reported the exact same uncommanded pitch excursion issue described in this incident - likely a fairly large portion of the fleet that has completed the upgrade. Since the uncommanded pitch change problem is apparently random; unpredictable; and recurring; the FD in this aircraft cannot be trusted to control the autopilot appropriately. I will only handfly the aircraft until Garmin; Daher-Socata; and the Service Centers determine a solution; and I strongly urge any other TBM operators who have had a G600 retrofit to do the same.

Data retrieved from NASA's ASRS site 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.