Narrative:

I was on an IFR flight plan at cruise altitude FL270. I had entered light IMC at approximately -34C about 10 minutes before the incident happened. I encountered no icing on any of the surfaces of the airframe (I am fully de-iced equipped and would have activated de-ice measures if any symptoms had surfaced). I started showing an airspeed drop; very fast; on the garmin G3X and then got an airspeed warning (impending stall). The garmin autopilot immediately took stall evasive action by pitching the plane down. As I noticed the altitude tape start rolling downward I realized for the first time my autopilot was connected to airspeed; not just the adahrs (attitude and heading reference system) or internal gyros; like in my previous plane. I immediately disconnected the autopilot as I knew we could not have possibly slowed the plane this fast and the autopilot was actually getting me into a dangerous position by diving the plane; possibly to the point of exceeding vne. About this time the first call came from ATC asking me what was going on with my altitude. I replied I am dealing with an issue to which he responded to call him with an update as soon as I could.I then turned on the pitot heat; as that was about the only thing that came to mind quickly that might correct the loss of airspeed indication on the EFIS. Within about 30 seconds the airspeed came alive and climbed very fast into the vne area. I immediately pulled power and went to full forward pitch on the prop. Unfortunately; it now appeared the autopilot had run the pitch servo into an extreme pitch down condition. I was still in IMC and concerned about the plane coming apart so I reactivated the autopilot and hit the 'level' button. I knew the autopilot servos were set to torque values that likely would not exceed the airframe capabilities. While this was all happening; I then got a warning in my headset to 'don oxygen mask'. I have a cabin audible alert wired into my headset that warns of cabin pressurization loss. I then realized pulling off all engine power had now resulted in no bleed air to pressurize the aircraft. I glanced at the cabin pressure and saw I still had 2 psi; so elected to keep the power off until getting the aircraft back under control. The plane leveled off at 23;000. At this time I got my second call from ATC and I was able to give him a preliminary update on what had just happened. I resumed power and prop condition to normal and requested a climb back to FL270. At this point it was clear there must have been some ice on the pitot tube. This whole event happened within 60-90 seconds. I looked over the aircraft again; looking at the windshield; wings and horizontal stabilizers; and had absolutely no ice anywhere.some very important lessons were learned through this event; and this was clearly the most life threatening event I have ever had in 3;500 hours of flying. I have never lost control of an airplane in IMC; and especially under a scenario where my instrument indications were not reliable enough to get me out of the situation. Applying emergency procedures just taught to me during transition training a few months ago (power back; prop full forward) likely saved my life. The second contributing factor to a safe outcome was the autopilot's option of 'level'; which just levels the plane off. I have had pitot tube icing in previous aircraft resulting in momentary airspeed loss until applying pitot heat; but always had the autopilot fly the airplane normally during the event. This new garmin autopilot is not gyro based; and in addition has a safety feature that tries to keep the plane from stalling; which is based on airspeed input. Loss of airspeed on this new autopilot will result in a pitch down command to the autopilot; whether the aircraft actually needs that or not. How will this experience change the way I fly? I always turn on the pitot heat when in IMC at any temp below +5C; regardless if there is no indication of ice anywhere else on the plane. If I ever suddenly lose airspeed indication again; during autopilot operation; I will immediately disconnect the autopilot before it can adjust the pitch servo outside the current condition position (level; climbing or descending modes). I think it's also critical to disseminate this major change in autopilot operation from the older systems that are gyro based with no connection to airspeed; versus newer glass versions that attempt to avoid stall or over-speed conditions that are directly connected to the airspeed tape. I am fairly sure I will not be the last pilot to experience this condition with this new garmin/G3X autopilot systems. The next one may not be as lucky as me.

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Original NASA ASRS Text

Title: Lancair IV pilot reported erroneous airspeed at FL270 due to pitot icing led the G3X autopilot to attempt to maintain airspeed by pitching down.

Narrative: I was on an IFR flight plan at cruise altitude FL270. I had entered light IMC at approximately -34C about 10 minutes before the incident happened. I encountered no icing on any of the surfaces of the airframe (I am fully de-iced equipped and would have activated de-ice measures if any symptoms had surfaced). I started showing an airspeed drop; very fast; on the Garmin G3X and then got an airspeed warning (impending stall). The Garmin autopilot immediately took stall evasive action by pitching the plane down. As I noticed the altitude tape start rolling downward I realized for the first time my autopilot was connected to airspeed; not just the ADAHRS (Attitude and Heading Reference System) or internal gyros; like in my previous plane. I immediately disconnected the autopilot as I knew we could not have possibly slowed the plane this fast and the autopilot was actually getting me into a dangerous position by diving the plane; possibly to the point of exceeding VNE. About this time the first call came from ATC asking me what was going on with my altitude. I replied I am dealing with an issue to which he responded to call him with an update as soon as I could.I then turned on the pitot heat; as that was about the only thing that came to mind quickly that might correct the loss of airspeed indication on the EFIS. Within about 30 seconds the airspeed came alive and climbed very fast into the Vne area. I immediately pulled power and went to full forward pitch on the prop. Unfortunately; it now appeared the autopilot had run the pitch servo into an extreme pitch down condition. I was still in IMC and concerned about the plane coming apart so I reactivated the autopilot and hit the 'Level' button. I knew the autopilot servos were set to torque values that likely would not exceed the airframe capabilities. While this was all happening; I then got a warning in my headset to 'don oxygen mask'. I have a cabin audible alert wired into my headset that warns of cabin pressurization loss. I then realized pulling off all engine power had now resulted in no bleed air to pressurize the aircraft. I glanced at the cabin pressure and saw I still had 2 PSI; so elected to keep the power off until getting the aircraft back under control. The plane leveled off at 23;000. At this time I got my second call from ATC and I was able to give him a preliminary update on what had just happened. I resumed power and prop condition to normal and requested a climb back to FL270. At this point it was clear there must have been some ice on the pitot tube. This whole event happened within 60-90 seconds. I looked over the aircraft again; looking at the windshield; wings and horizontal stabilizers; and had absolutely no ice anywhere.Some very important lessons were learned through this event; and this was clearly the most life threatening event I have ever had in 3;500 hours of flying. I have never lost control of an airplane in IMC; and especially under a scenario where my instrument indications were not reliable enough to get me out of the situation. Applying emergency procedures just taught to me during transition training a few months ago (power back; prop full forward) likely saved my life. The second contributing factor to a safe outcome was the autopilot's option of 'level'; which just levels the plane off. I have had pitot tube icing in previous aircraft resulting in momentary airspeed loss until applying pitot heat; but always had the autopilot fly the airplane normally during the event. This new Garmin autopilot is not gyro based; and in addition has a safety feature that tries to keep the plane from stalling; which is based on airspeed input. Loss of airspeed on this new autopilot WILL result in a pitch down command to the autopilot; whether the aircraft actually needs that or not. How will this experience change the way I fly? I ALWAYS turn on the pitot heat when in IMC at any temp below +5C; regardless if there is no indication of ice anywhere else on the plane. If I ever suddenly lose airspeed indication again; during autopilot operation; I will immediately disconnect the autopilot before it can adjust the pitch servo outside the current condition position (level; climbing or descending modes). I think it's also critical to disseminate this major change in autopilot operation from the older systems that are gyro based with no connection to airspeed; versus newer glass versions that attempt to avoid stall or over-speed conditions that are directly connected to the airspeed tape. I am fairly sure I will not be the last pilot to experience this condition with this new Garmin/G3X Autopilot Systems. The next one may not be as lucky as me.

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.