MU2 pilot reported altimeter and airspeed indicator errors due to moisture in the pitot static syste...

Attributes
ACN	1544753
Time
Date	201805
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	MU-2
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Pitot/Static Ice System
Person 1
Function	Captain Pilot Flying Single Pilot
Qualification	Flight Crew Commercial Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 20 Flight Crew Total 2000 Flight Crew Type 250
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Inflight Event / Encounter Weather / Turbulence

Narrative:

I climbed to my assigned altitude of FL200. All was fine. I then noticed that my altitude was decreasing very slowly on my G600 even though I had my autopilot on. All icing equipment was on. I tried to trim up to get back to FL200 but the plane's altimeter showed a slow descent. Around 19;700 ft; I contacted ATC and asked them what they were showing as my altitude. They confirmed that I was 300 ft low. I told them that my altimeter was not acting correctly. My airspeed was also fluctuating; at times showing 150 knots to 270 knots. The controller asked if I wanted 18;000 ft which I accepted. Once I got to 18;000 ft; the altimeter still showed I was descending and I was back in ice as well. I [advised ATC] because of the incorrect readings I was getting from the G600 and because I was in IMC. The controller gave me lower altitudes; all the way down to 12;000 ft at which point I finally got out of the IMC and the instruments began to give me correct indications. I cancelled the emergency and landed at ZZZ. The problems were caused by moisture in the pitot static system which froze at altitude.

Original NASA ASRS Text

Title: MU2 pilot reported altimeter and airspeed indicator errors due to moisture in the pitot static system.

Narrative: I climbed to my assigned altitude of FL200. All was fine. I then noticed that my altitude was decreasing very slowly on my G600 even though I had my autopilot on. All icing equipment was on. I tried to trim up to get back to FL200 but the plane's altimeter showed a slow descent. Around 19;700 ft; I contacted ATC and asked them what they were showing as my altitude. They confirmed that I was 300 ft low. I told them that my altimeter was not acting correctly. My airspeed was also fluctuating; at times showing 150 knots to 270 knots. The controller asked if I wanted 18;000 ft which I accepted. Once I got to 18;000 ft; the altimeter still showed I was descending and I was back in ice as well. I [advised ATC] because of the incorrect readings I was getting from the G600 and because I was in IMC. The controller gave me lower altitudes; all the way down to 12;000 ft at which point I finally got out of the IMC and the instruments began to give me correct indications. I cancelled the emergency and landed at ZZZ. The problems were caused by moisture in the Pitot Static system which froze at altitude.

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.