Narrative:

I was in cruise flight at FL190. It was a dark; moonless night. I was on top of an overcast. Outside air temperature was -25C. I flew into IMC and immediately begin accreting mixed rime ice on the leading edges and windshield. I turned on the tks de-ice system (which didn't slow the rate of ice accumulation); pitot heat; and the windshield defroster. After about 1 minute in IMC; I had approximately 1/2' mixed rime ice on the leading edge. Indicated airspeed decreased from approximately 135 knots to about 115 KTS. I requested and received from approach a lower altitude and vectors to the nearest airport with an instrument approach. A serious of clearances brought me down to 2000 MSL. At about 5000 MSL; I lost the ice accumulation on the aircraft. I did not have approach plates for ZZZ and was given vectors to the final approach course; along with the FAF crossing altitude; DH; and the tower frequency. I broke out at about 400 AGL in light rain. Taxi and parking was uneventful. In planning the flight I based my decisions on information obtained from the NOAA aviation website and from a standard briefing from a FSS weather briefer. I believed that the top of the system I experienced enroute was at about 16000 MSL. I did not expect to fly into weather at my final altitude of FL190. Nor; with a predicted OAT of -25C did I believe that if I were to fly into IMC; that I would accumulate ice. I believe that much of the icing I encountered was supercooled water droplets that didn't freeze until it came in contact with the airframe. I have been flying with commercial electronic charts; printing out only those charts I need for the origin; destination and alternate. I didn't have the laptop computer which contains the commercial charts setup in the cockpit. Because I had neither the paper nor electronic charts needed for an expected diversion; I had to rely on ATC for all approach information. In a no-communications situation; this could have been problematic; although the G1000 system does have much of this information. In the future; I will ensure that I have at least the electronic charting available for any possible diversion that could occur along my route of flight. Secondly; I shouldn't have attempted to beat the storm system moving into the northeast that evening. I believed; based on forecasts; that I was going to be able to stay clear of ice and arrive prior to the arrival of the storm. I was wrong on both accounts.

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Original NASA ASRS Text

Title: C206 pilot at FL190 reports encountering unforecast clouds and icing conditions. All anti-ice systems were turned on including a TKS airfoil deice system. Ice continued to build on the wings and descent and diversion are requested from ATC. Once below the freezing level the ice was shed and a normal landing ensued. Reporter also laments the lack of approach charts for this unplanned diversion.

Narrative: I was in cruise flight at FL190. It was a dark; moonless night. I was on top of an overcast. Outside air temperature was -25C. I flew into IMC and immediately begin accreting mixed rime ice on the leading edges and windshield. I turned on the TKS de-ice system (which didn't slow the rate of ice accumulation); pitot heat; and the windshield defroster. After about 1 minute in IMC; I had approximately 1/2' mixed rime ice on the leading edge. Indicated airspeed decreased from approximately 135 knots to about 115 KTS. I requested and received from Approach a lower altitude and vectors to the nearest airport with an instrument approach. A serious of clearances brought me down to 2000 MSL. At about 5000 MSL; I lost the ice accumulation on the aircraft. I did not have approach plates for ZZZ and was given vectors to the final approach course; along with the FAF crossing altitude; DH; and the tower frequency. I broke out at about 400 AGL in light rain. Taxi and parking was uneventful. In planning the flight I based my decisions on information obtained from the NOAA Aviation website and from a standard briefing from a FSS weather briefer. I believed that the top of the system I experienced enroute was at about 16000 MSL. I did not expect to fly into weather at my final altitude of FL190. Nor; with a predicted OAT of -25C did I believe that if I were to fly into IMC; that I would accumulate ice. I believe that much of the icing I encountered was supercooled water droplets that didn't freeze until it came in contact with the airframe. I have been flying with commercial electronic charts; printing out only those charts I need for the origin; destination and alternate. I didn't have the laptop computer which contains the commercial charts setup in the cockpit. Because I had neither the paper nor electronic charts needed for an expected diversion; I had to rely on ATC for all approach information. In a no-communications situation; this could have been problematic; although the G1000 system does have much of this information. In the future; I will ensure that I have at least the electronic charting available for any possible diversion that could occur along my route of flight. Secondly; I shouldn't have attempted to beat the storm system moving into the Northeast that evening. I believed; based on forecasts; that I was going to be able to stay clear of ice and arrive prior to the arrival of the storm. I was wrong on both accounts.

Data retrieved from NASA's ASRS site as of April 2012 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.