Narrative:

Prior to flight; the tks system was topped off and the sight gauge checked to ensure the system was full. The cockpit tks gauge also indicated full. Forecast icing charts (cip/fip) were reviewed. Widespread pockets of sld were forecast throughout the southwestern; central and eastern area of the state for all altitudes up to 21;000 ft. Forecast icing severity showed light to occasional moderate icing throughout the same aforementioned coverage area and altitudes with heavier icing conditions for altitudes above 7;000 ft. The aircraft was deiced using types 1 and 4 fluids. Prior to start of takeoff roll from runway the tks normal mode was selected on. About six minutes after takeoff while climbing from 4;000 ft through 6;000 ft; moderate icing was encountered and the tks 'high' mode was selected. Upon reaching my final requested altitude of 7;000 ft; the icing conditions returned to light intensity and tks was placed back in 'normal' mode. A short while later the icing conditions again became moderate in intensity and 'high' mode was again selected. This scenario repeated itself over an 18 minute period. About twenty five minutes after takeoff the icing conditions changed to moderate intensity and the tks had to remain in 'high' mode for the remainder of the flight. On approximately three occasions during the flight; 'maximum flow airframe' had to be briefly used. While being vectored for the ILS runway 24 approach the yellow caution light on the tks illuminated; but moderate rime icing conditions remained and the continued use of 'high' mode was required to keep the aircraft nearly free of ice accumulation. During the initial ILS approach; at 2;600 ft MSL; I experienced a false glide slope indication and I executed the published missed approach procedure. I was then given vectors for another ILS approach which was completed successfully. After landing and while turning clear of the runway; the red warning light of the tks system illuminated and tks was then selected off. During my post flight inspection of the aircraft; I observed that the tks system fluid supply was completely depleted. Throughout the entire flight duration the cockpit tks gauge indicated full. Overall; the tks system seemed to be very effective in keeping the aircraft mostly clear of ice accumulation; but the limitations of its 'high' mode is to me; somewhat concerning. Granted; an icing event such as this is an unusual occurrence for this geographical area; but this proves that it does indeed happen and the endurance of the tks system; in 'high' mode proved to meet the demand. However; if the weather at my destination had dropped below approach minimums while I was being vectored for the second approach; or if a diversion to another airport had become necessary; for any reason; this would have necessitated the declaration of an emergency as there would not have been any ice protection remaining. Therefore; while flights are normally limited in duration due to fuel supply and regulatory requirements; tks brings with it the same considerations concerning its fluid supply while planning and conducting flight operations in icing conditions. This means that my future selection of an alternate airport should be within the tks 'high mode' fluid flow range as well as required fuel range and depending on meteorological conditions; this may not be possible. This; combined with the lack of an adequate operational cockpit tks gauge; causes me to remain very skeptical about the reliance on the tks system to provide a safe flight operation when escape from icing conditions may not be available. In such meteorological conditions; the need to constantly recalculate remaining tks fluid quantity and endurance creates a huge distraction from other flight duties and at least significantly increases pilot workload. Also; regarding the sight gauge in the tks fluid tank; it lacks adequate markings to accurately indicate fluid quantities between 11.7 and 20.8 gallons; andi am told that even if the 'ball' is at the top of the sight gauge; the system may still not be completely full. Depending on the slope and area of the parking ramp; an accurate reading using the sight gauge may not be possible; leaving determination of fluid quantity to be a 'west.a.G.' at best. I understand that it is for this reason that pilots must always have the system topped off prior to the aircraft's next departure; but this too could prove to be problematic and may cause operational delays. A larger tks fluid tank capacity should be available to provide a safe operational margin when the conditions herein described occur. In this day and age of engineering and technology; there is no valid reason that an accurate tks cockpit gauge can't be developed or installed. The fact remains that the tks cockpit gauge is a part of a system installed in the aircraft and by regulation; like all other equipment installed in the aircraft; it must either be operational; or MEL provisions for its inoperative status be available.

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Original NASA ASRS Text

Title: A C208 pilot reported the TKS anti-icing system quantity indication system was unreliable; and because the HIGH mode was selected during prolonged moderate conditions the fluid was exhausted during flight. Reporter stated quantity gauge read 'full' for the entire flight.

Narrative: Prior to flight; the TKS system was topped off and the sight gauge checked to ensure the system was full. The cockpit TKS gauge also indicated full. Forecast Icing charts (CIP/FIP) were reviewed. Widespread pockets of SLD were forecast throughout the southwestern; central and eastern area of the state for all altitudes up to 21;000 FT. Forecast icing severity showed light to occasional moderate icing throughout the same aforementioned coverage area and altitudes with heavier icing conditions for altitudes above 7;000 FT. The aircraft was deiced using types 1 and 4 fluids. Prior to start of takeoff roll from runway the TKS Normal mode was selected ON. About six minutes after takeoff while climbing from 4;000 FT through 6;000 FT; moderate icing was encountered and the TKS 'HIGH' mode was selected. Upon reaching my final requested altitude of 7;000 FT; the icing conditions returned to light intensity and TKS was placed back in 'Normal' mode. A short while later the icing conditions again became moderate in intensity and 'High' mode was again selected. This scenario repeated itself over an 18 minute period. About twenty five minutes after takeoff the icing conditions changed to moderate intensity and the TKS had to remain in 'High' mode for the remainder of the flight. On approximately three occasions during the flight; 'MAX FLOW Airframe' had to be briefly used. While being vectored for the ILS Runway 24 approach the Yellow Caution light on the TKS illuminated; but moderate rime icing conditions remained and the continued use of 'High' mode was required to keep the aircraft nearly free of ice accumulation. During the initial ILS approach; at 2;600 FT MSL; I experienced a false glide slope indication and I executed the published missed approach procedure. I was then given vectors for another ILS approach which was completed successfully. After landing and while turning clear of the runway; the Red Warning light of the TKS System illuminated and TKS was then selected OFF. During my post flight inspection of the aircraft; I observed that the TKS system fluid supply was completely depleted. Throughout the entire flight duration the cockpit TKS gauge indicated full. Overall; the TKS system seemed to be very effective in keeping the aircraft mostly clear of ice accumulation; but the limitations of its 'HIGH' mode is to me; somewhat concerning. Granted; an icing event such as this is an unusual occurrence for this geographical area; but this proves that it does indeed happen and the endurance of the TKS system; in 'HIGH' mode proved to meet the demand. However; if the weather at my destination had dropped below approach minimums while I was being vectored for the second approach; or if a diversion to another airport had become necessary; for any reason; this would have necessitated the declaration of an emergency as there would not have been any ice protection remaining. Therefore; while flights are normally limited in duration due to fuel supply and regulatory requirements; TKS brings with it the same considerations concerning its fluid supply while planning and conducting flight operations in icing conditions. This means that my future selection of an alternate airport should be within the TKS 'HIGH Mode' fluid flow range as well as required fuel range and depending on meteorological conditions; this may not be possible. This; combined with the lack of an adequate operational cockpit TKS gauge; causes me to remain very skeptical about the reliance on the TKS System to provide a safe flight operation when escape from icing conditions may not be available. In such meteorological conditions; the need to constantly recalculate remaining TKS fluid quantity and endurance creates a huge distraction from other flight duties and at least significantly increases pilot workload. Also; regarding the sight gauge in the TKS Fluid Tank; it lacks adequate markings to accurately indicate fluid quantities between 11.7 and 20.8 gallons; andI am told that even if the 'ball' is at the top of the sight gauge; the system may still not be completely full. Depending on the slope and area of the parking ramp; an accurate reading using the sight gauge may not be possible; leaving determination of fluid quantity to be a 'W.A.G.' at best. I understand that it is for this reason that pilots must always have the system topped off prior to the aircraft's next departure; but this too could prove to be problematic and may cause operational delays. A larger TKS fluid tank capacity should be available to provide a safe operational margin when the conditions herein described occur. In this day and age of engineering and technology; there is no valid reason that an accurate TKS cockpit gauge can't be developed or installed. The fact remains that the TKS cockpit gauge is a part of a system installed in the aircraft and by regulation; like all other equipment installed in the aircraft; it must either be operational; or MEL provisions for its inoperative status be available.

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.