Carburetor Ice Flight Testing: Use of an Anti-Icing Fuel Additive

Abstract

THE effectiveness of an anticarbureto r-icing fuel additive was tested in-flight in a light twin-engine airplane. Fuel containing 0.15% ethylene glycol monomethyl ether was used in one while the other used stock aviation gasoline. The results show that the additive was very effective in preventing carburetor ice during cruise. Both the maximum severity of the icing and the range of environmental conditions conducive to its formation were reduced. During descents, the results are inconclusive. While the average rate of carburetor ice formation was reduced with the additive, it did not appear to be effective under certain conditions of temperature and dew point. Contents Carburetor ice has historically been a major cause of light airplane stoppages. Caused by the cooling of moist air within the carburetor, ice formation disrupts the flow of fuel and air to the engine. This cooling is a result of the expansion of the air as it passes through the carburetor venturi and around the throttle butterfly and is augmented by vaporization cooling as the fuel is added to the airstream. The static temperature drop can reach 70°F. During the period 1969-1975, the National Transportation Safety Board (NTSB) files show a total of 468 accidents where the probable cause was cited as engine failure— carburetor/induction system icing. These accidents caused 44 fatalities, 202 serious injuries, and destroyed 75 aircraft. Surprisingly, 62% of these accidents occurred during climb and cruise. Only 26% happened during the supposedly carburetor-ice prone phases of flight: descent and landing. In addition to these accidents where carburetor ice has been cited, there are approximately 180 accidents per year caused by stoppages where the specific cause of the stoppage was not determined. It has been suggested that many of these accidents might actually be caused by carburetor ice. A review of one year's undetermined failure accidents to Cessna 150 aircraft showed that 43% exhibited all of the symptoms of carburetor ice—the power loss followed a carburetor ice pattern, weather conditions were conducive, and the pilot failed to use carburetor heat. In 29% of the accidents, only two of these factors were present, so carburetor ice can only be considered probable. Based on these observations, we conclude that approximately one-half of the Cessna 150 carburetor ice accidents are not identified as such by the NTSB. By extension to other types, carburetor ice statistics should be scaled up by a factor of about two. While the problem remains a serious one, preventive measures have changed little since the NACA work of the 1940's.1>2 The conventional approach to protect engines from carburetor ice is an exhaust heat-exchanger to preheat the