飞机燃油系统中的结冰是由于燃油中存在溶解和未溶解的水。在特定系统中，溶解水或碳氢化合物燃料溶液中的水在总水势中所占比例相对较小，溶解量主要取决于燃料温度和燃料的水溶性特征。未溶解水的一种情况是夹带水，例如，由于自由水的机械搅拌或通过温度降低使溶解水转化而悬浮在燃料中的水颗粒。这可被视为类似于乳液状态。未溶解水的另一种情况是自由水，这可能是由于加油或夹带水的沉淀而引入的，夹带水在燃油箱底部聚集，容易被检测到，通过连续界面与上面的燃油分离。 空气通过通风系统进入燃油箱时产生的冷凝也可能会引入水。假设提升的燃油质量良好，通过飞机通风系统的蒸汽是一种重要的水引入机制。夹带的水在静态条件下会及时沉淀，根据其转化为自由水的速率，可能会或可能不会被排出。一般来说，在野外条件下，不可能将所有夹带的水从燃料中分离出来。沉降速率取决于一系列因素，包括温度、静止和液滴大小。液滴的大小取决于形成机理。通常，颗粒很小，肉眼看不见，但在极端情况下，可能会导致燃油中出现轻微的朦胧。如果低点排放规定足够，并遵循建议的维护措施，则可以从燃油箱中排放游离水。 溶液中的水不能被除去，除非通过脱水或通过降温将其转化为夹带水，然后再转化为游离水。在航空燃料中，水在溶液中并不是一个严重的问题，只要它仍在溶液中。夹带水和游离水问题最大，因为燃油系统表面可能结冰。此外，夹带的水将在冷燃料中冻结，并倾向于在溶液中停留更长时间，因为冰的比重与碳氢燃料的比重大致相同。在实际可行的范围内，在燃料储存、处理和输送系统以及飞机燃料系统中消除不溶水，可以减少或消除结冰问题的可能性。在受控结冰条件下对燃油系统、子系统和部件进行适当测试，可以建立飞机燃油系统在此类结冰条件下安全运行的信心。 测试的目的不一定是证明不会结冰，而是证明结冰的影响不会造成危险条件。本文讨论了控制潜在结冰问题的这些措施的注意事项。

Ice formation in aircraft fuel systems results from the presence of dissolved and undissolved water in the fuel. Dissolved water or water in solution with hydrocarbon fuels constitutes a relatively small part of the total water potential in a particular system with the quantity dissolved being primarily dependent on the fuel temperature and the water solubility characteristics of the fuel. One condition of undissolved water is entrained water, such as water particles suspended in the fuel as a result of mechanical agitation of free water or conversion of dissolved water through temperature reduction. This can be considered as analogous to an emulsion state. Another condition of undissolved water is free water which may be introduced as a result of refueling or the settling of entrained water which collects at the bottom of a fuel tank in easily detectable quantities separated by a continuous interface from the fuel above. Water may also be introduced as a result of condensation from air entering a fuel tank through the vent system. Assuming good quality of uplifted fuel, vapor passing through the aircraft vent system is a significant water introduction mechanism.Entrained water will settle out in time under static conditions and may or may not be drained, depending on the rate at which it is converted to free water. In general, it is not likely that all entrained water can ever be separated from fuel under field conditions. The settling rate depends on a series of factors including temperature, quiescence, and droplet size. The droplet size will vary depending upon the mechanics of formation. Usually the particles are so small as to be invisible to the naked eye, but in extreme cases can cause a slight haziness in the fuel.Free water can be drained from a fuel tank if low point drain provisions are adequate and recommended maintenance actions are followed. Water in solution cannot be removed except by dehydration or by converting it, through temperature reduction, to entrained, then to free water.Water strictly in solution is not a serious problem in aviation fuel so long as it remains in solution. Entrained and free water are the most problematic because of the potential of freezing on the surfaces of the fuel system. Further, entrained water will freeze in cold fuel and tend to stay in solution longer since the specific gravity of ice is approximately the same as that of hydrocarbon fuels.The elimination of undissolved water, to the extent it is practical, in fuel storage, handling, and delivery systems, as well as in aircraft fuel systems, can reduce or eliminate the potential for icing problems. Appropriate testing of fuel systems, subsystems, and components under controlled icing conditions can establish confidence in the safe operation of the aircraft fuel system in such icing conditions. The objective of testing is not necessarily to demonstrate that no icing will occur but rather that the effects of the icing will not create a hazardous condition. Considerations for these measures to control potential icing problems are addressed herein.