14-51 Figure 14-94.  A hand-held refractometer with digital display  measures the amount of fuel anti-ice additive contained in a fuel load.  Figure 14-93.  A sump drain tool used to open and collect fuel and  contaminants from the fuel system sumps. Daily sump draining is  part of the procedures needed to remove water from fuel that is to  be delivered to the engine(s). Checking for Fuel System Contaminants Water aviation. Personnel handling fuel or maintaining fuel systems  should be properly trained and use best practices to ensure  that the fuel, or fuel system, are not the cause of an incident  or accident.  Continuous vigilance is required when checking aircraft  fuel systems for contaminants. Daily draining of strainers  and sumps is combined with periodic filter changes and  inspections to ensure fuel is contaminant free. Turbine  powered engines have highly refined fuel control systems  through which flow hundreds of pounds of fuel per hour  of operation. Sumping alone is not sufficient. Particles are  suspended longer in jet fuel due to its viscosity. Engineers  design a series of filters into the fuel system to trap foreign  matter. Technicians must supplement these with cautious  procedures and thorough visual inspections to accomplish  the overall goal of delivering clean fuel to the engines.  Keeping a fuel system clean begins with an awareness of the  common types of contamination. Water is the most common.  Solid particles, surfactants, and microorganisms are also  common. However, contamination of fuel with another fuel  not intended for use on a particular aircraft is possibly the  worst type of contamination.  Water can be dissolved into fuel or entrained. Entrained  water can be detected by a cloudy appearance to the fuel.  Close examination is required. Air in the fuel tends to cause  a similar cloudy condition but is near the top of the tank.  The cloudiness caused by water in the fuel tends to be more  towards the bottom of the tank as the water slowly settles out.  As previously discussed, water can enter a fuel system via  condensation. The water vapor in the vapor space above the  liquid fuel in a fuel tank condenses when the temperature  changes. It normally sinks to the bottom of the fuel tank  into the sump where it can be drained off before flight.  [Figure 14-93]  However, time is required for this to happen.  On some aircraft, a large amount of fuel needs to be drained  before settled water reaches the drain valve. Awareness  of this type of sump idiosyncrasy for a particular aircraft  is important. The condition of the fuel and recent fueling  practices need to be considered and are equally important. If  the aircraft has been flown often and filled immediately after  flight, there is little reason to suspect water contamination  beyond what would be exposed during a routine sumping.  An aircraft that has sat for a long period of time with partially  full fuel tanks is a cause of concern.  Note that the fuel anti-ice additive level should be monitored  so that recommended quantity for the tank capacity is  maintained. After repeated fueling, the level can be obscured.  A field hand-held test unit can be used to check the amount  of anti-ice additive already in a fuel load.  [Figure 14-94] It is possible that water is introduced into the aircraft fuel  load during refueling with fuel that already contains water.  Any suspected contamination from refueling or the general  handling of the aircraft should be investigated. A change  in fuel supplier may be required if water continues to be  an issue despite efforts being made to keep the aircraft fuel  tanks full and sumps drained on a regular basis. Note that fuel  below freezing temperature may contain entrained water in  ice form that may not settle into the sump until melted. Use  of an anti-icing solution in turbine fuel tanks helps prevent  filter blockage from water that condenses out of the fuel as  ice during flight.