Fueling and Defueling Procedures Maintenance technicians are often asked to fuel or defuel  aircraft. Fueling procedure can vary from aircraft to  aircraft. Tanks may need to be fueled in a prescribed  sequence to prevent structural damage to the airframe. The  proper procedure should be confirmed before fueling an  unfamiliar aircraft.  Always fuel aircraft outside, not in a hangar where fuel vapors  may accumulate and increase the risk and severity of an  accident. Generally, there are two types of fueling process:  over-the-wing refueling and pressure refueling. Over-the- wing refueling is accomplished by opening the fuel tank  cap on the upper surface of the wing or fuselage, if equipped  with fuselage tanks. The fueling nozzle is carefully inserted  into the fill opening and fuel is pumped into the tank. This  process is similar to the process used to refuel an automobile  gas tank. When finished, the cap is secured, and subsequent  tanks are opened and refilled until the aircraft has the desired  fuel load onboard. Pressure refueling occurs at the bottom,  front, or rear of the fuel tank. A pressure refueling nozzle  locks onto the fueling port at the aircraft fueling station. Fuel  is pumped into the aircraft through this secured and sealed  connection. Gauges are monitored to ascertain when the tanks  are properly loaded. An automatic shutoff system may be  part of the aircraft system. It closes the fueling valve when  the tanks are full.  [Figure 14-103] Fueling Most importantly, the choice of the correct fuel for an aircraft  should never be in question. No one should ever put a fuel  into an aircraft fuel tank unless absolutely certain it is the  correct fuel for that aircraft and its engine(s). Personnel  involved in fuel handling should be properly trained. All  potential contamination situations should be investigated  and remedied. Precautions should be used with either type of fueling.  First and foremost, it is absolutely essential that the correct  fuel be put in the aircraft. The type of fuel to be used is  placarded near the fill port on over-the-wing systems and at  the fueling station on pressure refueled aircraft. If there is  any question about which fuel to use, the pilot in command,  other knowledgeable personnel, or the manufacturer’s  Onboard aircraft fuel systems must be maintained and  serviced according to manufacturer’s specifications. Samples  from all drains should be taken and inspected on a regular  basis. Filters should be changed at the specified intervals.  The fuel load should be visually inspected and tested from  time to time or when there is a potential contamination  issue. Particles discovered in filters should be identified and  investigated if needed. Inspection of the fuel system during  periodic inspections should be treated with highest concern. Contamination control begins long before the fuel gets  pumped into an aircraft fuel tank. Many standard petroleum  industry safeguards are in place. Fuel farm and delivery truck  fuel handling practices are designed to control contamination.  Various filters, testing, and treatments effectively keep fuel  contaminant free or remove various contaminants once  discovered. However, the correct clean fuel for an aircraft  should never be taken for granted. The condition of all storage  tanks and fuel trucks should be monitored. All filter changes  and treatments should occur regularly and on time. The fuel  supplier should take pride in delivering clean, contaminant- free fuel to its customers. with clean fuel of the correct type. Many contaminants, both soluble and insoluble, can contaminate an aircraft’s fuel  supply. They can be introduced with the fuel during fueling  or the contamination may occur after the fuel is onboard. Figure 14-102.  Laboratory tests of fuel samples are available. Figure 14-103.  A float switch installed in a fuel tank can close the  refueling valve when the tanks are full during pressure fueling of an  aircraft. Other more sophisticated automatic shutoff systems exist. 14-56