Hence, a pump of adequate capacity at all engine operating  conditions has excess capacity over most of the range of  operation. This is the characteristic that requires the use of  a pressure relief valve for bypassing excess fuel back to the  inlet. A typical two-stage turbine engine driven pump is  illustrated in  Figure 2-60 . The impeller, which is driven at a  greater speed than the high pressure elements, increases the  fuel pressure depending upon engine speed. tachometer reading is divided by the percent trim speed  obtained from the curve. The result is the new engine trim  speed in percent, corrected to standard day (59 °F or 15 °C)  temperature. The new trim speed in rpm may be calculated  when the rpm at which the tachometer reads 100 percent is  known. This value may be obtained from the appropriate  engine manual. If all these procedures have been performed  satisfactorily, the engine has been properly trimmed. Engine trimming should always be carried out under precisely  controlled conditions with the aircraft headed into the wind.  Precise control is necessary to ensure maintenance of a  minimum thrust level upon which the aircraft performance  is based. In addition, precise control of engine trimming  contributes to better engine life in terms of both maximum  time between overhaul and minimum out-of-commission  time due to engine maintenance requirements. Engines should  never be trimmed if icing conditions exist. The fuel is discharged from the boost element (impeller)  to the two high-pressure gear elements. A relief valve is  incorporated in the discharge port of the pump. This valve  opens at a predetermined pressure and is capable of bypassing  the total fuel flow. This allows fuel in excess of that required  for engine operation at the time to be recirculated. The bypass  fuel is routed to the inlet side of the second stage pump.  Fuel flows from the pump to the fuel metering unit or fuel  control. The fuel control is often attached to the fuel pump. The fuel pump is also lubricated by the fuel passing through  the pump, and it should never be turned without fuel flow  supplied to the inlet of the pump. As the engine coasts down  at shutdown, the fuel pump should be provided with fuel until  it comes to a stop. Most electronic control fuel control systems do not require  trimming or mechanical adjustments. Changes to the EEC  in the FADEC system is normally accomplished through  software changes or changing the EEC. Gas turbine engine fuel systems are very susceptible to the  formation of ice in the fuel filters. When the fuel in the aircraft  fuel tanks cools to 32 °F or below, residual water in the fuel  tends to freeze, forming ice crystals. When these ice crystals  in the fuel become trapped in the filter, they block fuel flow to  the engine, which causes a very serious problem. To prevent  this problem, the fuel is kept at a temperature above freezing.  Warmer fuel also can improve combustion, so some means  of regulating the fuel temperature is needed. Main fuel pumps deliver a continuous supply of fuel at  the proper pressure and at all times during operation of the  aircraft engine. The engine-driven fuel pump must be capable  of delivering the maximum needed flow at appropriate  pressure to obtain satisfactory nozzle spray and accurate  fuel regulation. These engine driven fuel pumps may be divided into two  distinct system categories: Their use depends on where in the engine fuel system they  are used. Generally, a nonpositive displacement (centrifugal  pump) is used at the inlet of the engine-driven pump to provide  positive flow to the second stage of the pump. The output of  a centrifugal pump can be varied as needed and is sometimes  referred to as a boost stage of the engine-driven pump. The second or main stage of the engine-driven fuel pump  for turbine engines is generally a positive displacement  type of pump. The term “positive displacement” means that  the gear supplies a fixed quantity of fuel to the engine for  every revolution of the pump gears. Gear-type pumps have  approximately straight line flow characteristics, whereas fuel  requirements fluctuate with flight or ambient air conditions.  Figure 2-60.  Dual element fuel pump.  Fuel Heater Main Fuel Pumps (Engine Driven) Engine Fuel System Components 2-43 1.  Constant displacement 2.  Nonconstant displacement