stem out of the notch in the poppet stem. A spring behind  the poppet forces it back onto its seat. A characteristic of  solenoid-operated fuel valves is that they open and close  very quickly.  [Figure 14-45] center of the pump, allowing a back-and-forth motion to pump  the fuel rather than a full revolution of the vanes as is common  in electrically driven or engine-driven vane-type pumps. Figure 14-46  illustrates the mechanism found in a wobble  pump. As the handle is moved down from where it is shown,  the vane on the left side of the pump moves up, and the vane  on the right side of the pump moves down. As the left vane  moves up, it draws fuel into chamber A. Because chambers  A and D are connected through the bored center, fuel is also  drawn into chamber D. At the same time, the right vane forces  fuel out of chamber B, through the bored passage in the center  of the pump, into chamber C and out the fuel outlet through  the check valve at the outlet of chamber C. When the handle  is moved up again, the left vane moves down, forcing fuel  out of chambers A and D because the check valve at the inlet  of the A chamber prevents fuel from flowing back through  the fuel inlet. The right vane moves up simultaneously and  draws fuel into chambers B and C.  Other than aircraft with gravity-feed fuel systems, all aircraft  have at least one fuel pump to deliver clean fuel under pressure  to the fuel metering device for each engine. Engine-driven  pumps are the primary delivery device. Auxiliary pumps are  used on many aircraft as well. Sometimes known as booster  pumps or boost pumps, auxiliary pumps are used to provide  fuel under positive pressure to the engine-driven pump and  during starting when the engine-driven pump is not yet up  to speed for sufficient fuel delivery. They are also used to  back up the engine-driven pump during takeoff and at high  altitude to guard against vapor lock. On many large aircraft,  boost pumps are used to move fuel from one tank to another. There are many different types of auxiliary fuel pumps in  use. Most are electrically operated, but some hand-operated  pumps are found on older aircraft. A discussion of the various  pump types found in the aviation fleet follows. Some older reciprocating engine aircraft have been equipped  with hand-operated fuel pumps. They are used to back up the  engine-driven pump and to transfer fuel from tank to tank. The  wobble pumps, as they are known, are double-acting pumps  that deliver fuel with each stroke of the pump handle. They are  essentially vane-type pumps that have bored passages in the  While simple with little to go wrong, a hand-operated pump  requires fuel lines to be run into the cockpit to the pump,  creating a potential hazard that can be avoided by the use of an  electrically driven pump. Modern light reciprocating-engine  aircraft usually use electric auxiliary pumps, but they often  make use of a simple hand pump for priming the engine(s)  during starting. These simple devices are single-acting piston  pumps that pull fuel into the pump cylinder when the primer  knob is pulled aft. When pushed forward, the fuel is pumped  through lines to the engine cylinders.  [Figure 14-47]  Hand-Operated Fuel Pumps Fuel Pumps 14-28 Figure 14-46.  A hand-operated wobble pump used for engine  starting and fuel transfer on older transport category aircraft.  Figure 14-45.  A solenoid-operated fuel valve uses the magnetic  force developed by energized solenoids to open and close a poppet. Closing solenoid Spring Fuel in Fuel out Opening solenoid control wires Opening solenoid Spring Fuel in Fuel out D B C A