portable oxygen system. High performance and air transport  category aircraft often have built-in oxygen systems that  contain plumbing designed to refill gaseous oxygen cylinders  while they are in place. A general discussion of the procedure  to fill this type of installation follows. under pressure flow from high pressure to low pressure.  Thus, by connecting the cart to the aircraft and systematically  opening oxygen cylinders with increasingly higher pressure, a  slow increase in oxygen flow to the aircraft can be managed.  The following is a list of steps to safely fill an aircraft oxygen  system from a typical oxygen refill cart. Before charging any oxygen system, consult the aircraft  manufacturer’s maintenance manual. The type of oxygen  to be used, safety precautions, equipment to be used, and  the procedures for filling and testing the system must  be observed. Several general precautions should also be  observed when servicing a gaseous oxygen system. Oxygen  valves should be opened slowly, and filling should proceed  slowly to avoid overheating. The hose from the refill source  to the oxygen fill valve on the aircraft should be purged of air  before it is used to transfer oxygen into the system. Pressures  should also be checked frequently while refilling. Airline and fixed-base operator maintenance shops often use  oxygen filler carts to service oxygen systems. These contain  several large oxygen supply cylinders connected to the fill cart  manifold. This manifold supplies a fill hose that attaches to  the aircraft. Valves and pressure gauges allow awareness and  control of the oxygen dispensing process.  [Figure 16-31]  Be  sure all cylinders on the cart are aviator’s breathing oxygen  and that all cylinders contain at least 50 psi of oxygen pressure.  Each cylinder should also be within its hydrostatic test date  interval. After a cart cylinder has dispensed oxygen, the  remaining pressure should be recorded. This is usually written  on the outside of the cylinder with chalk or in a cylinder  pressure log kept with the cart. As such, the technician can  tell at a glance the status of each oxygen bottle.  No pump or mechanical device is used to transfer oxygen  from the fill cart manifold to the aircraft system. Objects  16-18 Figure 16-31.  Typical oxygen servicing cart used to fill an aircraft  system.  12. Disconnect the refill line from the refill port on the  aircraft. Cap or cover both. 11. The aircraft oxygen cylinder valve(s) should be left in  the proper position for normal operations. Remotely  mounted cylinders are usually left open. 10. Close the aircraft refill valve and close all cylinders  on the cart.  9.  Continue the procedure in step 8 until the desired  pressure in the aircraft cylinder is achieved. 8.  When the cylinder pressures equalize, close the  cylinder on the cart, and open the cart cylinder with  the next highest pressure. Allow it to flow into the  aircraft cylinder until the pressures equalize and flow  ceases. Close the cart cylinder and proceed to the cart  cylinder with the next highest pressure. 7.  Open the aircraft oxygen system refill valve. Oxygen  will flow from cart cylinder (manifold) into the aircraft  cylinder. 6.  Observe the pressure on the aircraft bottle to be filled.  Open it. On the refill cart, open the cylinder with the  closest pressure to the aircraft cylinder that exceeds it. 5.  Connect the cart hose from the cart manifold to the  aircraft fill port. Purge the air from the refill hose with  oxygen before opening the refill valve on the aircraft.  Some hoses are equipped with purge valves to do this  while the hose is securely attached to the aircraft.  Other hoses need to be purged while attached to the  refill fitting but not fully tightened.  4.  Ground the refill cart to the aircraft. 3.  Take all oxygen handling precautions to ensure a safe  environment around the aircraft. 2.  Check pressures on all cylinders on the cart and  in the aircraft. If pressure is below 50 psi, replace  the cylinder(s). On the aircraft, this may require  purging the system with oxygen when completed.  Best practices dictate that any low-pressure or empty  cylinder(s) on the cart should also be removed and  replaced when discovered.  1.  Check hydrostatic dates on all cylinders, especially  those that are to be filled on the aircraft. If a cylinder  is out of date, remove and replace it with a specified  unit that is serviceable.