Oil tank Supply Pressure Vent Return Drain Oil Temp Oil Temp Oil Press. Oil Press. Scupper drain Oil tank vent Oil cooler Scavenger pump Oil pressure pump Engine breather Oil tank drain valve Flexible weighted internal hose assembly Oil pressure gauge Oil temperature gauge Hopper tank Baffles Figure 6-4.  Oil system schematic.  Figure 6-5.  Oil tank with hopper.  The opening at the bottom of the hopper in one type and  the flapper valve-controlled openings in the other allow oil  from the main tank to enter the hopper and replace the oil  consumed by the engine. Whenever the hopper tank includes  Most aircraft oil systems are equipped with the dipstick-type  quantity gauge, often called a bayonet gauge. Some larger  Generally, the return line in the top of the tank is positioned  to discharge the returned oil against the wall of the tank in a  swirling motion. This method considerably reduces foaming  that occurs when oil mixes with air. Baffles in the bottom  of the oil tank break up this swirling action to prevent air  from being drawn into the inlet line of the oil pressure pump.  Foaming oil increases in volume and reduces its ability to  provide proper lubrication. In the case of oil-controlled  propellers, the main outlet from the tank may be in the form  of a standpipe so that there is always a reserve supply of  oil for propeller feathering in case of engine failure. An oil  tank sump, attached to the undersurface of the tank, acts as a  trap for moisture and sediment.  [Figure 6-4]  The water and  sludge can be drained by manually opening the drain valve  in the bottom of the sump. the flapper controlled openings, the valves are operated by  differential oil pressure. By separating the circulating oil from  the surrounding oil in the tank, less oil is circulated. This  hastens the warming of the oil when the engine was started.  Very few of these types of tanks are still in use and most are  associated with radial engine installations. 6-6