combustion chambers are the can type, fuel drains by gravity  down through the flame tubes or interconnector tubes until  it gathers in the lower chambers, which are fitted with drain  lines to the drain valve. If the combustion chamber is of the  basket or annular type, the fuel merely drains through the air  holes in the liner and accumulates in a trap in the bottom of  the chamber housing, which is connected to the drain line. After the fuel accumulates in the bottom of the combustion  chamber or drain lines, the drain valve allows the fuel to  be drained whenever pressure within the manifold or the  burner(s) has been reduced to near atmospheric pressure.  A small spring holds the valve off its seat until pressure in  the combustion chamber during operation overcomes the  spring and closes the valve. The valve is closed during engine  operation. It is imperative that this valve be in good working  condition to drain accumulated fuel after each shutdown.  Otherwise, a hot start during the next starting attempt or an  afterfire after shutdown is likely to occur. Fuel quantity units vary from one installation to the next. A  fuel counter or indicator, mounted on the instrument panel,  is electrically connected to a flowmeter installed in the fuel  line to the engine. The fuel counter, or totalizer, is used to keep record of fuel  use. When the aircraft is serviced with fuel, the counter is  manually set to the total number of pounds of fuel in all  tanks. As fuel passes through the measuring element of the  flowmeter, it sends electrical impulses to the fuel counter.  These impulses actuate the fuel counter mechanism so that  the number of pounds passing to the engine is subtracted  from the original reading. Thus, the fuel counter continually  shows the total quantity of fuel, in pounds, remaining in the  aircraft. However, there are certain conditions that cause the  fuel counter indication to be inaccurate. Any jettisoned fuel  is indicated on the fuel counter as fuel still available for use.  Any fuel that leaks from a tank or a fuel line upstream of the  flowmeter is not counted. Fuel Quantity Indicating Units 2-48