Propane fuel is used to heat the air which generates buoyancy  for flight. The propane is stored in one or more fuel tanks  located in the basket. A withdrawal tube attached to the liquid  tank valve permits liquid propane to be drawn from the bottom  of the fuel tanks. The liquid propane is supplied to the burner  assembly through the fuel hoses that connect the fuel tanks to  the heater assembly (commonly referred to as the burner). The  fuel system also provides propane to the pilot light.  There are two types of pilot light systems: liquid and vapor.  In a liquid pilot light system, liquid propane is diverted from  the main supply line at the heater via a pilot shut-off valve.  The fuel goes through a vapor converter and regulator, and  is distributed through a pilot head. A piezo-electric system  ignites the vapor at inflation for most heaters, but many  balloonists choose to use a striker. In a vapor pilot light, a second fuel hose is used to supply  vapor to the heater assembly pilot light from the pilot light  tank valve located on top of the fuel tank. A regulator is used  to decrease the pressure of the propane vapor for proper pilot  operation. A pilot light valve located on the heater controls  the flow of propane vapor to the pilot light.  The main liquid tank valve controls the flow of liquid propane  to the burner, while the blast valve controls fuel flow at the  heater. With the liquid tank valve open, opening the burner  blast permits liquid propane to enter the heat exchange coil  where it is either completely or partially vaporized. After  exiting the heat exchange coil through the orifices in the  lower portion of the coil, the propane is ignited by the pilot  light.  [Figure 2-8] To meet redundancy requirements, heaters have a secondary  system which allows for operation at a reduced efficiency  should a problem develop with the main blast valve. These  backup, or bypass systems generally have proprietary names  unique to the individual manufacturers. Pilots should consult  individual flight manuals for an explanation of their use. The heaters typically have an output of approximately 20  million British thermal units (BTU) in use. There is a power  loss associated with altitude, generally considered to be four  percent per thousand feet of altitude. This is particularly  important when dealing with higher density altitudes.  Pilots accustomed to flying at lower altitudes are frequently  surprised by the decreased performance of their balloon upon  their first flight at a higher altitude. Wicker is the preferred material for the passenger compartment  basket of a hot air balloon because it is sturdy, flexible, and  relatively lightweight.  [Figure 2-9]  The flexibility of wicker  Basket Figure 2-9.  Representative sport basket configuration. 2-6 Figure 2-8.  Fuel system schematic. 200 140 120 100 0 250 PSI 0 40 60 80 160 180 Pilot valve Pressure gauge Pilot light Liquid valve Blast valve Nozzle Heat exchangers