per Title 14 of the Code of Federal Regulations (14 CFR)  part 91, section 91.319. Experimental balloons also require  specific documentation when flown outside of an area of 50  miles from its home port. Pilots of special shape balloons with  an Experimental Airworthiness Certificate should coordinate  their activities with their local Flight Service District Office  to avoid problems. A thermal airship combines the characteristics of a hot air  balloon, with respect to lifting force, and an airship, with  respect to its capacity of being steered while in flight. To  develop proficiency in this aircraft, knowledge is required  of not only hot air balloon operations and physics, but also  of airship operations. It is estimated that there are currently  less than 10 of these aircraft in the United States, and there  is no specific pilot certificate for thermal airships. These  aircraft are extremely expensive to purchase, and have some  significant operating limitations with reference to winds. Any  further discussion is outside the scope of this handbook.  The heater system consists of propane burners (one or more),  fuel tanks that store liquid propane, and fuel lines that carry  the propane from the tanks to the burners. The burners convert  ambient air into hot air, which in turn provides the lift required  for flight.  [Figures 2-6  and  2-7]  Figure 2-7.  Typical double heater (burner). 2-5 Heater System Thermal Airships Figure 2-6.  Typical single heater (burner). VALVE BLOCK  ASSEMBLY BURNER CAN COIL ASSEMBLY 6 5 4 3 2 1 4 1 6 5 4 3 2 1 6 5 LIQUID FIRE JET  ASSEMBLY IGNITOR ASSEMBLY PILOT LIGHT ASSEMBLY SINGLE BURNER CONFIGURATION DOUBLE BURNERS CONFIGURATION BURNER CAN 1 COIL ASSEMBLY 2 LIQUID FIRE JET ASSEMBLY 3 IGNITOR ASSEMBLY 4 PILOT LIGHT ASSEMBLY 5 COIL POST 6 LIQUID FIRE VALVE ASSEMBLY 7 HANDLE TUBE 8 MAIN VALVE ASSEMBLY 9 SWIVEL ASSEMBLY 10 GIMBAL BLOCK CAP LOWER 11 GIMBAL BLOCK CAP UPPER 12 PRESSURE GAUGE ASSEMBLY 13 GIMBAL TENSION ADJUSTMENT SCREW 14 GIMBAL MOUNTING SCREW 15 13 12 11 10 2 1 8 9 15 7 14 3 4 5 6