Helium is expensive but is more commonly used in the  United States while hydrogen is more common in Europe.  Helium provides slightly less lift than hydrogen but is the  more stable gas. (See subsection titled Flying in Inversions  on page 11-7 for a discussion of stability.) Balloon systems  must not be prone to generating static electricity if they are  to be used with hydrogen. There are at least six factors to consider when choosing a  lifting gas. These are:  Normally, the gas selection process starts with the type  of balloon. Hydrogen can be used if the balloon system  is hydrogen compatible, hydrogen is available, and local  ordinances allow its use. If one of these conditions is not  satisfied, as is often the case in the United States, then helium  is the likely choice. If a very short training or pleasure flight  of several hours is planned, the more economical alternative  of ammonia or methane (natural gas) may be considered. When inflating or landing with hydrogen or methane, care  must be taken to ensure that no flame or material with the  potential to generate sparks is present in the launch locale.  Lighted cigarettes, cigars, nylon clothing, cell phones, and  other electronic devices are examples of forbidden items.  Only essential personnel should be allowed in the launch and  landing areas. For long, competitive flights, the increased  stability of helium is a factor in its favor. Gas balloon systems can be broken into four parts: The maneuvering valve  [Figure 11-4]  is at the apex of the  envelope. It allows for the controlled release of a small  amount of gas to initiate a descent. It is usually spring  actuated and controlled via a line that runs from the valve  down through the envelope into the gondola. On some  The connection between envelope and gondola may be made  of rope, flat tape, or steel cable. The total strength of the  11-4 A trail rope is slung on the outside gondola along with much  of the remaining support equipment. The trail rope is typically  about 150 feet of natural fiber rope, one inch in diameter and  weighs about 40 pounds. Its use is described in the landing  paragraph.  Support Cabling Envelope $_{Figure 11-5.}$  Typical gondola layout. 4.  Other equipment 3.  Support system to connect the envelope to the  gondola 2.  Gondola for carrying pilots and equipment 1.  Envelope to contain the lifting gas Components of the Gas Balloon Gondola  balloons, a gas tight parachute top may be used instead of a  valve. The envelope is also equipped with either a rip panel  or deflation port for rapid, total deflation during high wind  landings. More information on proper use of the valve and  deflation ports is provided in the section The Practice of Gas  Ballooning, page 11-8. The gas balloon’s gondola  [Figure 11-5] is typically  somewhat larger than ones used for sport hot air ballooning,  with four by five feet being a typical size. A foldable cot or  sleeping pad normally runs along the long side of the gondola  and a “kick-out” panel in the side wall at one end of the cot  may be used to provide additional legroom for sleeping. 1.  Compatibility with the balloon system being used  2.  Cost  3.  Lifting capacity 4.  Availability  5.  Locale of flight  6.  Inherent gas stability $_{Figure 11-4.}$  Maneuvering valve on quick fill gas ballon.