Aviation insurance is usually written by carrier pools or individual companies that specialize in this business. Most coverage is written for general aviation, a diverse industry that includes all forms of flying except commercial airlines and military operations. The four major classes of aircraft owners/operators are industrial aid, business and pleasure, flying clubs, and fixed base operators. Aircraft hull and liability coverage is similar to the coverage an automobile policy provides because it includes physical damage, liability, and medical payments coverages. Under hull insurance, the two types of coverage available are physical damage coverage when the aircraft is not in flight and physical damage coverage when it is. Liability coverage is similar to automobile liability, except that it is divided into two separate insuring agreements, one for bodily injury excluding passenger liability, and the other for passenger bodily injury liability. Medical payments coverage may be added if passenger liability coverage is provided.