Most collections relating to the field of aviation focus on one of four categories—commercial airlines, dirigibles, famous aviators, or generic images of aircraft.
Early American airlines depended on government subsidies for carrying mail. By 1930, five international and 38 domestic airlines were operating in the United States. A typical passenger load was 10 people. After World War II, four-engine planes with a capacity of 100 or more passengers were introduced.
— Harry L. Rinker
“Official Price Guide to Collectibles”