As of May 2015, based on trailing 12-month data, the average long-term debt/equity ratio of airline companies is 91.53. Airline companies are included in the major airlines and regional airlines subsectors, which are a part of the transportation sector.

The debt/equity ratio measures a company's financial stability and leverage and is calculated by dividing a company's total liabilities by its shareholders' equity. If a company has a high debt/equity ratio, it typically indicates that the company has a high debt level per each dollar of shareholders' equity. Therefore, investors favor companies with low debt/equity ratios.

The average long-term debt/equity ratio of companies in the major airline industry is 104.89, which indicates that for every $1 of shareholders' equity, the average company in the industry has $104.89 in total liabilities. Since the major airline industry is highly capital-intensive, companies in this industry tend to have high debt/equity ratios.

Similarly, the average long-term debt/equity ratio of companies in the regional airline industry is 78.16, indicating that the average company in the regional airline industry has $78.16 in debt per $1 of shareholders' equity.

The average of the long-term debt/equity ratios of companies in the airline sector is 91.53, or (104.89 + 78.16) / 2. This average includes the long-term debt/equity ratios of large-, mid- and small-cap companies. Delta Air Lines Inc. has a long-term debt/equity ratio of 105.82; American Airlines Group Inc. has 684.59; United Continental Holdings, 408.44; Spirit Airlines Inc., 30.39; and Virgin America Inc., 28.39. American Airlines and United Continental are using high debt levels to finance their growth compared to the overall sector.