Many tourism sectors utilize capital equipment which is indivisible, aircraft being obvious examples. The firm often posses a set amount of equipment and thus fixed capacity over the short run and incurs fixed costs in maintaining it; it can be altered only in the long run where re-capitalization occurs. As capital is used, variable costs arise, for example, in the form of fuel and staff costs in the case of air transport. In the short run, economics argues that a firm, even if does not cover its total cost, should continue to trade as long as it covers its variables costs. However, in the long run the firm must generate sufficient revenue to cover both its fixed and variable costs if it is to remain in business. There may also be sunk costs which are not recoverable if the firm leaves the sector. They normally relate to indivisible inputs and can affect the contestability of the market as they deter entry into a sector if wholly irrecoverable.