The three major operating expenses for most airlines are fuel, labor, and maintenance/ownership costs. Fuel is often the largest and most volatile expense, frequently accounting for 25–35% of total costs; it is high because it is tied to global oil prices and the massive energy requirements of lifting heavy aircraft. Labor (salaries for pilots, flight attendants, and ground crew) is the second-largest cost, driven by the specialized training required and strong union contracts. Maintenance and Ownership (including aircraft leases and repairs) are high because aviation parts must meet extreme safety standards and are produced by a limited number of global suppliers (like Boeing or Airbus), making them incredibly expensive. These costs are exacerbated by "fixed" elements like airport landing fees and government taxes, leaving airlines with very thin profit margins, often as low as 3–5%, making them highly sensitive to economic shifts or price spikes in any of these three categories.