Fleet age directly imposes fuel-cost penalties on US single-aisle operators, with legacy carriers absorbing double-digit burn penalties compared to newer fleets. The Running Fuel Efficiency Index confirms fleet age is a price, not a number, as older aircraft consume significantly more fuel, creating a structural cost disadvantage. Fuel represents 31% of total spend, and the efficiency gap between aging legacy fleets and modern competitors will not close without major renewal. Supply chain delays leave a shortfall of over 5,000 fuel-efficient replacements, forcing operators to retain older planes. Legacy carriers, typically hedged 30–50% for 2026, still face persistent efficiency gaps regardless of hedging, undermining profitability and competitive positioning.