some of the challenges experienced by commercial aircraft and then discusses other strategies for improved fuel efficiency. Since the preceding NRC report dealt with improving engine efficiency, an important determinant of fuel consumption, that strategy is not covered here.³

The aging and service use of commercial aircraft and jet engines take a toll, reducing aerodynamic and propulsion efficiency, as evidenced by increased fuel burn. As aircraft age and material wears, or suffers minor damage, fuel efficiency tends to decline because of external repairs, increased air leakage from the fuselage, weight gain from the entry of moisture and from years of modification programs, and engine deterioration. It is common for new commercial aircraft types to experience fuel burn increases over the specification (or “book”) level of 2-4 percent within 4 years of entry into service. The regulatory agencies and internal technical organizations that certify continued airworthiness set the allowable in-service expansion of the original by tight manufacturing tolerances to accommodate the effects of normal wear and tear on commercial machinery.

Then, too, owners and operators of aircraft often push the performance limits of their equipment to achieve greater payload, range, endurance, or takeoff performance. Regardless of the specifications that prevailed when the aircraft were procured, political, regulatory, economic, or demographic influences open up prospects for new missions or markets that lie tauntingly just beyond the existing capabilities of existing in-service aircraft. Aircraft operators must then either seek new equipment with the required performance or attempt to improve the performance of existing equipment, through modification, to accommodate those new missions and markets. Specific strategies to take on these challenges are discussed below.

Lessons learned from the commercial airplane industry suggest that aerodynamic improvements using strategies other than wingtip modification are worth consideration for the Air Force’s fleet of aircraft. Many of the its transport aircraft were designed in the early days of swept-wing trans-