The Depot-Level Maintenance of DoD's Combat Aircraft: Insights for the F-35

Intended to replace older models of aircraft used by the U.S. Air Force, Navy, and Marine Corps, the F-35 is a fighter aircraft with stealth capabilities that reduce the chance of detection by radar and heat-seeking missiles. The Department of Defense (DoD) plans to spend almost $350 billion (in fiscal year 2017 dollars) from 1994 through 2044 to develop and procure F-35s. The department expects to spend almost twice as much to operate and support the aircraft over their lifetimes; a sizable portion of that spending will go toward the aircraft’s depot-level maintenance. This report examines the depot-level maintenance experiences of aging combat aircraft currently in use by DoD and provides insights for the Congress and DoD to consider as the F-35 fleet enters service.

The depot-level maintenance of aircraft consists of in-depth maintenance that is beyond the capability of maintenance staff at aircraft’s operating locations—for example, disassembly, inspection, repair, rebuilding, repainting, and flight testing. Whereas aircraft receive maintenance at their operating locations throughout their service lives, depot-level maintenance is provided only intermittently.

Depot-level maintenance can be costly and time-consuming, and aircraft are unavailable to operators while the maintenance occurs. Nevertheless, adequate depot-level maintenance is essential to ensuring an aircraft’s safe operation and capability to perform missions. The extent of such maintenance can also influence whether an aircraft’s life can be extended cost-effectively.

Funding for depot-level maintenance is provided by the Congress through operation and maintenance (O&M) appropriations, as well as through procurement appropriations for aircraft modification programs. Should the decision be made to replace, rather than continue to maintain, an existing system, the Congress would also provide appropriations for the replacement.

Different approaches to depot-level maintenance have been used for different combat aircraft, and outcomes have differed among those aircraft as they have aged. Those prior experiences can inform decisions about F-35s’ depot-level maintenance, which will have long-term implications for the costs, availability, and longevity of the F-35 fleet. This report focuses on depot-level maintenance practices for the Air Force’s F-15, F-16, and A-10 combat aircraft as well as the Navy’s F/A-18 fighter aircraft as sources of insights for the F-35. Experience with those aircraft suggests that adequate depot-level maintenance throughout the F-35’s life should enhance the aircraft’s performance as it ages and make extending its service life more feasible.

How Has DoD Provided Depot-Level Maintenance for Combat Aircraft?

DoD takes varying approaches to scheduling depot-level maintenance. Generally, such maintenance is calendar-based (for example, occurring once every six years) or modification-based (for example, accomplished in conjunction with the fleetwide installation of an upgrade).

The Air Force’s F-15s receive a type of calendar-based maintenance called programmed depot maintenance (PDM). Under that system, the aircraft undergo depot-level maintenance on a regular basis, typically after each 61 to 72 months of operation. The Navy also uses a calendar-based system, called planned maintenance intervals, for its F/A-18 fighter aircraft.

Air Force F-16 fighter aircraft, by contrast, have received depot-level maintenance that is governed by orders for modifications or repairs. The F-16 program office coordinates with major commands to identify necessary modifications and repairs and plans the aircraft’s visits to depots accordingly. F-16s’ depot visits have been scheduled more irregularly, but also more frequently, than F-15s’.

The Air Force’s A-10 aircraft do not receive calendar-based or modification-based maintenance. Instead, A-10s follow a system known as risk-based scheduling, under which they visit depots on the basis of flying hours, with adjustments to account for stress to the airframe incurred during their missions.

How Have Different Combat Aircraft Performed As They Have Aged?

As the four types of combat aircraft have aged, they have all experienced declines in the percentage of their fleets that is available to operators and capable of performing missions. All four fleets have also experienced declines in annual flight hours per aircraft. Those declines have been particularly severe in recent years for older models (C/D variants) of the Navy’s F/A-18.

The F/A-18C/D fleet’s recent decreases in availability have resulted from lengthy depot-level maintenance on many of the aircraft. CBO's analysis suggests that the increased maintenance currently required by the aircraft may be attributable to a relative lack of depot-level maintenance earlier in their service lives. In the 1990s, F/A-18C/Ds received less O&M funding per flying hour and spent fewer hours in depot-level maintenance per flying hour than did the A-10s, F-15C/Ds, or F-16C/Ds.

What Are the Implications for the F-35’s Depot-Level Maintenance?

In recent years, F-15C/Ds (using PDM) and F-16C/Ds (using modification-based maintenance) have had higher availability rates than F/A-18C/Ds. This suggests that either approach could be successful for the F-35.

How much depot-level maintenance aircraft receive may be more important than how that maintenance is scheduled. The F/A-18C/D appears to have been undermaintained, and the Navy has had difficulty extending its service life. By contrast, the Air Force has been able to extend the life spans of its F-15 and F-16 fleets with smaller declines in their availability. Those observations suggest that sufficient depot-level maintenance throughout the F-35’s life should improve the aircraft’s performance as it ages and make extending its life span easier—which could delay the need to procure a replacement aircraft, resulting in long-term cost savings. However, increased depot-level maintenance would require greater O&M funding throughout the F-35’s life.