Availability and Use of Aircraft in the Air Force and Navy

This report shows trends in the availability and use of the Air Force’s and Department of the Navy’s (DoN’s) aircraft since 2001. (DoN includes the Navy and the Marine Corps, whose aircraft were not separated in the data the Congressional Budget Office analyzed.) The report also shows how the military services’ aircraft have performed since the March 2020 onset of the coronavirus pandemic.

How CBO and the Department of Defense Measure Availability Rates

In this analysis, CBO uses a measure of a fleet’s availability rate—the percentage of time its aircraft are fit to fly missions for operating squadrons—that differs from the measure used by the Department of Defense (DoD). Both CBO and DoD use data from the Air Force’s Reliability and Maintainability Information System (REMIS) and DoN’s Decision Knowledge Programming for Logistics Analysis and Technical Evaluation (DECKPLATE) to calculate average availability rates. The two databases track the monthly performance of individual aircraft, including flying hours and the number of hours aircraft are “mission capable” (the term REMIS and DECKPLATE use for “available”).

CBO and DoD define aircraft as available if they are identified in the databases as both “mission capable” and “possessed by operators”—that is, not currently undergoing depot-level maintenance or in storage. (For discussion of other ways to measure availability, see the appendix.)

The availability rate of a fleet can be measured by dividing the number of hours that aircraft are available in a month by the total number of aircraft hours in that month. DoD and CBO use different measures of total hours. In defining total hours, DoD excludes the hours aircraft spend in depot-level maintenance or storage. DoD’s availability-rate formula is:

DoD’s measurement captures only the availability rate of aircraft in operational squadrons. For example, it would not account for an unusually large backlog of aircraft in a modification program at a depot.

CBO uses the same numerator as DoD (the number of hours that aircraft are both mission capable and in the possession of operational squadrons) and divides it by total possible hours of availability for the entire fleet, including aircraft receiving maintenance in a depot and aircraft in storage. CBO’s availability rate formula is:

CBO’s measure of the availability rate would show, for example, when there is an increase in the percentage of a fleet that is in storage or in depot-level maintenance, and it cannot be tailored by recategorizing unflyable aircraft in the squadrons as being in depot status.

How CBO’s Measure of Availability Rates Compares With the Department of Defense’s Measure

To illustrate the differences between the methods CBO and DoD use to calculate the availability rate of military aircraft, CBO analyzed 2019 data on the Air Force’s F-15C fighter jet.

Trends in Aircraft Availability and Use

This section presents CBO’s calculations of annual availability rates and flying hours per aircraft for the Air Force and DoN from 2001 to 2020 using CBO’s measure of availability. CBO also tracked a complementary metric, flying hours per aircraft per year. The two metrics are slightly different: Availability rates show what share of the fleet could have been flown in a given period, whereas flying hours show the amount of flight that actually occurred. After presenting fleetwide trends for both services, CBO shows availability rates and annual flying hours per aircraft for fighters and attack aircraft, helicopters and tiltrotor aircraft, and fixed-wing training aircraft.

Fleetwide Trends

In general, fleetwide availability rates have declined for both services, but they have declined more for DoN. Average annual flight hours per aircraft have followed a similar trend.

Fighters and Attack Aircraft

The trends for fighters and attack aircraft in each service mirror the trends observed for the services’ fleets as a whole. Of particular note is the steep decline in the availability rate for DoN’s fighters and attack aircraft, which drives the decline in the availability rate of the total fleet. For DoN, the aircraft CBO analyzed included the AV-8B, EA-6B, EA-18G, F-14, and F/A-18 aircraft; for the Air Force, the aircraft CBO analyzed included the A-10, F-15, F-16, and F-22 aircraft. REMIS does not accurately track availability or flying hours for the F-35A, and the data CBO received from DECKPLATE on the F-35B and F-35C did not match other reports of the availability of those aircraft, so F-35s are not analyzed here.

Specific Types of Fighters

CBO looked at some of the most numerous types of fighters to illustrate the difference in the availability rates for the Air Force and DoN. Fighters’ availability has declined for both services since about 2006. There has been an especially marked decline in the availability of DoN’s F/A-18C/Ds, known as legacy Hornets, caused by considerable delays in the successful completion of “high flight hour inspections,” which are not solely inspections but also a series of actions intended to extend the Hornets’ operating life.

Helicopters and Tiltrotor Aircraft

Both the Air Force and DoN operate helicopters and tiltrotor aircraft. However, helicopters and tiltrotor aircraft make up a much larger fraction of DoN’s fleet, representing 34 percent of its total fleet compared with just 3 percent of the Air Force’s fleet. The trends for those aircraft are consistent with fleetwide trends in both services.

For the Air Force, the helicopters and tiltrotor aircraft CBO analyzed included the CV-22, HH-60, and UH-1 aircraft. For DoN, the helicopters and tiltrotor aircraft the agency analyzed included variants of the H-1, H-3, H-46, H-53, H-58, H-60, H-72, TH-6, TH-57, and V-22 aircraft.

Fixed-Wing Training Aircraft

Both the Air Force and DoN operate fixed-wing aircraft for pilots to train in before they fly operational aircraft. The fixed-wing training aircraft CBO analyzed included the Air Force’s T-1, T-6, and T-38 aircraft and DoN’s T-2, T-6, T-34, T-38, T-39, T-44, T-45, U-1, and X-26 aircraft.

Effects of the Pandemic

In the early months of the pandemic, fleetwide availability increased and flying hours per aircraft declined in both the Air Force and DoN. CBO analyzed Air Force data through March 2021 and DoN data through September 2020.

Effect on Availability Rates

Availability peaked in April 2020 for the Air Force and in May 2020 for DoN. With the services flying fewer hours, more spare parts may have been available to complete maintenance, increasing the number of aircraft that were available. Or fewer flying hours may have reduced the chances that available aircraft would experience problems and need repairs.

Effect on Flying Hours per Aircraft

To evaluate the effect of the pandemic on flying hours, CBO computed five-year monthly averages for the years preceding the pandemic. (CBO averaged the data over the past five years because monthly flying hours may vary considerably from year to year.) The agency compared those averages with the corresponding months in fiscal years 2020 and 2021 (in months for which CBO had REMIS data).

AppendixAlternative Measures of Availability

The availability rate for military aircraft can be measured in many ways. This appendix explores two alternatives that differ from the measures used by the Congressional Budget Office and the Department of Defense (DoD). One alternative would modify CBO’s measure by imposing a stricter standard for availability, resulting in lower availability rates than CBO or DoD found. The other would modify DoD’s approach by treating aircraft that are receiving depot-level maintenance as part of the total fleet. That would result in an availability rate that was lower than DoD’s measure but higher than CBO’s.

A Stricter Modification of CBO’s Availability Rate

Under CBO’s and DoD’s measures, it is possible that some aircraft that could not be flown might be counted as available. That is because those measures count as available all of the aircraft that are shown by the military’s databases to be “operator possessed”—that is, not currently undergoing depot-level maintenance or in storage—and “mission capable,” or able to perform at least some of the missions that might be assigned. However, those same databases, the Air Force’s Reliability and Maintainability Information System and the Department of the Navy’s (DoN’s) Decision Knowledge Programming for Logistics Analysis and Technical Evaluation, also show months when some aircraft did not fly even though they met CBO’s definition of availability. Given the demands associated with overseas operations for much of the period CBO analyzed as well as the demands for pilot training proficiency during that period, it is striking that some available aircraft apparently did not fly at all during a month.

CBO therefore explored a stricter measure of availability in which specific aircraft were counted as available in a particular month only if they flew in that month. That definition would be more accurate than CBO’s measure if the reason aircraft did not fly was because they could not. But the alternative measure would be less accurate if the reason aircraft did not fly was because they were not needed or because pilots or funding were not available. The data CBO had access to did not allow the agency to determine why specific aircraft did not fly in a given month.

CBO calculated availability rates in 2019 using its method and the stricter method (see Table A-1). The differences between the availability rates calculated by the two methods were larger for the Air Force than for DoN. Under the stricter measure, the availability rate for the Air Force’s entire fleet was 4.6 percentage points lower than it was under CBO’s measure, and the availability rate for DoN was 2.1 percentage points lower. Looking across multiple years, CBO found that the difference between the rates would have been consistently larger for the Air Force than for DoN: That is, it has been more common for the Air Force’s mission-capable-coded aircraft not to fly. The differences between the two methods varied by type of aircraft; Table A-1 shows the differences associated with the two services’ largest fleets in 2019.

A Modification of DoD’s Availability Rate

Another way to measure availability would be to exclude storage-coded aircraft from the calculation (as DoD’s measure does) but to include depot-coded aircraft (which DoD’s metric does not). Calculating availability that way would account for the active portion of a fleet (those aircraft in operational squadrons or in depot-level maintenance) and exclude aircraft that have entered long-term storage, a status from which only a few aircraft have ever returned to operational use. In effect, this measure finds the share of the fleet receiving operational funding that is capable of flying missions. By contrast, the measure CBO used in this report indicates the share of all aircraft purchased—excluding those destroyed in accidents, scrapped, or otherwise disposed of—that are capable of flying missions.

Not surprisingly, omitting aircraft in storage results in availability rates that are higher than the rates found under CBO’s measure and the first alternative measure but lower than the rates found using DoD’s measure (see Figure A-1).

About This Document

This Congressional Budget Office report was prepared at the request of the Chairman and Ranking Member of the Subcommittee on Readiness of the House Armed Services Committee. In keeping with CBO’s mandate to provide objective, impartial analysis, the report makes no recommendations.

Edward G. Keating, R. Derek Trunkey, John Kerman (formerly of CBO), and Kathryn McGinnis prepared the report with guidance from David Mosher. David Arthur, Ron Gecan, Shannon Smith, and Natalie Tawil provided assistance. Adebayo Adedeji fact-checked the report. J. J. Gertler of the Congressional Research Service and S. Craig Goodwyn of CNA provided comments. The assistance of external reviewers implies no responsibility for the final product; that responsibility rests solely with CBO.

Jeffrey Kling and Robert Sunshine reviewed the report. Elizabeth Schwinn edited it, and R. L. Rebach created the graphics and prepared the text for publication. The report is available at www.cbo.gov/publication/57433.

CBO seeks feedback to make its work as useful as possible. Please send comments to communications@cbo.gov.

Phillip L. Swagel

Director

January 2022