The US Government Accountability Office (GAO) has found that the US Navy has not fully mitigated attack submarine fleet maintenance and other challenges affecting readiness.
The GAO said that, over the last 10 years, the US Navy has lost more than 15,000 operational days due to maintenance delays and idle time on active attack submarines.
These delays resulted in an estimated US$3.4 billion in costs to sustain crews and submarines that provided no operational capability.
Lost operational days include delays completing depot maintenance and active idle time. Active idle time describes a period when an attack submarine and its crew must remain pier-side because they are no longer certified to conduct normal operations and cannot be inducted into a maintenance period.
The GAO said that, in addition to lost operational days, the US Navy faces significant challenges with managing inactive idle time for attack submarines.
Inactive idle time describes when an attack submarine designated for decommissioning cannot be inducted into a dry dock in a shipyard. As a result, the submarine and its crew must remain pier-side until a shipyard has the capacity in a dry dock to defuel their nuclear reactor.
While the US Navy has made recent progress in reducing active idle time, inactive idle time for submarines has grown and is projected to worsen. Without mitigation, 15 attack submarines will enter inactive idle time from fiscal year 2026 through fiscal year 2030—during which time the navy could incur more than 14,000 days of inactive idle time and US$3.1 billion in costs to operate and support these inactive submarines.
The GAO said the navy has not fully evaluated alternative options to more efficiently decommission attack submarines and develop and implement an inactivation plan that could allow them to reduce inactive idle time for crews and save billions.
According to the US Navy, its 44 attack submarines, as of fiscal year 2025, provide the United States an asymmetric advantage to gather intelligence undetected and attack enemy targets, among other missions. These capabilities make attack submarines some of the most requested assets by the global combatant commanders.
House Report 118-529 includes a provision for the GAO to assess the readiness and availability of the US Navy’s attack submarine fleet. This report discusses the extent to which the navy has effectively maintained its attack submarine fleet and has mitigated any challenges. The GAO analysed US Navy attack submarine maintenance data from fiscal years 2016 through 2025, reviewed US Navy documentation, and interviewed officials.
This report is a public version of a classified report GAO issued July 16, 2026. The GAO said it omitted information the Department of Defense deemed classified or sensitive, such as attack submarine operational availability and how the US Navy’s approach to attack submarine force generation compares to strategic competitors’ approaches.
The GAO has made two recommendations including that the US Navy fully evaluate alternative options to more efficiently decommission attack submarines and develop an attack submarine inactivation plan that reduces inactive idle time. The navy verbally concurred with the GAO’s recommendations, but did not provide written comments.
