F-35 STOVL Series

The F-35 STOVL Series Modification Program is a major line item within the Navy's Aircraft Procurement budget, specifically focused on the Short Take Off and Vertical Landing (STOVL) variant of the Joint Strike Fighter, the F-35B. The goal of this program is to sustain, modernize, and improve the operational safety, reliability, and combat capability of the F-35B fleet. The program supports both near- and long-term modifications, with a focus on correcting documented deficiencies, implementing new capabilities, and extending the service life of the aircraft. The FY2026 budget request is justified by the need to accelerate Block 4 kit procurements and increase installation rates as depot capacity expands, aiming to avoid aircraft grounding due to life limits and to support a total active inventory objective of 243 aircraft.

F-35 STOVL Correction of Deficiencies (OSIP 004-19) is a core component of this line item, targeting safety, reliability, maintainability, and structural life limitations. The primary objective is to address deficiencies that could impact flight safety or operational availability, with a significant focus on the Structural Life Limited Parts (SLLP) program. This effort involves the procurement and installation of retrofit solutions for up to 68 different Engineering Change Proposals (ECPs) that remove life limits below 8,000 flight hours. Modifications are performed by contractor field teams and organic depot artisans, with expanded capacity at Fleet Readiness Center East enabling higher throughput. Funding covers material, installation labor, and support costs across multiple depots, with the SLLP effort projected to continue through 2047.

F-35 STOVL Block 4 Upgrade (OSIP 012-20) is designed to retrofit advanced capabilities onto fielded F-35B aircraft to ensure they remain effective against evolving threats. Block 4 upgrades are delivered through a Continuous Capability Development and Delivery approach, involving both hardware and software improvements. The program includes over 66 individual capability enhancements, such as Technical Refresh 3 (TR-3), Next Generation Distributed Aperture System (DAS), advanced electronic warfare modules, and enhanced communication/navigation systems. Retrofit kits are tailored to bring early production aircraft up to the latest configuration, with manufacturing and installation schedules adjusted to align with production and part qualification timelines. Integrated Logistics Support and pilot training device upgrades are also funded to support Block 4 changes.

F-35 STOVL APG-85 Radar (OSIP 015-26) represents a significant capability improvement, replacing the legacy APG-81 radar with the new APG-85 system in U.S. F-35B aircraft. The APG-85 radar is expected to provide enhanced performance and threat detection capabilities, although specific milestone and delivery details are maintained at a controlled unclassified level. FY2026 funding supports the procurement of radar upgrade kits, with installations scheduled to begin as production and delivery timelines are met.

F-35 STOVL Propulsion (OSIP 010-20) addresses engine-related readiness degraders, production deficiencies, obsolescence, and life-cycle cost reductions for the F-35B's F135 engine. The program funds hardware and software upgrades to improve reliability, maintainability, and availability, including modifications such as bushing replacements, stator upgrades, scavenge tube durability improvements, gearbox redesigns, and other critical component enhancements. Installations are managed through both field and depot retrofit activities, with support from manufacturers. The aim is to ensure the F-35B propulsion system remains credible and capable throughout its service life.

F-35 STOVL Reliability and Maintainability (OSIP 009-21) focuses on operational safety improvements that reduce cost per flight hour and address readiness degraders. This program funds the identification and correction of reliability and maintainability issues through numerous small ECPs, which are typically installed at the unit or depot level. The initiative is expected to have a positive impact on the overall platform, supporting sustained operations and reducing life-cycle costs.

Inactive/Completed OSIPs serve as a placeholder for prior year funds associated with completed modification programs, such as the Block 3i/3F Upgrade and Concurrency corrections. These efforts are no longer active, but their inclusion in the budget line item provides transparency regarding historical investments in F-35B modernization and sustainment.