The Government Accountability Office has recommended the Defense Department and U.S. Navy issue an updated or new set of guidelines for developing plans to sustain legacy weapon systems.

GAO said in a report published Monday that the Navy inconsistently documented its sustainment strategies for C-2A, E2C, EA-18G and F/A-18A-D aircraft since 2012.

Meanwhile, the DoD lacks clear guidance on whether developing a sustainment strategy is required and needs to be modified every five years for weapons systems that are already being operated and supported.

The GAO noted that clarifying requirements for documenting sustainment plans for aging systems will allow agencies to check the availability and operating and support costs of aircraft and any associated risks.

For the study, the agency assessed the aircraft availability goals and O&S cost trends of the Navy and U.S. Air Force for a selected pool of fixed-wing planes between 2011 and 2016.