The Maritime Self-Defense Force's P-1 aircraft, the country's first domestically made patrol aircraft, is underused mainly due to engine corrosion and problems with electronic devices, the government's Board of Audit has said.

Reporting the outcome of its probe into the matter Friday, the board called for making maximum use of knowledge gained from past malfunctions and conducting appropriate tests to improve the performance of the aircraft.

The board stopped short of disclosing details of the low operational status, such as the number of aircraft active at present, citing national security concerns.

The importance of monitoring submarines and military vessels in waters around Japan is increasing, especially after the recent close encounters between Japanese patrol and Chinese military aircraft

On June 7, a fighter jet based on a Chinese aircraft carrier made a dangerously close approach to an MSDF P-3C patrol aircraft monitoring the ship in the Pacific Ocean. A similar incident happened the next day.

The P-1 "has not been used as effectively as possible," a Board of Audit official said.

Japan started to deploy the P-1, developed as successor to the aging P-3C, in 2013.

The value of contracts related to the development and operations of the P-1 concluded by the government by fiscal 2023 totaled ¥1.8 trillion.

The board examined 35 P-1 aircraft deployed at two MSDF air bases as of the end of fiscal 2023 and concluded that only a limited number could be used to carry out missions without a hitch.

Reasons for the low operational status included engine corrosion, the unavailability of some electronic devices for use in intelligence gathering and the long time needed to procure some parts, according to the report.

"We've been working to improve the operational status of the P-1, but we take the Board of Audit's findings seriously and will continue our efforts to maximize the utilization of the P-1," said an official of the Acquisition, Technology and Logistics Agency's Project Management Department.