The US Air Force now has the minimum number of 12 Lockheed Martin F-35A fighters with completed modifications required before the first squadron can be declared initially capable for operations, service officials announced on 13 July.

Since 2013, the USAF has planned to declare initial operational capability (IOC) with 12-24 F-35As assigned to the 34th Fighter Squadron at Hill AFB, Utah, from 1 August to 31 December this year.

But that timeframe appeared to be threatened with the discovery of a faulty fuel tank last September. In certain flight conditions, USAF testers found that air can enter the siphon fuel tank, potentially causing the tank to over-pressurise and rupture in-flight.

The USAF restricted the delivered F-35A fleet to 3g maneouvres when carrying a fuel load of fuel. Only when more than half of the fuel tank was empty could the F-35A perform manoeuvres up to 7g’s, the maximum allowable for USAF variant with Block 2B software. The Block 3F version scheduled for release next year will allow the F-35A to operate the full flight envelope with manoeuvres up to 9gs.

Workers at the Ogden Air Logistics Center at Hill AFB are installing a modification in the F-35A fleet to prevent air from entering a fuel transfer line. By adding a relief line controlled by a solenoid valve, the siphon tanks can vent the air that could cause an over-pressurisation, according to a 2015 report by the Defense Department’s office of test and evaluation.

The path to declaring IOC for the F-35A is now driven by a requirement to complete training for pilots and maintainers, the USAF says. The aircraft must be able to carry either a mix of two GBU-31 and two GBU-12 bombs or two AIM-120 AMRAAMs, and perform basic close air support, air interdiction and suppression or destruction of air defences.

The US Marine Corps declared IOC with the F-35B variant last July, standing up the VMFA-121 squadron at Yuma MCAS, Arizona. The navy, meanwhile, plans to achieve IOC between August 2018 and February 2019.