Lockheed Martin’s F-35 Joint Strike Fighter jet has faced more delays in initial operational testing and will not come any sooner than 2018, a US government official says.

US Air Force Lieutenant General Chris Bogdan, the F-35 program chief, said Tuesday that the delay was taking place in order to install the final software on the 23 airplanes needed for testing.

The mostly software-driven aircraft, in the works since 2001, has yet to achieve full combat capability due to its many deficiencies.

Defense Acquisition Chief Frank Kendall told reporters that the fifth-generation stealth aircraft was supposed to enter the testing phase in the second half of 2017, months behind the October 2016 schedule that was set in 2012, when the program was reorganized.

Michael Gilmore, the Pentagon’s top weapons tester, says that the final phase of testing should be delayed as the current timeline was unrealistic.

Earlier in May, Gilmore told Congress that a software issue grounded five out of six US Air Force F-35s during a test and only one of them was able to successfully boot its software and become airborne.

Gilmore said this was not the only instance of software bugs crippling the jet as just recently another group of F-35s had to abort their test mission due to software stability issues.

In a previous report, Gilmore had noted that the US Air Force was not satisfied with the expensive jet because of “inherited deficiencies” with its software and “new avionics stability problems,” as well as a major radar flow.

A fuel system deficiency, faulty diagnostic systems, cracks in wing spars, lack of high-fidelity simulators for combat missions, and a pilot escape system that could kill ejecting pilots were among the problems that Gilmore cited for F-35.

The warplane’s immunity against hacking attacks has yet to be verified, amid the growing threat of cyber warfare across the world.

F-35 variants

With a cost of around $400 billion, the F-35 is the most expensive US weapon ever and is projected to cost another $1 trillion during its 55-year life cycle.

Lockheed is producing three models of the jet which will be used by the US and at least 9 other countries.