It has so far cost over $3.5bn, and is eight years late.

Hailed as the most expensive weapon in history, the controversial F-35 stealth fighter jet is undergoing rigorous testing at California's Edwards Air Force Base.

Now, it has emerged the jets complex radar system has a problem - it keeps crashing.

Hailed as the most expensive weapon in history, the controversial F-35 stealth fighter jet complex radar system has a problem - it keeps crashing.

The software glitch that interferes with the ability of the Lockheed Martin F-35 Lightning II Joint Strike Fighter's AN/APG-81 AESA radar working in flight.

This poses the greatest threat to delaying US Air Force (USAF) plans to declare its jets operationally deployable, a top service official told Janes.

Major General Jeffrey Harrigian, director of the air force's F-35 integration office at the Pentagon, described the problem as 'radar stability - the radar's ability to stay up and running'.

'What would happen is they'd get a signal that says either a radar degrade or a radar fail -something that would force us to restart the radar,' Maj Gen Harrigian said

New software that corrects the error is expected to be delivered to the USAF at the end of March.

A recent Pentagon report has revealed a massive list of potentially lethal bugs still facing the jet.

A December memo by Michael Gilmore, the Department of Defense's director for Operational Test and Evaluation warned of 'significant ongoing challenges' - and these have now been revealed.

The Air Force is currently scheduled to announce their version of the plane is ready to begin flying, known as 'initial operating capability,' in August or December at the latest, according to Defence One.

A recent Pentagon report has revealed a massive list of potentially lethal bugs still facing the jet.

The Marines have already claimed their version of the plane, known as block 2B, is ready.

However, the report reveals serious problems with the computer software, including 'in fusion, electronic warfare, and weapons employment result[ing] in ambiguous threat displays, limited ability to respond to threats, and a requirement for off-board sources to provide accurate coordinates for precision attack.'

Earlier this year researchers revealed the hugely delayed and over budget project has finally fired its first shots in the air.

The F-35A Lightning II completed the first three airborne gunfire bursts from its internal Gun Airborne Unit (GAU)-22/A 25mm Gatling gun system during a California test flight on October 30th.

The F-35 also has a smart parts system to ensure everything is working within its limits, and can warn when parts need to be replaced.

However, the computerized maintenance management System, or CMMS, 'incorrectly authorizes older/inappropriate replacement parts.' the report said.

THE EJECTOR SEAT THAT COULD KILL Pilots under 136 pounds aren't allowed to fly any F-35 variant. Pilots under 165 pounds have a 1-in-4 chance of death and 100 percent chance of serious neck injury upon ejecting, according to the testing office. 'The testing showed that the ejection seat rotates backwards after ejection. This results in the pilot's neck becoming extended, as the head moves behind the shoulders in a 'chin up' position. When the parachute inflates and begins to extract the pilot from the seat (with great force), a 'whiplash' action occurs. The rotation of the seat and resulting extension of the neck are greater for lighter weight pilots,' the report states. Sophisticated: Footage from ground testing of the F-35A stealth jet at Edwards Air Force Base in California, shows the awesome firepower of the four-barrel Gatling gun embedded in the left wing Advertisement

It also fails to detect if it's been flying too fast and 'randomly prevented user logins' into its computerised control system.

In the first live firing test, three bursts of one 30 rounds and two 60 rounds each were fired from the aircraft's four-barrel, 25-millimeter Gatling gun.

In integrating the weapon into the stealthy F 35A airframe, the gun must be kept hidden behind closed doors to reduce its radar cross section until the trigger is pulled.

'The successful aerial gun test sortie was a culmination of several years' planning, which intensified in the first half of 2015 at the Edwards F-35 Integrated Test Force (ITF) Flight Test Squadron with a team of Air Force, Lockheed Martin, Pratt & Whitney, General Dynamics, and Northrop Grumman personnel,' said Mike Glass, Edwards ITF flight test director.

'The results of this testing will be used in future blocks of testing, where the accuracy and mission effectiveness capabilities will be evaluated.'

The 25mm gun is embedded in the F-35A's left wing and is designed to be integrated in a way to maintain the F-35's very low observable criteria.

It will provide pilots with the ability to engage air-to-ground and air-to-air targets.

'At the end of the program's system development and demonstration phase in 2017, the F-35 will have an operational gun.

The first phase of F-35 gun testing started in June, when initial shots were fired from the ground at the Edwards Air Force Flight Test Center's gun harmonizing range.

The gun system will be further tested with a production F-35A next year for integration with the jet's full mission systems capabilities.

The test team will demonstrate the gun's effectiveness in both air-to-air and air-to-ground employment when integrated with the next generation fighter's sensor fusion software, which will provide targeting information to the pilot through the helmet mounted display.

At the end of the program's system development and demonstration phase in 2017, the F-35 will have an operational gun.

The F-35 is designed with the entire battlespace in mind, bringing new flexibility and capability to the United States and its allies. F35.com

The first phase of the gun testing began on June 9, and the amount of munitions fired has been gradually increased, until the 181 rounds were fired on August 17.

The ground tests were designed using software to replicate being in flight, using a production version of the GAU-22/A gun.

The tests, using the target practice PGU-23/U which does not explode on impact, showed the gun's ability to spin up and down correctly.

Further testing will be carried out next year, to integrate the GAU-22/A system with the jet's full avionics and mission systems capabilities.

The following stage will be to observe the qualitative effects, including muzzle flash – the visible light emitted by the blast of a firearm – the human factors, and the flying qualities.

'Flying computer': U.S. military leaders have extolled the virtues of the F-35 jets, which are intended to 'combine advanced stealth capabilities with fighter aircraft speed and agility'

But the production of the latest breed of stealth jet – one of the most highly anticipated advancements in military history – has had more than its fair share of problems.

Despite costing the US military more than $350billion, the jet has so far failed to live up to expectations.

The cutting-edge F-35, which is meant to be the most sophisticated jet ever, was embarrassingly outperformed by a 40-year-old F-16 jet in a dogfight in July.

The test pilot condemned the jet's performance at the time, claiming it performed so appallingly that he deemed it completely inappropriate for fighting other aircraft within visual range.

The Pentagon leapt to the defense of its new toy, insisting that the aircraft used in the test was not equipped to the same standard of its front-line aircraft, and did not have its 'stealth coating'.

But even so, the dismal result of the dogfight against an aircraft designed in the 1970s did little to restore confidence in the F-35.

He even criticised the half-million-dollar custom-made helmet, supposedly designed to give the pilot a 360-degree view outside the plane, but which he claimed made it difficult to move his head inside the cramped cockpit.

Plagued by setbacks: The F-35 production has already cost the U.S. military $350billion and been delayed by more than eight years

'The helmet was too large for the space inside the canopy to adequately see behind the aircraft,' he wrote in his five-page report.

A series of setbacks has delayed production by up to eight years and put it $263billion over budget, so far.

The spiralling costs are due to a number of factors, including engine problems that caused one jet to burst into flames during take-off last May.

Air Force Lt. Gen. Chris Bogdan, who is in charge of the F-35 programme, said the planes had been plagued by simple mistakes. These included everything from wingtip lights that did not meet Federal Aviation Administration (FAA) standards to tires that could not cope with the landings.

But military bosses have been quick to extol the virtues of the stealth multirole fighter, which is predicted to be vastly superior to its fourth-generation predecessors.

Marine Lt Gen. Robert Schmidle said the planes were like flying computers and that they could detect an enemy five to 10 times faster than the enemy could detect it.

And Lt Col David Burke told 60 Minutes last year: 'I'm telling you, having flown those other airplanes, it's not even close at how good this airplane is and what this airplane will do for us.'

Its creators at Lockheed Martin boast that the stealth jet 'combines advanced stealth capabilities with fighter aircraft speed and agility, fully fused sensor information, network-enabled operations and advanced logistics and sustainment'.

BUT HOW DOES IT COMPARE WITH ITS 1970S PREDECESSOR, THE F-16 JET? F-35 Role: Stealth multirole fighter First flight: December 15, 2006 Unit cost (not including engine): F-35A - $98million F-35B - $104million F-35C - $116million Number built: 115 (as of November 2014) Length: 15.67m Wingspan: 10.7m Height: 4.33m Max speed (F-35A): 1,930kph Armament: One of the most highly-anticipated features of the F-35 armament is the Small Diameter Bomb II (SBD II).The bomb is able to guide towards its target using laser, imaging infrared or radar homing. It can hit moving or stationary targets in any weather, or at night, with unprecedented reliability and accuracy. The 'super weapon' is predicted to be the most versatile air-to-ground munition in the Pentagon's air combat inventory. The cost per SDB II is said to be around $250,000 and the US military plans on buying as many as 17,000 of them. F-16 Role: Multirole fighter First flight: January 20, 1974 Unit cost: F-16A/B - $14.6million F-16C/D - $18.8million Number built: 4,540+ Length: 15.06m Wingspan: 9.96m Height: 4.88m Max speed (F-16C): 2,120kph Combat history: The F-16 has served in the Air Forces of 26 nations, including the U.S., Israel, Egypt, the Netherlands, Denmark and Norway. During Operation Desert Storm, the 1991 assault on Iraq, F-16s flew over 13,000 operations, more than any other Coalition aircraft. The U.S. has employed the F-16 in operations over the Balkans, Afghanistan and Libya. At its production peak in 1987, the F-16 team in Fort Worth was also making history, by producing 30 F-16s in just 30 days. Thanks to frequent upgrades improving and incorporating new technologies into the cockpit, avionics, sensors and weapons, the aircraft has become more reliable over its 40 years. Advertisement

The f35.com website continues: 'The F-35 is designed with the entire battlespace in mind, bringing new flexibility and capability to the United States and its allies.

'Reliance on any single capability – electronic attack, stealth, etc – is not sufficient for success and survivability in the future.'

There are three main variants to the F-35: F-35A conventional takeoff and landing; the F-35B short take-off and vertical landing; and F-35C carrier-based catapult-assisted take-off but arrested recovery aircraft.

I'm telling you, having flown those other airplanes, it's not even close at how good this airplane is and what this airplane will do for us. Lt Col David Burke on 60 Minutes

The fifth-generation aircraft is designed to excel in electronic warfare, air-to-surface combat and air-to-air combat.

Its stealth technology allows it to avoid radar detection that previous fourth generation fighters cannot. It also carries its weapons and fuel internally so it cannot be 'detected and tracked'.

The Royal Air Force has so far ordered eight F-35s to be delivered by next year.

The jets were meant to enter service in 2012, but Sir Nick Harvey, who served as the Minister of State for Armed Forces between 2010 and 2012, said that there was 'not a cat in hell's chance' that the jet would be in British service by 2018.

'I don't recall…having heard anyone suggesting that these things could be used in combat before 2020.'

In total, Britain is expected to purchase 138 of the jets from the US, but at present costs that would add up to a total of $19billion.

The delays and escalating costs mean that once the British Tornado jets – which have been in service since 1979 - are retired in three years' time, the UK will be left with an 'offensive capacity' of just 60 planes.

But the UK is not the only country waiting for the costly fighter jet to be fixed.