The F-35 Joint Strike Fighter, the world's most expensive weapon, has begun its much anticipated and highly controversial 'fly off' against the Air Force's 40 year old A-10 Thunderbolt II.

The series of tests is designed to find out if the $400bn F-35, which has faced massive problems and delays, is better than the A-10 for close air support and combat search and rescue missions.

However, there are claims military bosses are manipulating the tests to favor the F-35

'Our troops deserve better than a surreptitious test rigged in favor of a weapon that can't do the job and against the one that can,' Dan Grazier of the Straus Military Reform Project at the Project on Government Oversight (POGO) wrote.

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Showdown: the 40 year old A-10 (left) is set to take on the new F-35 (right) in a series of war games.

'They are staging an unpublicized, quickie test on existing training ranges, creating unrealistic scenarios that presuppose an ignorant and inert enemy force, writing ground rules for the tests that make the F-35 look good.

'According to sources closely associated with the fly-off, not a single event includes ground troops, or any kind of fluid combat situation, which means these tests are hardly representative of the missions a close air support aircraft has to perform.

'Rather than telling us whether or not the F-35 can actually provide the kind of close support our ground forces need to survive and prevail, this grossly inadequate test has been designed to mislead. '

The evaluation began on July 5, 2018, and will last just one week, ending on July 12, 2018.

Only four of those days involve actual flying.

The F-35 is supposed to to take over the A-10's 40 year role of supporting ground forces with its titanium armor and powerful nose cannon.

However, experts believe that for many missions, the older aircraft may actually perform better - and say the two could even fly together in some missions.

HOW THEY MATCH UP: THE WARTHOG VS THE JSF

A-10 WARTHOG F-35 JOINT STRIKE FIGHTER Top speed 439 mph 1,199 mph Introduced March 1977 TBD Wingspan 58′ 0″ 35′ 0″ Main weapon 30mm cannon Four-barrel 25 mm GAU-12 Equalizer cannon Unit cost $11.8mn $98m Range: 800 miles 1,379 mi Engine type General Electric TF34 Pratt & Whitney F135

Michael Gilmore, the former director of the Defense Department operational test and evaluation office, previously said during Senate testimony the battlefield comparison 'makes common sense'.

'To me, comparison testing just makes common sense,' Gilmore said.

'If you're spending a lot of money to get improved capability, that's the easiest way to demonstrate it is to do a rigorous comparison test.'

'We're going to do it under all the circumstances that we see CAS [close air support] conducted, including under high-threat conditions in which we expect F-35 will have an advantage and other conditions requiring loitering on the target, low-altitude operations and so-forth,' Gilmore told the Senate Armed Services Committee.

The F-35 Joint Strike Fighter has been hailed as the 'most expensive weapon in history.' But despite a price tag of $400 billion for 2,457 planes, the fifth-generation fighter has been plagued with issues. Pictured is a F-35B aircraft prepares for a landing at Marine Corps Air Station Yuma, Arizona

'There are a lot of arguments that ensue over which aircraft might have the advantage, the A-10 or the F-35, but that is what the comparison test is meant to show us,' Gilmore said.

The Pentagon plans to spend close to $400 billion to buy nearly 2,500 F-35s for the Air Force, Navy and Marine Corps.

The budget request for fiscal year 2017 includes $8.3 billion to purchase 63 aircraft.

The F-35, which is being designed to meet the specific requirements of each of the services, cost over $100 million each.

The F-35 Joint Strike Fighter's record on cost, schedule and performance has been a scandal and a tragedy, Senator John McCain has previously told senior Pentagon officials.

A-10: THE 40 YEAR OLD 'UGLY WARTHOG' It is the ugliest aircraft in the Air Force's arsenal. The A-10, often called a warthog, was designed to destroy Soviet tanks and troops on the ground. Officially the Thunderbolt II, it was quickly nicknamed the Warthog for its unusual looks, It was specifically designed around its main weapon, a 30mm cannon which fires 4,000 rounds a minute. The plane can fly low and slow, coming down to 50ft to shoot at or drop bombs on enemy positions. Its top speed is just above 400mph but it can go as slow as 150mph and 'loiter' for hours above targets making it an effective deterrent as well as an attack plane. The Warthog is covered in 1,200lbs of titanium armor, making it invulnerable to attack from anything but heavy weapons. Even when hit it is designed to fly home on one engine, with no tailfin and half a wing missing. On board the single-seater the pilot has at his controls the cannon, which is accurate to 4,000ft, and fires depleted-uranium shells, as well as Maverick air-to-surface missiles, 500lb free fall bombs, and Hydra air-launched rockets. Despite USAF attempts to retire the fleet, it is expected to remain in service into the 2020s. At one stage when the Air Force suggested retiring its more than 300 A-10s the Army indicated it would take them over as soldiers are so keen on its close support capabilities. Advertisement

He said the aircraft's development schedule has stretched to 15 years, deliveries of the F-35 have been delayed, and costs have skyrocketed.

'It's been a scandal and the cost overruns have been disgraceful,' McCain said.

'And it's a textbook example of why this Committee has placed such a high priority on reforming the broken defense acquisition system.'

In March Air Force Chief of Staff Gen. Mark Welsh testified to the Senate committee that the F-35 would not replace the A-10, according to Military.com.

During the hearing Tuesday, Sen. Kelly Ayotte, R-N.H., said the general's earlier testimony appeared to contradict statements on the fighter jet program's website and its longtime aim to take over the Warthog responsibilities.

The Pentagon's top weapons buyer denied any contradiction.

'Both statements are correct. We will in fact replace the A-10s with F-35s, that is the plan,' said Frank Kendall, under secretary of defense for acquisition, technology and logistics.

But Kendall said there should be no expectation that the F-35 will perform in the same way as the A-10 on the battlefield.

'The A-10 was designed to be low and slow and close to the targets it was engaging, relatively speaking,' he said. 'We will not use the F-35 in the same way as the A-10, so it will perform the mission very differently.'

Last month it was revealed the U.S. Air Force is shelving plans to retire the aging A-10 'Warthog' aircraft, a heavily armored tank killer, because of its role in the fight against Islamic State.

The 40-year-old ground attack aircraft is popular with soldiers, Marines - and some U.S. lawmakers.