Countries that have ordered the F-35 are still trying to determine if they can afford to operate the new fighter long-term as a debate over the cost per flying hour for the stealthy aircraft continues between the Pentagon and manufacturer Lockheed Martin. But, as they try to sort out the ownership cost for the aircraft, a grounding of the entire test fleet due to an engine issue has once again brought flight-testing to a halt.

In January, Air Force Chief of Staff Gen. Mark Welsh said his staff and Lockheed Martin were working toward a single cost-per-flying-hour figure for the F-35A. Though the Air Force will be the largest operator of the F-35, its F-35A is also slated for use by 10 partner nations that are also eager to better understand ownership cost before they commit to a fleet size. The company's view of the ownership cost is lower than that of the Air Force, Welsh said: “It was characterized in a different way, a different format.” Air Force officials declined to release Lockheed's and its own numbers, saying they are not finalized. The final number is slated for release in the annual selected acquisition report, which will come out next month.

At issue, Welsh says, is obtaining an “apples-to-apples” comparison between the estimated F-35A cost and the price of operating legacy aircraft, such as the F-16 and A-10, that it will replace. The service needs this to estimate how much more money, if any, F-35 use would entail and if procurement numbers would need to be cut to reduce ownership cost.

Lt. Gen. Burt Field, Air Force deputy chief of staff for operations, plans and programs, said last month that he assumes the F-35 is going to be a “little more expensive” than the F-16 to operate. This contradicts Lockheed Martin's marketing promise that customers could operate the new aircraft more cheaply than legacy fleets across their lives.

An industry official says the company's estimate differs from the service's because the Air Force includes some items the company omits. In aiming for an accurate “apples-to-apples” comparison, Lockheed Martin did not include the cost of operating the electro-optical targeting system or information technology used to support the aircraft, for example, the industry official says. This is because the F-16 cost-per-flying-hour figure lacks data on the cost of operating its targeting pods and supporting computer systems. Also skewing the numbers is that the Air Force's legacy aircraft flying hour accounts are not fully funded, so the cost is below what an optimal value would be. With the F-35 estimate, the service assumes full funding for the accounts.

This is a “work in progress,” the industry source says. “We agree the cost per flying hour will exceed that of the F-16.” But Lockheed expects the anticipated total lifetime cost will be less than that of legacy aircraft, the source adds.

Meanwhile, an investigation by F135 engine manufacturer Pratt & Whitney indicates the fighter program has potentially dodged a bullet. Officials say an analysis of an F135's cracked third-stage low-pressure turbine (LPT) appears to show it was an isolated failure, rather than a recurrence of the more serious high-cycle fatigue design issue that hobbled the $400 billion program in 2007 and 2008. Had the fatigue issue resurfaced, it would have inflicted cost and schedule problems the F-35 can ill afford. The crack was found during a routine inspection of F-35 tail number AF-2 on Feb. 19; the entire F-35 fleet was grounded as a precautionary measure.

Early analysis suggests the crack was likely caused by either a manufacturing defect during the casting of the blade or possibly a creep rupture. This latter failure occurs when deformation begins under constant load and high temperatures and over time reaches a point where the blade material abruptly fails or ruptures. Casting issues can also trigger such creep ruptures by introducing microstructural weakness in the blade.

As with the recently discovered blade crack, the failures 5-6 years ago also struck blades in the third LPT stage, forcing a redesign of the module. However, those failures cropped up on engines configured for the F-35B short-takeoff-and-vertical-landing (Stovl) variant, rather than the standard F-35A/C engine variant. The work contributed to delays of the F-35B's first flight as well as significant cost overruns to the overall F135 engine development program.

The first incident occurred in an early Stovl test engine, FX635, on Aug. 30, 2007, after a total run time of 121 hr. The second failure occurred on Feb. 4, 2008, on the sixth flight-test engine, which was being used for flight clearance “proof-test” ground runs prior to installation in the first Stovl F-35, BF-1. The tests were aimed at proving, on an engine-by-engine basis, that the specific unit was safe to use in BF-1, and that the engine did not exhibit the same combination of assembly characteristics, tolerances and other factors that led to a similar failure.

The second failure occurred after only 18 hr. of total run time, proving the fatigue was triggered by a resonant response to aerodynamic excitation caused by the upstream stator vanes between the second and third turbine stages. A root cause was the long length of the unshrouded blade, which the engine maker acknowledged at the time was outside its “family of experience.” Pratt said it “had neither the standard work nor the advanced analytical techniques to address this phenomenon.” In other words, the design should have been tested, but Pratt's “standard work” design rules did not call for it.

The original fracture, which occurred during high-stress, powered-lift runs for the Stovl engine on a test stand, was traced to an internal crossover cooling hole in the affected blade. The redesign involved reducing stress through changes to the internal configuration of the third-stage blades, and a revised asymmetric vane configuration, which broke up the resonance between the stages.

Although reassured that this latest problem is manageable, program officials will be focusing on whether high-cycle fatigue played a part in either the initiation or propagation of the crack, and what caused the low, steady stress at the failure location.

F-35 Program Executive Officer U.S. Air Force Lt. Gen. Christopher Bogdan is both optimistic in suspecting that the blade problem is not major and cautious in not seeing an immediate return to testing. “It is not a great idea to speculate before you do all the testing on a component that has failed,” he said. “I would caution my Pratt & Whitney brothers to take a conservative approach until they see what is going on. . . . It would be bold to predict a return to flight-testing [this] week. But it is not bold to expect to [have found out] the root cause of the problem this [past] weekend.”

Testing on the failed component, a cracked turbine blade, had included cold-flow, X-ray, white-light and ping tests. As Bogdan spoke, coating was to be stripped from the blade, after which there would be a grain etching test and a fractology test.

Bogdan's concerns about Pratt are not limited to engine performance; he also expresses continued frustration with the aircraft prime and engine maker over their approach to contract negotiations. “What I see Lockheed Martin and Pratt & Whitney doing today is behaving as if they are getting ready to sell the very last F-35 and the very last engine, and are trying to squeeze every nickel out of that last engine and that last airplane,” Bogdan told reporters at the Australian International Airshow near Melbourne last week. “The behavior I want to see is that they want to and intend on and are knowledgeable that they are going to sell 3,000 airplanes and 3,000 engines, and take the long view on this program.”

He complains that the low-rate, initial-production Lot 5 negotiations with Pratt & Whitney have been delayed six months because of disagreement on the award fee. “You would think that a company like Pratt & Whitney that was just given the greatest Christmas gift [the exclusion of competitors General Electric and Rolls-Royce from the program] you could ever, ever, ever get for a company would act a little differently,” he said.

The negotiations over fees, therefore, seemed inappropriate, he suggested.

More generally, Bogdan wants the prime contractors to accept greater risk and invest in cost reductions.

Bogdan is encouraging the Air Force to consider declaring initial operational capability (IOC) for the aircraft with its rudimentary 2B software package, which lacks a wider set of weapons capabilities that will come in the 3F software release.

The service risks having a growing fleet of aircraft unsuitable for operations if it does not consider this option before the 3F release is out. Last month, Field, said Air Combat Command is open to allowing for operational capability with the 2B, but an official ruling has not been made.

“Given that we're producing airplanes today—[and] the U.S. Air Force is going to take delivery of a lot of airplanes between now and 2015, 2016, 2017—if I can give them invasive war-fighting capability that at least allows them to do some missions, and they have enough airplanes out there, then I think that's a decision they need to look at,” Bogdan said. “If they don't declare IOC, then fundamentally those airplanes are going to be used for training and operational exercises.”

The U.S. Marine Corps is expected to declare IOC with the 2B software, owing to an urgency to retire its inferior and costly AV-8B Harrier aircraft. The 2B is equivalent to what foreign customers are also expecting at first to accept, though it includes a hardware update, called 3I. The final software standard, 3F, will allow the F-35 to launch up to 15 types of weapons from internal and external stations, Bogdan said. It is due to be completed with the rest of the development program on Oct. 31, 2017.