Lockheed’s plan for the F-35B to survive probation

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One year ago on 18 March, Graham Tomlinson notched the first vertical landing of the Lockheed Martin F-35B, a milestone hailed then as proving the most troublesome of the F-35's three variants had turned a corner.

Events have not entirely gone to plan. In the 12 months since, the F-35B has been grounded from flight tests, cracked during ground tests, cancelled by one customer and placed on two-year probation by another. Yet, supporters of the short take-off and vertical landing variant remain unwavering in their confidence that - despite the declared doubters, which now include the UK Royal Navy and US Secretary of Defense Robert Gates - the problems are already being overcome.

Backing the supporters is a resurgence of vertical landings - the key test for a STOVL fighter. After making only 10 landings in roughly the first six months after 18 March, the programme's test pilots have recorded nearly 50 in the past six months.

In recent interviews, Lockheed officials have candidly described the F-35B's four critical flaws, along with their plans for addressing them.

It remains too early to know whether Lockheed's plan will be enough to save the world's only STOVL fighter in development, but it might be the F-35B's best chance for survival beyond 2013, when Gates's probation expires.

The blame for last year's three-month halt to vertical landing tests falls on two doors, says Steve O'Bryan, Lockheed vice-president for business development. These auxiliary air inlet (AAI) doors allow enough air to flow into the lift-fan as the F-35B enters low-speed and hover mode. But the doors are actually opened when the F-35B's forward speed is 250kt (460km/h), and proved too weak to handle the disruptive air flowing around the cover of the lift-fan, O'Bryan says. For a near-term fix, Lockheed modified the software for the inlet doors to keep them closed until needed at very low speeds, O'Bryan says. But that is not the long-term solution.

By the end of this year, Lockheed plans to complete a redesign for the AAI doors, which could involve strengthening the hinges or the doors themselves to survive vibration at 250kt, O'Bryan says. Lockheed has identified a near- and long-term solution to fix the bulkhead, which is numbered 496 on Lockheed's design sheets, which cracked in durability testing. At the point where the wing attaches to the rear of the fuselage, this bulkhead that absorbs some of the F-35B's heaviest aerodynamic loads failed less than 10% through a durability test.

In the short term, Lockheed can keep the STOVL test fleet flying despite the risk of structural failure, O'Bryan says. By blending one of the bulkhead's hard edges into a curve, Lockheed's analysis shows the structure will survive to 1,500h. In an overall flight-test programme expected to involve 7,500 flights by more than 13 aircraft, that should give the BF-series fleet plenty of margin to complete all the test points necessary for certification. But the 1,500h lifespan of the 496 bulkhead falls short of the required 8,000h service life for operational fighters, so a long-term repair is also necessary. A 3.2-3.6kg (7-8lb) steel patch has been designed to strengthen the bulkhead, O'Bryan says. Lockheed also is considering a lighter-weight, composite patch.

Another major problem involves a performance measurement called vertical lift bring-back (VLBB). To land vertically without having to jettison stores or fuel, the F-35B either needs to reduce weight or increase thrust, or some combination of both, O'Bryan says. Internally, there is still no agreement on the amount of margin required to meet future weight growth. "We can bring 10 engineers in the room and get 11 different opinions," O'Bryan says. Previously, William Boley, president of Pratt & Whitney military engines, has described a need to increase VLBB by as much as 180kg. O'Bryan, however, said that figure is "a lot" when describing the actual VLBB margin.

Although Boley has advocated increasing the thrust of the P&W F135 engine to achieve the margin, Lockheed officials continue to resist that approach. "More thrust I will say generally comes with more heat," O'Bryan says. "More heat equals less lifetime, which equals more cost."

Lockheed is studying ways to get round the problem without raising the F-35B's price tag. "That's the nirvana where you can pull weight out and save cost," O'Bryan says. "We have funds to find those types of investments."

In basic terms, some of the F-35B's parts are failing too fast. Lockheed has traced the root cause to mistakes by suppliers. In the short term, Lockheed must solve the problem by buying more spare parts. In the long term, the parts have to become more reliable to keep the aircraft affordable. Says O'Bryan: "You have to do both."