A spot-on rotation after an 8s take-off roll should have excited - or at least relaxed - the high-ranking programme officials lining the runway at the US Navy's Patuxent River NAS in Maryland on 17 March.

BAE Systems test pilot Graham Tomlinson had just proved that the Lockheed Martin F-35B can indeed take off from amphibious carriers and small runways. Checking-off the test point also would clear the way for Lockheed to stage the F-35B's first vertical landing the next day.

Coming six weeks after a major restructuring caused by a $100 billion increase in cost projections, the F-35 Joint Strike Fighter programme undoubtedly felt pressure to score a landmark success in the long-delayed flight-test programme.

At the moment of Tomlinson's rotation to flight, however, the mood on the flightline was more like anxiety. For a large, black object was observed blasting from the tail of the aircraft down the runway in the opposite direction just as test aircraft BF-1's nose tilted skyward.

Lockheed Martin F-35B JSF
 © Lockheed Martin

Doug Pearson, Lockheed's vice-president for the integrated flight-test force, feared the surprise object signalled a problem on the now-airborne F-35B with Tomlinson still on board.

"It really got my attention," Pearson says. "When you see something else flying off after your airplane, you think, 'Oh my gosh, what came off'?"

Obviously, the scare turned out to be nothing. The first short take-off flight was followed on 18 March by the first vertical landing, again flawlessly executed by Tomlinson.

The mysterious object seen propelled backwards from the F-35B was determined to be a rubber grommet. The black, hollow ring was fitted - but not glued or sealed - around a light fixture embedded into the runway, Pearson says.

The grommets around such lights have survived countless take-offs and landings by jets at Patuxent River, including several by flight-test aircraft in the F-35B programme. But its ejection during the first short take-off tests suggests the power from the F-35B's exhaust nozzle, which is vectored perpendicular to the runway on short take-offs.

Yet the incident does not appear to reinforce concerns about the heat or force generated by the F-35B exhaust aboard a carrier, as the grommet was not attached to the ground or the light fixture.

 Mystery object ejected on F-35 takeoff test solved

"The exhaust from the thrust vectored down blew it right out of the hole because they're not glued in. They're just sitting there," Pearson says.

But the moment does symbolise so many minor and major perils faced by any flight-test programme, which, by definition, are characterised by moments of unanticipated - but never unwelcome - discovery.

The 42-month history of the F-35 flight-test programme so far is no exception. A previously seven-year series of flights to confirm the airworthiness and mission capability of three variants has been extended to 10 years.

Few of the reasons for the delays appear to have been caused by unanticipated discoveries during the flights themselves, with few exceptions. An errant grommet on the first short take-off of an F-35B is not a real concern.

More worrying within the programme no doubt was an electrical short-circuit experienced on Flight 19 in May 2007. A poorly spaced wire touched the lid of a box, knocking out power to the right horizontal stabiliser. The F-35's 270V, power-by-wire flight-control system responded appropriately, adjusting the authority for other control surfaces to maintain normal flight. A wiring redesign later would fix the wiring problem.

Flight 19's electrical snag remains a blip in an otherwise fairly uneventful flight-test programme in the air.

But the real problem for the F-35 programme has been getting aircraft into the air at all. The programme had by mid-June completed fewer than 250 flights, mostly because of setbacks on the ground.

Lockheed Martin F-35C JSF 
 © Lockheed Martin

The slow pace has marred what is now one of the most intense and lengthy flight-test programmes in history. A series of 5,000 flight tests must be completed with the JSF's three different variants.

Breakdowns in production appear to be the prime cause for extending the seven-year schedule into a 10-year programme.

Deliveries of flight-test aircraft are running essentially a year behind the previous schedule. Lockheed celebrated the debut flight of the first F-35C carrier variant named CF-1 in early June. But that event occurred nearly 11 months after "roll-out" and about nine months after it was originally scheduled.

Moreover, the first short take-off and vertical landing aircraft were delivered three years ago, but were grounded for a year. A second-stage turbine blade for the Pratt & Whitney F135 engine failed fatigue testing and was grounded until the manufacturer completed a fix.

The flight-test programme is now back on track, but faces a reduced set of goals. While the previous schedule called for flying more than 1,240 flights in fiscal year 2010, the programme's restructuring earlier this year produced a revised plan. Lockheed is now expected to complete only 394 flight tests in the calendar year of 2010, increasing to about 1,000 next year, Pearson says.

Six months into the new schedule, it appears that the company has finally found solid footing. Sortie rates had ramped up from about 30 for the first three months of 2010 to about 30 each in April and May. Lockheed was required to complete only 28 flights in June, but was tracking ahead of schedule through the first half of the month.

By design, the F-35 is not faster or vastly more agile than the aircraft that it will replace, with the exception of the US Marine Corps' Boeing AV-8B Harrier II. Unlike the US Air Force's Lockheed F-22, the F-35 is not seeking to invent the concept of a supersonic, stealthy aircraft.

That is not to argue that the flight-test programme should be easy. Despite the programme's ambition to manage risk, Lockheed was given the task of solving two problems.

First, the F-35's flight-test team must demonstrate that a largely common design can perform three different missions for the USAF, US Navy and USMC, with the latter variant requiring the first operational test of a shaft-driven lift fan in a Western combat aircraft.

Secondly, the F-35 introduces a supersonic package of high-fidelity sensors and flight-control systems managed by more than five million lines of software code on board the aircraft.

Yet Lockheed's design for the F-35 remains almost entirely intact since major changes were made in 2004 to counter weight growth. A few tweaks have been incorporated however, particularly on the F-35C. Fatigue tests revealed that the carrier variant's keel beam needed a redesign, and Lockheed has also added a spoiler to the model's larger wing.

Such tweaks seem minor so far in comparison with previous flight-test programmes. McDonnell Douglas had to redesign the F-15 wingtip and Boeing encountered a wing-drop problem with its F/A-18E/F Super Hornet.

"I'm very confident that we've got the shape right," says Pearson, a retired USAF major general who previously commanded the service's flight test centre.

Pearson cites static testing on the F-35 ground test aircraft. This concluded on the conventional take-off and landing F-35A in early June with "no significant setbacks", he says.

"That relieves the burden on the flight-test programme. If we can get the load paths correct on the CTOL we can have high confidence we have the design load-path on the other two variants."

Lockheed Martin F-35A Pair JSF
 © Lockheed Martin

Although 42 months have passed, Lockheed remains in the beginning stages of the flight-test programme. Test pilots so far have aimed at validating the middle of the flight envelope, a term that describes an aircraft's ability to manoeuvre at different speeds and altitudes.

Tests have shown so far that the aircraft flies "very nicely" at 350kt (647km/h) and 20,000ft (6,100m) altitude, Pearson says.

Flight tests also have evaluated the F-35's performance in the transonic zone between Mach 0.95 and M1.05, with the CTOL and STOVL variants both having achieved up to M1.07 airspeed. Top speed for the F-35 is listed as M1.6, but the programme is focusing on the transonic zone first, which, Pearson adds, is the "most interesting".

"There's no reason we didn't go faster," he says. "We didn't need to yet."

The airspeed tests offer an example of the flight-test programme's philosophy. Rather than evaluate each capability to its logical extreme, the integrated test team is taking samples and extrapolating the data.

With fewer than 250 flights achieved so far, Lockheed is still focused on confirming the aircraft's basic flying qualities and systems. A key issue is the aircraft's unique power-by-wire flight-control system, which uses electrohydrostatic actuators powered by a 270V electrical system.

"The F-35's [USAF-coined] nickname 'Lightning' is very, very appropriate if you relate lightning to electricity," Pearson says.

The F-35's electrical loads generate an unprecedented amount of heat in a combat fighter of its size. That heat must be dissipated. However, to preserve the F-35's infrared signature, dumping it overboard is not an option. Instead, the F-35 channels thermal loads into its fuel, which serves a dual role as heat sinks.

Ensuring the right balance is a challenge, as demonstrated by the F-22's well-chronicled thermal management problems.

On the F-35, thermal management remains a concern within the flight-test programme. First flight for the third CTOL flight-test aircraft - AF-3 - was delayed by several weeks in June because of excess heat generated by "a couple of components".

"We're working a couple of issues on the airplane related to particular issues," Pearson says. "It's not a design problem. It's more about the performance of a couple of components."

More thermal management problems should be solved when P&W delivers planned improvements for the F135 engine. The F-35's current sole powerplant "puts a lot of thermal load into the system", Pearson says. Upgraded fuel pumps and valves for the F135 will help to further reduce the excess heat-loads, he adds.

Overall, however, the F-35 is starting at a higher level of maturity with the thermal management system than the F-22, Pearson says. At the beginning of the Raptor's flight-test programme, time limits were placed on operating the aircraft on the ground because of overheating problems in its electrical system. The F-35 flight-test fleet has no such limits placed on it, so far.

"Thermal management has turned out to be an interesting issue and one that we've worked very hard," Pearson says. "The dividends are paying off."

Flight test, of course, is not necessarily supposed to go perfectly smoothly. The goal is to discover the "unknown-unknowns" in the design of the aircraft. Part of Pearson's job is to be prepared to respond as flight tests reveal new problems. As noted earlier, Flight 19 provided a vivid example.

"I thought the response from Flight 19 was pretty good," Pearson says. "We learned from it."

Indeed, Flight 19 offers a template to understand Lockheed's process for responding to such events. After completing its own investigation, Lockheed invited the USAF to "look at the incident" as well, Pearson says.

"If we have a serious incident in the future, we'll ask one or both of the services to come in and look at what occurred," he says. "We'll review and learn precisely what happened and what can we do to prevent it in the future."

New discoveries are possible on any flight during this phase of the programme, but - after the first vertical landing on 18 March - the next most serious test for the F-35 will be trials aboard the USS Wasp amphibious carrier in the second quarter of FY2011.

The test will see the STOVL variant's BF-1 test aircraft fly out to the ship and perform short take-offs and vertical landings. The Wasp's deck will be evaluated for how it survives the exhaust air of the F-35B in STOVL mode.

It has taken nearly four years for the F-35 flight-test programme to reach the stage of daily flight tests involving several different aircraft and all three variants. But with less than 5% of the flight-test sorties having been completed, there is plenty of room for more discovery to come.

"This is the beginning of the movie," Pearson says, "and it's looking pretty good."

Source: Flight International