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LONDON—The engine fire on a Lockheed Martin Corp. LMT +0.79% F-35 Joint Strike Fighter that has grounded the fleet might have been a somewhat isolated incident, the Pentagon's top arms buyer said Sunday.

"We don't see at this point what I call a systemic problem," said Frank Kendall, U.S. undersecretary of defense for acquisition, technology and logistics.

The grounding occurred after an F-35A jet suffered a fire on June 23 on the ground at Eglin Air Force Base in Florida.

The operational hiatus has kept F-35s from making a planned trans-Atlantic flight to the U.K. for participation at the Royal International Air Tattoo in England during recent days and means the plane will be a no-show when the Farnborough air show opens outside London on Monday.

Safety investigators have reviewed the fire, saying that excessive friction occurred among blades and other internal parts of an engine built by the Pratt & Whitney arm of United Technologies Corp. UTX +0.83%

When one of the blades failed, Mr. Kendall said, it sparked the fire.

Safety authorities are now discussing whether to allow the plane to return to flight. Mr. Kendall said he was "hopeful" the F-35 could still make an appearance at the air show in the coming days.

Some friction among components is acceptable.

For reasons that aren't fully known, the level of interaction exceeded design margins in the plane that caught fire.

Mr. Kendall said evidence so far suggests it was a unique incident, although it is too early to draw that as a final conclusion.

The grounding comes as the government and Lockheed Martin also are trying to ramp up efforts to reduce costs.

Lockheed Martin said on Thursday that it, along with partners, would spend $170 million to find ways to lower long-term costs.

Mr. Kendall said the initiative "is a positive step."

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""FARNBOROUGH - The blade rubbing that prompted a fire in an F-35A – leading to a fleetwide grounding July 3 – took place in the third-stage fan of the integrally bladed rotor (IBR) in the low pressure section of the F135 engine, according to Lt. Gen. Christopher Bogdan, program executive officer for the U.S.–led F-35 program.

The three-stage IBR sits behind the front fan in the F135 and compresses the air before passing it into the high-pressure core. Each stage is separated by a stator and rotates within the casing, which is lined with an abradable strip to maintain tight clearances between the blade tips and the inner wall of the compressor casing. This enables tight tolerances while reducing pressure loss and some rubbing is acceptable. In the engine for AF-27, the blades were rubbing far in excess of the design, creating excessive heat and microcracking in the blades. The resulting high cycle fatigue failure forced the section to “come apart,” Bogdan says, prompting the fire June 23 at Eglin.

The pilot safely egressed and the flames were extinguished with foam.

Senior Pentagon officials say the problem thus far appears isolated; officials have inspected all 98 Pratt & Whitney F135 engines in service, Bogdan says. “All 98 of the other engines did not indicate the same phenomena as the one that failed,” he said. “We have created a body of evidence now that we think is ample enough to fully understand what happened.”...

...Meanwhile, “there is a growing body of evidence that this is not a systemic, major design problem,” Kendall said.

“This is not related to any incident in the past,” said Pratt & Whitney President Paul Adams.

Pratt engineers were already in the midst of a redesign to the first stage fan in the IBR, embracing a solid bladed design over the hollow design. A ground-based test engine “blew” late last year, Bogban said, owing to cracking in the hollow blades. The second and third stage fans –- the area in question for the June 23 fire -– are both constructed of solid blades. Pratt has submitted plans to Bogdan’s office for approval of plan to produce solid blades in the first stage and they are under review. This work is unrelated to the June fire incident, Adams said.""

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Some friction among components is acceptable.

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"...
The company had already begun a redesign for the IBR as part of a movement to reduce overall engine costs, according to both Bogdan and Pratt spokesman Matthew Bates. The new design replaces the current hollow blade design with solid blades, which allow the company to use simpler, and hence cheaper, manufacturing techniques.

That redesign will add roughly six pounds to the engine; although the engine in question was a STOVL F-35B model, the IBR redesign will impact the engines for all three JSF variants. While weight is closely monitored on the F-35, Bogdan did not seem concerned about the extra weight....

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If the blades were hollow for the sake of cooling then that might mean that some other measure will have to be taken to prevent them from melting - with other undesirable trade-offs.

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"...The incident is the second in less than a year to affect a part of the engine known as the integrally bladed rotor, or IBR. The component contains three sets of fans to help compress air into the engine. A first-stage fan made of hollow titanium blew apart during ground testing in December and is undergoing redesign, while a third-stage fan made of solid titanium disintegrated during the June takeoff.

Officials said the two cases aren't related.

A review of almost 100 engines in the fleet didn't find evidence of the same problem affecting the third-stage fan, though several showed signs of mild rubbing, Bogdan said. The extent of the problem appears to be isolated to that particular aircraft,..."

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FARNBOROUGH England (Reuters) - U.S. military officials have lifted a fleetwide grounding order for Lockheed Martin Corp F-35 fighter jets, but will maintain some operating restrictions on the jets given an ongoing investigation into a massive engine failure last month, sources familiar with the situation said on Tuesday.

Discussions are still under way about whether the decision will clear the way for the newest U.S. warplane to travel to Britain to make its international debut at the Farnborough air show

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Safety remains the overriding priority.

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Two US fighter jets were due to make their international debuts this week at the year's most important aerospace event, the Farnborough Airshow. At the moment, only one of them is here.
The F-35 may be the world's most expensive, most advanced military jet programme, but it was a cheap and cheerful budget aircraft that managed the trans-Atlantic crossing to Farnborough.
The Scorpion costs about $20m (£12m) a throw, is built from off-the-shelf components, and went from drawing board to first flight in 23 months.
The F-35, costing three times as much and conceived in the early 1990s, is still in the US while engineers figure out what caused a fire that has grounded the entire fleet.
OK, making comparisons is unfair; the Scorpion and F-35 are lightyears apart in specification and functionality. But it is still slightly ironic

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