Monday, November 03, 2014

What caused the F-35 engine fires and how's it getting fixed?

If you've heard a whole lot about the F-35 engine fires but are a little unclear on what happened, the head of the program did a pretty good job of explaining the cause and the planned fixes.

Air Force Lt. Gen. Christopher Bogdan, who heads the F-35 program, said the engine failure and subsequent fire that halted testing of the F-35 were the result of micro fractures in one of the three-stage fan sections that compress air before it enters the engine, reported the Pentagon's news agency Friday. These sections are lined with a polyimide material designed to rub against fan blades to reduce pressure loss.

The investigation into the F-35 engine fire that temporarily caused the fleet of fighter planes to be grounded this summer found the third fan rubbed well above tolerance during maneuvers several weeks before the failure, causing the blades to heat to about 1,900 degrees — 900 more than ever expected, Bogdan said. This led to tiny fractures in the titanium part of the rotor, which grew over the next few weeks of flying before finally failing.

The result? The rotor separated from the airplane. And as pieces of the engine flew out through the fuel tank, the fire ignited.

The F-35 is made by Lockheed's Fort Worth-based Lockheed Martin Aeronautics unit at the company's massive facility there.

Engine manufacturer Pratt & Whitney came up with two short-term fixes that will keep the planes in the sky until a permanent solution comes. For the first, a new engine is worked to "burn in" the rubbing in a controlled environment. That process has already been used on four test airplanes.

The second fix involves a change to the manufacturing process of the engine, eliminating the rubbing on the polyimide lining completely by "pre-trenching" the lining. That worked great, Bogdan said, but the process of replacing the engine takes about a week. That makes the alternative burn-in approach essential for getting planes in the air in the meantime.

Long term, Pratt & Whitney is mulling a few options in addition to the pre-trenching approach, from changing the polyimide material to one that can handle more heat, to treating the tips of the titanium fan blades to withstand more heat, to some combination of the options.

Bogdan estimated it won't be until near the end of 2015 before engines are coming off the production line with the chosen solution. Once that happens, he added, any engines that are not in airplanes yet will be retrofitted.

Bethesda-based Lockheed Martin Corp. hasn't been hurt too much by the testing delays. Bruce Tanner noted during a media call to discuss third quarter earnings that despite being grounded for a month and a half, the F-35 was still close to meeting its delivery goals.

As for Pratt and Whitney, sure, the company will have to swallow the costs for the fixes. But as Reuters reported last week, it also announced it had won a contract to build an eighth batch of 48 engines for the F-35, bringing the total value of the deal to $1.05 billion.

- Source:    http://www.bizjournals.com

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