Hi barit1,
I am thinking this engine was in an aircraft being used to test various thermal paints (heat resistant) applied to some areas of tail that were receiving damage from the afterburner which swivels (not fixed in one plane). So they ran the afterburner in flight for excessive periods of time to determine which paint worked best or perhaps using a preselected paint to determine if it fixed the problem. Then at some point discovered the crack in one 3rd stage turbine blade. I am sure their first thought was oh-oh, the older blade cracking problem has returned.

Anyhow, all of this leads to some questions:
1. Why would prolonged afterburner usage have any effect on the LPT of the engine?
2. Wouldn't engine EGT monitoring result in the fuel schedule being automatically reduced if EGT was being exceeded?

A fairly early and low operating hour blade creep situation leading to creep rupture (cracking) on a single crystal turbine blade would seeming indicate one is perched on the edge of the cliff with not much margin. Perhaps this blade had a not so perfect crystal orientation that degraded creep life or the stress levels of the blade are higher than thought to be. It is hard to say, but there has to be more to the story. The reduced hours prior to borescope inspection is a good thing.

TD

Turbine D:

Good possibilities, good questions. And I agree the burner usage should not affect the LPT.

However, if the A/B fuel or nozzle schedules were not quite right, there could be a LP rotor rollback which might put the blade into an unwanted vibration region. I have observed this rollback on older engines.

If Pratt's control loops do not provide this protection - well, that's just speculation on my part.

'Turbine D' perhaps you missed the link (not well highlighted by me on previous page): http://www.pprune.org/military-aircr...ml#post7721369

F-35 Engine Issue Isolated: Fleet Cleared for Flight 01 Mar 2013 Amy Butler

"...The crack was found in AF-2, which has been used for testing the aircraft at the edge of its operational envelope. "The engine in question is part of the F-35 test aircraft fleet and had been operated for extended time in the high-temperature environment in its mission to expand the F-35 flight envelope," according to a statement from Lt. Cdr. Kyra Hawn, an F-35 spokeswoman. "Prolonged exposure to high levels of heat and other operational stressors on this specific engine were determined to be the cause of the crack."...

...AF-2 was used to test the new horizontal tail skin, and so it was run through many extra afterburner tests. Officials are sure to implement additional inspections and monitoring actions to understand more about the durability of these engines in extreme conditions...."

Hi barit1,

I would agree with you, there are a lot of subtle things that cause A/B issues including the one you mentioned. During the 2009 time period in testing the F-35, they ran into an A/B screech problem where they couldn't get the engine to full power and high vibrations were a problem. The problem generally occurs at lower altitudes and high speeds. It's a problem you can't wring out in a test cell per se, but can be fixed changing fuel schedules & software and modifications to A/B hardware confirmed by flight testing.

SpazSinbad,

I did see and read the article you posted, but it isn't the full story. A fuller story is available here:

F-35 Cleared For Flight | Defense News | defensenews.com

Keep in mind the bigger picture here, cost overruns, delays in the testing program, the general cost of this aircraft relative to the various aircraft it is replacing and funding issues have generated much pressure on DoD management. So I don't believe we will see anything but very guarded statements tending to be as "positive" as they can be in newsprint articles. The customer (Lt. Gen. Christopher Bogdan) has stated his general unhappiness with both aircraft and engine at this point in time verses what was promised even although the flight status has been cleared to resume, once again.

TD

A 3rd stage turbine blade is pretty far back from the high heat zone. However it is susceptible to induced fatigue from aero sources. Multiple AB mis-lights are just one of many (wrong nozzle area etc.).

Once identified easily fixed

More info:

F-35 Fan Blade Crack Raises F-35 Durability Questions Amy Butler 05 Mar 2013

"...The crack was found on a blade in AF-2’s engine. Bogdan says this was the “workhorse engine on the program,” which has been used for envelope expansion work and, thus, exposed to extremes in operation, including flights at 1.5 Mach; in low-level, supersonic conditions and at 7-8G. This aircraft was “doing the majority of the envelope expansion for the A-model fleet,” he says.

Two other aircraft in the conventional-takeoff-and-landing fleet, AF-3 and AF-6 remain grounded because they have been flown in similar – though not quite as rigorous – conditions.

Engine experts will require at least two more weeks to explore what implications there are for engine durability, Bogdan says. “What level of thermal stress would it take to get to that point on a normal airplane,” not exposed to such extremes, he says. “The issue is if it turns out that it is less than 100% of what we expect the life of the engine to be, then we have turbine blade having life limiting parts on it and we’ll have to deal with that.”

Should this become a problem, customers could be required to conduct additional inspections and/or additional maintenance over what was originally planned."

Engine crack that grounded F-35 traced to thermal creep 06 Mar 2013 Zach Rosenberg

Engine crack that grounded F-35 traced to thermal creep

"Bennett Croswell, president of Pratt & Whitney's military engine division, says a problem with F135 engines that grounded the Lockheed Martin F-35 is due to thermal creep, and is unlikely to affect the aircraft further as it returns to flight status.

The issue was a crack in a third-stage turbine blade on a single engine. As a precaution, the US military grounded all F-35 aircraft until a cause was discovered.

"During [an] inspection we found about 1/6-inch (4.2mm) crack on the turbine blade," says Croswell. "We felt we could continue to fly, and we took that recommendation to the (joint programme office), but on consultation with them we both came to the conclusion it was safer to suspend operations."

Thermal creep from high-temperature, high-intensity testing was found to be the cause of the crack. The engine the tenth engine built, powers the second F-35A, was tested extensively at supersonic speeds and low altitudes, generating significant more heat than expected.

"It was operating at levels 4X an operational mission, and 4X of levels we had qualified the engine for," says Croswell. "That was very good news, you don't want something like high-cycle fatigue or low-cycle fatigue." The issue is not expected to impact operational aircraft for months or years, depending on how the aircraft are flown...."

Er,

Does that mean the issue IS expected to impact operational sircraft?

Courtney, every use of material influences its longer term readiness. What they found here might give them more data points for identifying borescope inspection intervals based on engine usage, time in afterburner/reheat, etcetera. Whether or not the enging is modular enough to replace blades or various sections simply, in the field or at intermediate level, remains to be seen.