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I came across this extremely interesting article with details on a 2018 United Airlines Boeing 777 engine failure, which is also related to the more widely-publicized failure a few days ago: http://aerossurance.com/safety-management/ndi-failures-b777-pw4077-fbo/ . Amongst others, it contains the following:

The records for the TAI [Thermal Acoustic Imaging] inspection in July 2015 as well as an earlier TAI accomplished in March 2010 revealed a thermal indication in the same location as where the LCF crack occurred. The records for the fractured fan blade’s July 2015 TAI inspection was annotated ‘paint’ that, according to the inspector, was consistent with him accepting the indication because he thought it was an issue with the paint.

Flaking paint was a regular issue, affecting perhaps 25% of blades, requiring either a touch-up or a complete re-paint.

which begs the question: why are fan blades painted? The only reason I can think of is so that damage from small objects (or birds) sucked into the engine is easier to see? But, if I read the article correctly, the paint can also mask more serious issues - in this case, the inspector thought that the TAI indication was due to flaking paint, which is apparently common, when in fact it was a defect in the blade. So maybe it would make sense to leave the blades unpainted?

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  • $\begingroup$ This seems like an attempt to devise a new inspection process. Given the amount of effort that is put into creating a highly polished surface on fan blades (at least by other engine manufacturers), painting them and not removing the paint seems crazy. There are reliable methods of detecting internal cracks that don't rely on inventing Rube Goldberg technology. $\endgroup$
    – alephzero
    Feb 26 at 23:19
  • $\begingroup$ Note, fan blades are often painted with colored stripes when running a blade-off test, simply to make it easier for computer software to track the failure sequence from high speed video. But in that situation losing a few percent of aerodynamic efficiency is irrelevant. $\endgroup$
    – alephzero
    Feb 26 at 23:24
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The final report notes:

At the time of each TAI [thermal acoustic imaging], the inspectors attributed the indication to a defect in the paint that was used during the TAI process and allowed the blade to continue the overhaul process and be returned to service. [emphasis mine]

The paint being referred to is used for the inspection. Its function is to improve the emissivity of the object.

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  • $\begingroup$ So... the fan blades are painted just for the TAI inspection and the paint is then removed? $\endgroup$
    – rob74
    Feb 26 at 10:58
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    $\begingroup$ @rob74: Yep. Most probably washed away. I tried to check for details on PW's method, but other than it's a "new and emerging" technique, the details seem to be under wraps -- maybe that will change with the latest engine failure. $\endgroup$
    – ymb1
    Feb 26 at 12:50
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    $\begingroup$ @ymb1 Based on "inside information" working for another engine company at the time, the latest blade-off failure seems very similar to one of the (several) failed blade-off tests that PW had to repeat to achieve certification. Boeing were sufficiently concerned at the time to share some PW data, that would normally have been proprietary, with the other manufacturers offering engines for the 777. $\endgroup$
    – alephzero
    Feb 26 at 23:33

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