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The engines are always painted a certain colour but the front part is always grey.

Source

cfm56

And here on the XWB: xwb

Source

Why is it that?

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    $\begingroup$ Not directly related to your question but what's the red line on the side of both engines that sort of goes halfway around? $\endgroup$ – curious_cat Feb 7 '16 at 16:41
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    $\begingroup$ Red line is to indicate danger area. No personnel allowed in front of red line when engine is running. boeing.com/commercial/aeromagazine/articles/qtr_3_08/… $\endgroup$ – vasin1987 Feb 7 '16 at 16:52
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    $\begingroup$ @TomMcW According to the narration, a strap on his suit snagged on a probe and held him in position a few inches in front of the fan. The engines themselves are several feet aft of the front of the intake. $\endgroup$ – reirab Feb 7 '16 at 21:58
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    $\begingroup$ @reirab Ah. I didn't watch the whole thing. I've seen that video a thousand times. It's so freakish you can't stop watching. But I wouldn't want to see what the flight attendants saw that Casey mentioned. I guess the comp blades must be pretty far back from the inlet. He wouldn't get so lucky on a high bypass engine. The fan is right there. Like a giant Cuisinart! $\endgroup$ – TomMcW Feb 7 '16 at 22:03
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    $\begingroup$ @vasin1987 - I especially like how that article is entitled "Preventing engine ingestion injuries...." as if "being turned into mince" still falls under the "injury" banner $\endgroup$ – Jon Story Feb 8 '16 at 11:48
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The engine inlet lip is bare metal to facilitate anti-icing. The area is heated from within using hot engine bleed air.

This partial schematic (from Boeing AERO QTR_01.12) shows the engine anti-ice valve, which controls the air supply to the inlet lip: 747-8 Engine Bleed Schematic

Interesting side note: Even the 787 "no-bleed" system architecture uses bleed air for engine anti-ice.

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There are two reasons I can think of for this:

  1. Aerodynamic reasons. Believe it or not, even paint can change the dynamics of a plane, to the point that Boeing discourages multi-colored paint jobs or liveries on 787 nacelle, claiming they affect performance.

  2. If that part of the engine were painted, there would be risk of paint chips being sucked into the engines, potentially damaging them. While this may seem far-fetched, this account shows that even paint chips can pose a foreign object debris threat to an engine. By not painting this part of the nacelle, the risk of paint flecking off and into the engine is reduced.

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    $\begingroup$ The linked article is about chips of concrete and runway marking paint causing damage. The stuff they use to paint the runway markings is much more substantial than the paint used on planes. $\endgroup$ – David Richerby Feb 8 '16 at 4:24

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