There are some red windows on the 737 and the CS-300 as well. Why are they painted red?
Source 7378 cs100



2 Answers 2


A red/orange color is typically used to denote flight test instrumentation. Indeed, these windows do contain such instrumentation. Below you can see a similar window on a 747-8I flight test aircraft (note the 7th window from the nose):


And the corresponding inside view:

enter image description here

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    $\begingroup$ What is in those big tanks? $\endgroup$ Mar 26, 2016 at 9:50
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    $\begingroup$ @RockPaperLizard, those tanks hold water, which is used to create different center of gravity/weight conditions for flight test purposes. $\endgroup$ Mar 27, 2016 at 4:05
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    $\begingroup$ Makes sense. Thanks for the explanation. $\endgroup$ Mar 27, 2016 at 8:05

The aircrafts shown above are test aircraft and the windows are blanked for mounting test instrument. The image below shows the inside of Bombardier C-series test aircraft, which shows the mounting of test equipment.


Image from airwaysnews.com

Also, the screengrab below from this video about C series testing shows the interior of the C series test aircraft with blanked windows with attached instrumentation.

C series test

Also, see this thread that discusses about the same thing.


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