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These are found on the stbd aft fuselage. The yellow disc indicates that the squib has discharged and the red disc that the bottle has over-pressured. The pressure gauge is optional and may not be fitted on all airframes.

The second photo (bottom right) shows the condition of the discs shortly after the APU fire extinguisher had been discharged. Notice how the squib disc has only moved slightly and the over-pressure disc not at all. This could easily be missed on a walkaround.

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*** Updated 16 Sep 2016 ***

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