On the side of the engine cowling of the Avroliner 146-RJ85 in the photo below you can see a small square placard located below a small round hole (view it large!) that reads ......
Photo © Dion Fuchs
I suspect this feature/hole has something to do with the engine's fire bottles.
My questions are....... What is being indicated?
Is it that the fire extinguisher is fully charged?
Is it that the extinguisher has been used and thus needs servicing?
Is this object a pre flight "walk around" check for the pilots or strickly used by maintenance personnel?
What does this round object/hole do? Does it change colour? Does an object stick out of it when it's indicating?
Finally, do other jet airliner's engine's have these Extinguisher Indicators on the outside of their cowlings?