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There are some circles below the wings of these two aircraft. Look: enter image description here

Source: Airliners.net (modified)

enter image description here

Source: Airliners.net

On the second picture the circles are clearly visible on the wing. What are they? What is their purpose?

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    $\begingroup$ Clearly, they are not part of a collision avoidance system. Ouch! $\endgroup$ – RockPaperLizard Apr 13 '16 at 19:30
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    $\begingroup$ They look like inspection ports $\endgroup$ – Dan Pichelman Apr 13 '16 at 19:48
  • $\begingroup$ They are inspection ports as @DanPichelman says. $\endgroup$ – Ron Beyer Apr 13 '16 at 20:12
  • $\begingroup$ I was prepared to say "engines", but I guess you're talking about different circles... :) $\endgroup$ – FreeMan Apr 14 '16 at 12:52
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In the US, those are called either "Access Panels" or "Inspection Covers", depending on their function, but many people combine the two terms and call them "Inspection Panels". Based on the comments, other parts of the world apparently refer to them as "Inspection Ports".

We remove both panels and covers to perform routine inspections, but access panels cover system components that may be accessed more often for servicing. Both panels and covers are designed for quick removal, typically fastened with screws, as is the case in the aircraft depicted in the photos above.

Here is an inspection hole with the cover removed. This is on a Cessna 170 that I am in process of rigging. Note the aileron cable turnbuckle visible inside the wing. Through this access panel I will adjust the cable tension, among other things, and once the cable is properly tensioned I will safety wire this turnbuckle. Here the panel is removed for maintenance, but during an inspection I would remove this panel to inspect these same systems.

enter image description here

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These are inspection rings. From FAA Advisory Circular 43.13-1B (complete PDF is here):

Inspection Rings. Inspection access is provided adjacent to or over every control bellcrank, drag-wire junction, cable guide, pulley, wing fitting, or any other component throughout the aircraft which will be inspected or serviced annually. They are installed only on the bottom side of the wings except where installed on the top surface by the original manufacturer.

This article also mentions it from a student's point of view.

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    $\begingroup$ I'm sorry, they are not. Inspection rings are used in fabric aircraft construction, as you cited from 43.13 Chapter 2, which is the chapter on Fabric. Inspection rings are used to create inspection access on fabric covered aircraft. $\endgroup$ – J Walters Apr 14 '16 at 11:04

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