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What are these two holes at the rear of most 747s? I've found plenty of answers for the holes on the front of the wing box, but no answers about these back holes.

enter image description here

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    $\begingroup$ Does this answer your question? What is this device below the door of a 737 (and other aircraft)? $\endgroup$ – ymb1 Jun 14 at 1:47
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    $\begingroup$ What are these elements on the back of a Boeing 737? with a photo of what is behind the hole. $\endgroup$ – mins Jun 14 at 18:34
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    $\begingroup$ If this question had a picture of a 737, I'd agree that it's a dupe of those questions. But it's a different type of aircraft with two holes, while the 737 has one. To close as a dupe is to imply that those 2 on the 747 are the same thing as the 1 on the 737, although without actually SAYING so, nor having a source that TELLS us so. If the 747 does in fact have two outflow valves (believable; I don't know offhand), I'd think it's reasonable to provide an answer that documents this fact, rather than implying as much without having to support it by 'closing as dupe.' VTLO. $\endgroup$ – Ralph J Jun 14 at 19:05
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These are the cabin outflow valves for regulating cabin pressure.

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    $\begingroup$ Can you provide a source so that somebody who questions if the 747 does in fact have two outflow valves (while smaller aircraft have just one), has something they can see to verify what you've posted? I can believe that your answer is probably correct, but documentation would be good. $\endgroup$ – Ralph J Jun 14 at 19:07
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    $\begingroup$ Unfortunatelly the manual I am refering to is copy right protected but to quote a small section: "Two outflow valves are installed at the rear of the cabin. The valves normally operate in parallel. Cabin altitude and full ventilation rates can be maintained by either valve." (Boeing 747 Operations Manual) $\endgroup$ – tsg Jun 14 at 19:57
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    $\begingroup$ Here is the overhead panel section containing the valves controls. There are two sets of controls below the yaw damper switches. Each set corresponds to a valve. $\endgroup$ – mins Jun 16 at 14:53

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