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enter image description here
— wikimedia.org; cropped

Those have always caught my eye and I always dismissed them as equipment cooling vents.

But, after finally remembering to check the flight manual, it turns out the equipment cooling is vented either through the forward outflow valve (near the nose landing gear), or the aft outflow valve (near the rearmost left door).

Googling for those vents, I could not find any result, so what are those vents for?

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enter image description here

10 Left AFT Fuselage

  • AFT outflow valve
  • Water drains, service doors, waste vents
  • Air demand pump exhausts (2)

The only non-explicit hint in the flight manual is in the exterior inspection section above; searching the manual for "air demand" yields only one other result:

Sec. 6.2 Page 1 [...] Air Conditioning Packs

[...] Pack output is automatically increased during high pack demand periods (to compensate for a failed pack or recirculation fan), or limited during high bleed air demand periods (such as for gear retraction during takeoff).

Given the gear retraction remark, it must be related to the two "air driven hydraulic pumps" of the center hydraulic system. Knowing what to look for now, a Boeing paper on the 777's hydraulic system uses the engineering acronym ADP (air driven pump), which is not used in the flight manual:

[...] the EDP and ADP are interchangeable. The ADP is powered by the engine bleed air via the environment control system (ECS).

Using the engineering term to find photos yielded this from a maintenance manual, confirming they're the exhausts of the air used by the air driven hydraulic pumps:

enter image description here
— Boeing 777 "knocking" query, pprune.org

And this:

enter image description here
— reddit.com

This reminded me of the 767, which has one air driven pump, and sure enough, there is one such vent on the 767:

enter image description here
— airlinercafe.com

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  • $\begingroup$ Good find, it's strange they called them "air demand pump", I don't get the exact meaning of "air demand", "air driven pump" seems more consistent with EDP $\endgroup$
    – mins
    Oct 9, 2021 at 18:53
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    $\begingroup$ @mins Each 777 hydraulic system has primary and demand pumps. For L and R, the primary is engine driven and demand is electric. For C, the primary is electric and demand is air driven. The FCOM says "The demand pumps provide supplementary hydraulic power for periods of high system demand [and] also provide a backup hydraulic power source [...]" $\endgroup$
    – Bianfable
    Oct 9, 2021 at 20:18
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    $\begingroup$ @mins: Now that you mention it, I double checked: the paper uses both – "driven" for the acronym ADP, and "primary" and "demand" for the functions as @ Bianfable explained. The FCOM never says "ADP". $\endgroup$
    – user14897
    Oct 9, 2021 at 20:56

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