This is picture of a B737-800. When you open the bleed system a small amount of air is discharged from this hole (marked with an arrow, lower right). What is it for? And what happens if this hole gets clogged?

AMM Part I Reference 29-09-00, page 3.

hyd. reservoir press. line a small hole

enter image description here

Click images for higher-res.

  • $\begingroup$ Do you have a closeup of that?? $\endgroup$
    – acpilot
    Commented Feb 1, 2017 at 14:09
  • $\begingroup$ @Federico at the end of the pipe the red arrow points to I think. $\endgroup$
    – Notts90
    Commented Feb 1, 2017 at 14:10
  • 2
    $\begingroup$ @Notts90 thanks. in my brain it was only a red pipe. $\endgroup$
    – Federico
    Commented Feb 1, 2017 at 14:25
  • $\begingroup$ Red arrow shows to small hole. Small hole is end of the pipe plug. $\endgroup$ Commented Feb 1, 2017 at 14:32
  • 1
    $\begingroup$ Amm part 1 referance 29-09-00 page 3. $\endgroup$ Commented Feb 1, 2017 at 14:37

1 Answer 1


Answering based on ATA 49 system, B757/767 APU Surge Control Valve muscle air plumbing.

Some pneumatic sensing and pressure systems have a moisture bleed hole to prevent icing in the system. They are typically located at a low point or bend in the line to allow water to escape the system while maintaining pressure. If the hole gets plugged it can prevent moisture from escaping and ice to form in the line.


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