3 ms·

And this may be obvious to some, but why does use inhibit rust?

t0mas88 6y ago
In normal use these valves take air out of the turbine stage, so they see very very hot air with almost zero moisture in it in normal use. If you park them long enough I can imagine they get wet from rain or even just condensation.
- aunty_helen 6y ago
  According to the AD this is the 5th stage bleed air valve, so on the compressor side and cold air. >Bleed air is tapped at the 5th and 9th stages of the HP compressor and from the fan. https://hursts.org.uk/airbus-technical/html/ar01s19.html#idm45838557976224 https://hursts.org.uk/airbus-technical/html/ar01s19.html#idm... (Airbus doc but the engines are CFM56 on the 737 models listed) Also according to this page the CFM56 doesn't have a turbine bleed air takeoff.
  - benhurmarcel 6y ago
    When you say "cold air" I guess you mean that it's bled before the combustion stage. It's not cold at all after the compression. And bleed air is indeed always taken in the compressor, never in the turbine. First there's no point taking it later than the compressor, you just want compressed air. And it's ultimately used to ventilate the cabin, you don't want fuel vapors or smell.
    - aunty_helen 6y ago
      Stage 5 would be pretty early on. However generally when talking about the HPC you're talking about cold air. At this stage, cold air that's sprayed on the turbine housing when engine power is reduced to thermally shrink the metal to keep the turbine blades and casing within tolerance for efficiency's sake. All is explained in the link I posted, would recommend it as good introductory reading on the subject.
JanSolo 6y ago
From my cursory glance at the doc, the part in question is inside the bleed-air cutoff system. This is a flapper valve (a bit like in your toilet) that takes air from the engine and uses it for other systems in the plane (air conditioning, cooling, starting other engines, etc). When it's in use, the movement of the parts will prevent rust from building up in one spot. A little rust might accumulate, but it will be rubbed/wiped off when it's next used. The problem occurs when enough rust builds up that the flapper cannot move anymore.
bfuclusion 6y ago
Not specific to this part, but most mechanical items that move get oiled as part of their operation. When they don't move for a long time that oil evaporates, and now the moisture in the air can attack the surfaces. This is a common situation in general aviation if they're not flown regularly and can lead to early replacement of the engine.

In normal use these valves take air out of the turbine stage, so they see very very hot air with almost zero moisture in it in normal use. If you park them long enough I can imagine they get wet from rain or even just condensation.

aunty_helen 6y ago
According to the AD this is the 5th stage bleed air valve, so on the compressor side and cold air. >Bleed air is tapped at the 5th and 9th stages of the HP compressor and from the fan. https://hursts.org.uk/airbus-technical/html/ar01s19.html#idm45838557976224 https://hursts.org.uk/airbus-technical/html/ar01s19.html#idm... (Airbus doc but the engines are CFM56 on the 737 models listed) Also according to this page the CFM56 doesn't have a turbine bleed air takeoff.
- benhurmarcel 6y ago
  When you say "cold air" I guess you mean that it's bled before the combustion stage. It's not cold at all after the compression. And bleed air is indeed always taken in the compressor, never in the turbine. First there's no point taking it later than the compressor, you just want compressed air. And it's ultimately used to ventilate the cabin, you don't want fuel vapors or smell.
  - aunty_helen 6y ago
    Stage 5 would be pretty early on. However generally when talking about the HPC you're talking about cold air. At this stage, cold air that's sprayed on the turbine housing when engine power is reduced to thermally shrink the metal to keep the turbine blades and casing within tolerance for efficiency's sake. All is explained in the link I posted, would recommend it as good introductory reading on the subject.

JanSolo 6y ago

From my cursory glance at the doc, the part in question is inside the bleed-air cutoff system. This is a flapper valve (a bit like in your toilet) that takes air from the engine and uses it for other systems in the plane (air conditioning, cooling, starting other engines, etc). When it's in use, the movement of the parts will prevent rust from building up in one spot. A little rust might accumulate, but it will be rubbed/wiped off when it's next used. The problem occurs when enough rust builds up that the flapper cannot move anymore.

bfuclusion 6y ago

Not specific to this part, but most mechanical items that move get oiled as part of their operation. When they don't move for a long time that oil evaporates, and now the moisture in the air can attack the surfaces. This is a common situation in general aviation if they're not flown regularly and can lead to early replacement of the engine.