What's bleed air valves for?

[tifafan]

When we start up an cold aircraft ,we have to cycle bleed air knob through to NORM.

 

 

What's bleed air valves for?

 

Do we have to cycle it after right engine starting up?Why?

Because it affect left engine's startup?

[jonsky7]

Though I don’t know the specifics of how a F18s systems work, Bleed air is usually pressurised air ducted from the compressor stages of a gas turbine engine. This air is used to pressurise the cockpit/cabin, used for temperature control, Defog the windshield, and provide engine anti-icing.

 

You’d normally turn bleed air off for engine start.

Edited December 22, 2018 by jonsky7

[Repth]

In the f-18 the engines use bleed air from the apu to start. The fire test you perform closes off the bleed air valves in the aircraft. In order to provide bleed air for startup back to the engines you have to cycle them open again. That is the reason you have to move the knob around again.

[macedk]

cheeky answer ....bleed air ? ;)

[Rainmaker]

Though I don’t know the specifics of how a F18s systems work, Bleed air is usually pressurised air ducted from the compressor stages of a gas turbine engine. This air is used to pressurise the cockpit/cabin, used for temperature control, Defog the windshield, and provide engine anti-icing.

 

You’d normally turn bleed air off for engine start.

 

It doesnt really exist during engine start so not much of a need to turn it off. Most of your bleed air valves are goint to be fail safe close anyway and with no power/servo air they will be closed.

[Robin_Hood]

It won't prevent or hamper your second engine start, as APU air does not pass through a bleed air valve (it will however prevent a crossbleed start, obviously). But with bleed air valves closed, you will have no air conditionning or pressurisation.

 

Bleed air is air taken from the compressor stage of the engines and used for the ECS (Environment Control System). When you do the Fire test, the valves close and air is no longer taken from the engines. The idea is to get your ECS back up and running as soon as you can, which is as soon as you have AC power on the aircraft, therefore, after the first engine start.

 

 

[raelias]

It's simple: when you perform a firetest the bleed valves are forced shut, you just need to turn the knob to reset em to normal operation

 

If you were not to perform the fire test for whatever reason you don't need to touch them at all

Edited December 23, 2018 by raelias
further info

[Eagle7907]

Doesn’t bleed air provide avionics cooling as well? That MIGHT be important?!

 

 

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[Robin_Hood]

Yes ,you may get an AV HOT caution if you keep bleed air off, especially with radar, ECM, sensors, etc... running).

Note that in-flight, you can supplement avionics cooling with ram air via the AV COOL switch EMERG position. It opens a scoop that allows ram air to cool part of the avionics. However, once opened, it can only be closed manually on the ground. I have no idea if this is modeled in DCS as of now.

[Flamin_Squirrel]

When we start up an cold aircraft ,we have to cycle bleed air knob through to NORM.

 

 

What's bleed air valves for?

 

Do we have to cycle it after right engine starting up?Why?

Because it affect left engine's startup?

 

Bleed air comes from the engine compressor, and is quite hot (~300C). If the bleed air ducts ever leak, this hot air is a serious fire risk; the bleed air valves are there to shut off the bleed air to stop this from happening. This is why the bleed air valves form part of the fire protection system, and close during the test.

 

Bleed air is used for many different things including pressurisation of the cabin, fuel tanks, external fuel transfer, canopy sealing, G-suit pressurisation, gun gas purging, canopy anti-ice and some others I can't quite think of. It's cooled before being used for these things, of course!

[tifafan]

Thank you guys!