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31 votes

Is the bleed air passengers breathe tapped from the engines before or after it comes in contact with fuel? And why? Isn't this unhealthy?

The bleed will be tapped from one or two of the 3rd, 4th, 5th or 6th High Pressure Compressor stages. Usually there are two bleed ports. The highest pressure port will supply really hot air for ...
John K's user avatar
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25 votes
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How do bleedless passenger airliners keep cabin air breathable?

Bleedless aircraft still get air from outside. They just have dedicated compressors to compress outside air for cabin use, rather than relying on bleed air from the engines. The air compressors have ...
Chris's user avatar
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24 votes

Why is a dedicated compressor more efficient than using bleed air to pressurize the cabin?

The other answer is on the right track, but doesn't emphasize the real difference (IMO). Let's say it takes 100 hp (or kW if you prefer) to compress the amount of air you need for a given purpose. ...
Rob McDonald's user avatar
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17 votes

Why is a dedicated compressor more efficient than using bleed air to pressurize the cabin?

Using bleed air isn't free. In order to produce the same amount of power as an engine with no bleed air taken out, the engine needs to compress more air. This takes energy, and means more fuel used ...
Chris's user avatar
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12 votes
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Why is ram air not used for air conditioning in airplanes?

The problem is pressure. In order to use outside air for the cabin, it needs to be pressurized. Otherwise you could not reach higher pressures inside the cabin than outside. The advantage of using ...
Bianfable's user avatar
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11 votes
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Is the air in aircraft cabin toxic?

It seems this is a fairly new issue cropping up and I have been able to find some credible research on the topic (i.e. from official aviation authorities and not news outlets or lawyers websites...) ...
Dave's user avatar
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11 votes

How do bleedless passenger airliners keep cabin air breathable?

tl;dr: Through air intake holes using compression. For example, here they are on the fuselage of a Boeing 787, below the forward edge of the wing: Bottom intake air gets compressed (and thus heated) ...
einpoklum's user avatar
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11 votes
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Why is a dedicated compressor more efficient than using bleed air to pressurize the cabin?

Given all this extra hardware why is a "bleedless" design more efficient per passenger-km? Flexibility and adaptation But this can only be understood not focusing on the engines only rather ...
sophit's user avatar
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7 votes
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Do pilots breathe the same air as passengers?

In General, Yes. As your image already shows, the same air is distributed throughout the cabin and the flight deck. For example, in the 737 NG FCOMv2 2.30.4 (Air Systems - Air Conditioning ...
Bianfable's user avatar
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7 votes

Why can APU supply both bleed air and electrical power up to 10.000 feet but bleed air alone up to 17000?

It's a function of the relative demand each service puts on the APU. You have to remember that bleed is power consumed; you're just taking it in a different form. As a turboshaft jet engine, the ...
John K's user avatar
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6 votes

Do pilots get more fresh air than passengers?

Taking the B737 as an example, fresh air from the left pack flows directly into the cockpit (green airflow in the diagram), whereas the passenger cabin is served from the mix manifold only (yellow). ...
summerrain's user avatar
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6 votes

When are the engine bleed air, APU and packs turned on in the B737?

The general answer is, the engine bleeds stay on almost all the time. The times they're turned off: For a bleeds-off takeoff to get more power from the engines. During deicing on the ground, so that ...
Ralph J's user avatar
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6 votes

Where does the air used in an air cooled oil cooler come from in jet engines?

Kerosene burning high altitude airplanes generally use fuel as the oil cooling medium, because you need to heat the fuel anyway due to the low operating temperatures at altitude. It limits ice ...
John K's user avatar
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5 votes

Why and how can a fume event occur?

(1) How can can other fluids contaminate the bleed air? Engine Oil: The engine fan and compressor stages are rotating pieces of metal on metal and thus need lubrication. Engine oil is used for this ...
Bianfable's user avatar
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4 votes

Is the bleed air passengers breathe tapped from the engines before or after it comes in contact with fuel? And why? Isn't this unhealthy?

I have found a diagram (bbc.com) showing the location where the bleed air is tapped (red arrow). As can be seen in the image, the location is upstream of the combustion chambers, which means that ...
summerrain's user avatar
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4 votes

How much thrust is lost due to the bleed air system?

It's on the order of a few percent. Not huge but not negligible either.
Daniel K's user avatar
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4 votes

What is the function of this small hole on the B737's hydraulic reservoir line?

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 ...
Jay Brooks's user avatar
4 votes

In the B737, does Trim Air require that air supply (by the APU, engines or external) is provided in order to operate?

The Trim Air switch will accomplish nothing without bleed air. That switch exists when you have more temperature zones than packs -- in a 737-800, two packs & three zones (flight deck, forward ...
Ralph J's user avatar
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4 votes
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How does a C208B's climate control system that utilizes bleed air for cabin heat work?

There is lots of detailed information available on how these systems work, but in general once bleed air is tapped off the compressor section the lines split: One line stays hot, and the other runs ...
Michael Hall's user avatar
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4 votes
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Which of the major airliners are bleedless?

It's very unlikely to find bleedless ECS and Anti-ice on other airliner designs simply because doing it on the 787 required a massive effort to create the electrical support system to make it work, ...
John K's user avatar
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3 votes

From which stages is the bleed air from a GE90?

The base engine (up -94B rating) has 10 stages HPC, the growth engine (up to -115B) has 9 (reference https://www.geaviation.com/commercial/engines/ge90-engine). In both cases, the type ...
Daniel K's user avatar
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3 votes
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Why does the cabin air sometimes smell of kerosene?

You might have been smelling cooked turbo oil. Leaking seals can potentially allow oil traces into the compressor bleed discharge, especially on auxiliary power units. The other possibility is ...
John K's user avatar
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3 votes

Could you start both A320 engines with APU BLEED on?

I have been flying A320s for over two years now and this is the first time I am hearing this. Normally the engines are always started using the APU with its bleed providing air for the air starter ...
Anas Maaz's user avatar
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2 votes

What kind of filters are being used to filter the bleed air?

If you are speaking of the commercial airlines, like the 737, 757, 767, etc and Airbus equivalents, they are designed to cruise at above 30,000 feet where there (typically) is no debris to filter. ...
Tom Stagliano's user avatar
2 votes
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What is the maximum temperature of engine bleed air for de-icing the wings?

This is based on aircraft type and manufacturer approved limitations. Typically it’s limited by temperature or duration of use. Compressor bleed air is tapped at around 450° F, hotter than a typical ...
Romeo_4808N's user avatar
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2 votes

Is the bleed air passengers breathe tapped from the engines before or after it comes in contact with fuel? And why? Isn't this unhealthy?

To add to what John K already posted, cooling requirements prevent any bleed air being taken in the combustor and high pressure turbine sections. In these sections, thin film cooling is preformed ...
XRF's user avatar
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2 votes
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Air quality monitoring devices for a320

Far as I can tell, there are no passive or active air quality monitoring devices on the A320. The cabin air systems on modern airliners are designed against an air quality standard, and their ...
Juan Jimenez's user avatar

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