# Tag Info

31

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 things like anti-icing. The lower pressure port farther upstream will supply air for air conditioning/pressurization and other pneumatics. Some systems combine ...

16

On multiple PPRUNE threads they incorrectly say: The APU bleed is much cooler than the engine bleed system, and would be ineffective for WAI. But the reason is: The APU bleed temperature is unregulated and thus could damage the wing slats if used for anti-ice. Highlight shows regulation of engine bleed temperature.

16

For the Boeing 737-800: Bleeds off: If bleed air is turned off, it gives ~1% N1 increase. This higher N1 also means a 1 or 2 knot faster $V_1$ and/or $V_{mcg}$. Bleeds on adds about a 2.5-tonne correction to the gross weight. For the various anti-ice: The penalty can be as high as 5% N1. Note the single- and dual-sources figures. Dual means APU is running ...

15

Using air from the exhaust section either for anti-ice directly, or using a heat exchanger, would create many problems. The air must first be used to burn fuel before reaching a turbine bleed location. Air taken from the compressor section hasn't required any fuel to be burned (other than to compress that air of course). The air will be much, much hotter ...

13

The reason (back in the day) was an envisioned low-cost autopilot ownership in smaller planes,$^1$ but it was a troublesome system as it aged. The first production [Twin Otter] aircraft left the Downsview factory with an option for the Honeywell H-14 autopilot. This system used large pneumatic servos to control the aircraft. This was a popular option at ...

10

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...) but all of it is pretty recent. The CAA seems to be aware of the issue and has some information on it here. The two studies they link were published in 2017 so ...

8

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 bleed air is that this air is already pressurized, so you don't need to do any additional work. It is however too hot to be used as cabin air directly, so a PACK (...

7

Your first reason is the major one: Bleed air is abundant (jet engines generate far more than they "need" for operation), already hot (typically well over 100C), and produced at a relatively high pressure - that's why it's tapped for things like anti-icing systems, cabin air conditioning, and starting other engines. It takes little effort in design to ...

7

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 primary limitation on total power is the amount of heat the turbine can take. When you take bleed from the compressor, the turbine is forced to generate a lot of ...

6

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 deicing fluid doesn't get in the bleed air system. When you are using the APU bleed air, the #1 engine bleed switch needs to be off IF that engine is running ...

6

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 Description) it says: Since the flight deck requires only a fraction of the air supply provided by the left pack, most of the left pack air output is mixed with the ...

5

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). Therefore pilots receive 100% fresh air while passengers receive a mix of 50% fresh air and 50% re-circulated cabin air on average, across all aircraft types ...

5

Although typical HEPA filters are what's used (speaking from Boeing experience only), it's important to understand how bleed air is used in a typical aircraft environmental control system (ECS). Bleed air is taken from the compressor section of the engine before the fuel is introduced, so it's still relatively clean ambient air. As such, special filtering is ...

4

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 bleed air is tapped before it comes into contact with fuel:

4

It's on the order of a few percent. Not huge but not negligible either.

4

(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 purpose and could come into contact with the bleed air in case of a leakage (see mooveaviation.com for details). Hydraulic Fluid: Some components of the engine, ...

4

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 cabin, aft cabin). To make that work, the packs produce conditioned air at a temperature (i.e. the produce enough cold air) to satisfy the coldest temperature ...

3

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 cooked deicing fluid, which is quite common. Turbo oil and deicing fluid are the two major sources of petrochemical cabin stink. Another possibility would be an ...

3

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 certification is counting bleed starting from the first stage of the HPC (not the LPC). The reference to "10th stage" bleed in the type certification is misleading. There ...

3

Bleed Air is filtered using air filters. Various companies offer a lot of air filters, but High-efficiency particulate arrestance (HEPA) is a fairly common filter used to filter bleed air. Its working is as under: HEPA filters are composed of a mat of randomly arranged fibres. The fibres are typically composed of fiberglass and possess diameters ...

3

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 ...

3

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 motor. So, in normal operations, cross bleed is set at AUTO, APU bleed is turned on and both the engines are started with the help of the APU. Once the engines are ...

2

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 where a small amount of air is injected around the surfaces to prevent the hot combustion products from touching them. Even if the bleed air was being taken for a ...

2

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 performance is tested with equipment that is installed on the test aircraft for that specific purpose. EASA published the results of an air quality campaign test ...

2

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. The bleed air comes from one of the Compression stages of the engine (or from the APU unit when the aircraft is on the tarmac). The bleed air is used for a lot of ...

2

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 household oven. These high temperatures can begin to significantly affect the yield strength of aluminum alloys typically used in aircraft construction, ...

2

Nobody can tell you for sure without insider knowledge, and insider knowledge would be confidential in this case, but ultimately it boils down to risk management. Every new development carries a risk of budget and deadline overruns and of problems surfacing in operation and the manufacturer needs to be reasonably certain they can put the product to market ...

2

Anti icing systems that use bleed are either "run-wet" or "evaporative". Run-wet systems heat the surfaces to below the water boiling point, say 140-160F. Supercooled drops hitting the surface are kept from freezing on the leading edge, but can run back and refreeze on the unprotected areas. In that case the "lowest" ...

2

The answer is in the FCOM: So, the ENG bleed valve closes if: Reverse flow detected ENG 1 starter valve not closed APU bleed valve open Bad things (Overpressure / Overheat / Leak) ENG BLEED switch is off or FIRE pb pushed So, if APU is not ON, the APU BLEED valve is closed, and then the condition 3 is false. If all the others conditions are also false, ...

1

The stage number is counted from the front of the compressor the bleed is tapped from, in this case I'm assuming its the HP compressor so 4th, 7th and 10th stage HP air. Extraction locations depend really on what you want the air to do, turbine cooling, cabin conditioning, anti ice, start/surge/stall control etc etc, Whilst there are bleed point on some LP ...

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