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44 votes
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Why is the air inside airliners so dry (low humidity)?

What happens during a flight cycle is the humidity is high during departure, from ambient air and the moisture from the pax, but over time the air dries out because the bleed being supplied to keep ...
John K's user avatar
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38 votes

Why is the airflow in airline cabins downwards?

Temperature stratification is a vexing problem on lots of airliners. The big challenge is you need a distribution system that is effective when cooling (on the ground and at low altitude when the ...
John K's user avatar
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27 votes

Why is the airflow in airline cabins downwards?

Not at all. Cold air tends to go low and hot air high. If the cooled air flow were pumped in from the floor, then what we would have would be customers with cold feet and hot heads, making an already ...
CGCampbell's user avatar
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18 votes

Why is the air inside airliners so dry (low humidity)?

Why is the air so dry Its in part by design and in part a constraint of the system. All aircraft are at risk of corrosion not just big airliners. So keeping them dry is a good way to cut down on that ...
Dave's user avatar
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18 votes
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What does the phrase "electrically controlled and pneumatically operated" mean?

It means that an electrical switching signal is used to drive the pneumatic or air valves in the air conditioning system. Think about it like this: We want to use our garden hose to push open a cat ...
Craig's user avatar
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17 votes
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What powers the air-conditioning on commercial planes?

What powers the airconditioning on most airliners is APU or main engine compressor bleed, which is being supplied at fairly high pressure, say, 80 psi, and temperature, around 3-400F, to the air ...
John K's user avatar
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17 votes

Why do airplanes have complex air conditioning systems?

It's because for the Air Cycle process, to chill ambient air without refrigerant, to work, you have to take really hot high pressure air and and remove most of the heat while keeping the pressure up, ...
John K's user avatar
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16 votes

Do jet fighters have air conditioning systems?

Quick Introduction to the Airconditioning Unit In the A7-E the air-conditioning system takes high pressure bleed air and passes it through the primary heat exchanger. After here it enters compressor ...
Aaron'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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10 votes
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What's inside the packs? (Airbus a320)

Here's an answer for the B737, from own work compiled from several public sources. The A320 and other airliners are very similar. The air conditioning packs are Air Cycle Machines (ACM) which remove ...
Koyovis's user avatar
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10 votes

Why air pressure is raised twice in air cycle machines?

You have a system that takes hot high pressure air that was compressed in the engine, removes the heat while it is still at high pressure, then lets the pressure drop, reducing its temperature to ...
John K's user avatar
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9 votes

Why is the air inside airliners so dry (low humidity)?

200 humans exhaling persistently in an enclosed, relatively warm, high pressure atmosphere creates large quantities of condensation on the cold interior surfaces & structure of the aircraft hull, ...
Mackk's user avatar
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9 votes
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How common and dangerous are fume events?

The AVHerald article you linked to pretty much as the answer to your question: How common are they?Nearly 2,000 events a year in the US alone. [According] to the Kansas State University Research ...
abelenky'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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6 votes

Why is the airflow in airline cabins downwards?

As an addition to the other answers I want to mention that feet tend to stink - especially when lots of passengers take their shoes off. You don't want that stink to rise from the bottom to the top, ...
yar's user avatar
  • 161
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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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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5 votes

What does pneumatic operated and electrically controlled means?

Pneumatic operated: actuation by using air pressure. By opening a valve, high pressure air enters a cylinder, causing a piston to be pushed out. Typically, actuators of this type have two valves ...
Sanchises's user avatar
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4 votes

What does the phrase "electrically controlled and pneumatically operated" mean?

They're called "magnet valves". They're basically a valve that's operated by a solenoid. Also seen on locomotive electrical contactors. The solenoid admits air, which moves an air piston which ...
Harper - Reinstate Monica's user avatar
4 votes

How often is the cabin air replaced in A320?

Let's do some simplified arithmetic: The packs airflow in an A320 is 1.2 kg/s per pack in HIGH setting, but let's take the normal setting 1 kg/s, thus 2 kg/s considering both packs The cabin of an ...
ocirocir's user avatar
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4 votes

Why is it that some planes require the AC to be turned off during engine start and take off?

Assuming there's a heat pump system involved (as with home and auto air conditioning), it draws considerable power from the engine, and is also subject to unpredicted pump seizure which can result in ...
Zeiss Ikon's user avatar
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4 votes

Why is it that some planes require the AC to be turned off during engine start and take off?

More or less addressed in my answer to this similar question but aircraft POH data is typically derived from full power numbers which means the engine is driving power only to the propeller and the ...
Dave's user avatar
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3 votes

What dictates the A320's 'emergency ram air inlet' operation?

Please read carefully ... Extract taken from my FCOM, Info accurate at January 2020: An emergency ram air inlet ventilates the cockpit and cabin to remove smoke, or if both packs fail. The emergency ...
Pedro Fabio López Pascua's user avatar
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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2 votes

Which are the advantages of low pressure levels during flight? (i.e during cruise flight in a commercial aircraft)

When transporting animals, particularly larger (zoo, show) animals, it is often requested by the handler/vet to have higher pressure altitudes. This helps calm the animal, and makes them less active. ...
mongo's user avatar
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2 votes
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Which are the advantages of low pressure levels during flight? (i.e during cruise flight in a commercial aircraft)

You want the lowest pressure that can be tolerated by most of the population for the purpose of maintaining adequate blood oxygen levels, which is generally considered to be 8000 ft above sea level. ...
John K's user avatar
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2 votes

Do pilots get more fresh air than passengers?

I can confirm that similar air distribution systems also exist for: Boeing 777: The flight deck receives 100% fresh conditioned air from the left pack. The flight deck is maintained at a slightly ...
Bianfable'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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