Forgive me if this has been discussed, which I am sure it has, but the A-Net search engine still sucks.

My understanding is that cabin air is a mix of fresh air via the engines and recirculated cabin air through HEPA filters, correct? Any idea of approximate percentages for a 737NG when inflight?

My real question is that when you are on the ground, is there any fresh air being pulled into the cabin? Some potential scenarios:

1. At the gate. Is the airplane receiving power from the terminal; "plugged in" to the ground power unit or other OR is it using the APU? Is the airplane receiving any fresh air at this point? Does the APU provide fresh air to the cabin?

2. Taxing. The airplane is moving under it's engines power but not nearly at full throttle. How much fresh air is it receiving at this point? I assuming less then when inflight?

3. Other. If the airplane is indefinitly delayed, diverted or otherwise and just sitting on the tarmac, is it typically being powered by the APU for that time. Is the airplane receiving any fresh at this point?

Thank you.

Air in the cabin doesn't stream from ceiling to bottom, it is very turbulent in reality. Air is sucked from the bottom of the cabins, into the HEPA filters.

Comparing air going through the airco/ HEPA filters with the cabin volume is non-sense. It's mixing all the time. Airlines love to give you simplified incorrect impressions though.



https://www.researchgate.net/figure/Air ... _269399879

We give them ground power and air up until about 15 minutes before departure.

The manual wants it on as long as possible up to 5 minutes before departure but those are items that realistically get removed as soon as we have time. The air hose can be very bulky and time consuming to stow away so as soon as the APU is on and we have a few minutes to spare between loading bags we are getting the air pulled off. Power can stay on a bit longer as its quick to disconnect it and stow it. If it is a quick turn the odds of us even bothering to put the air on is quite slim, there are just many other items for us to do and the pilots tend to just keep the APU running for the full turn anyway.

At the gate, power is being provided by the jetway. Most airports also have a conditioned air hose or two that supply air directly to the aircraft ducting.

During taxi there is more than enough air volume from a single engine at idle to cool and provide fresh air.

When sitting a during a delay the crew can keep an engine running to provide air to the a/c packs. The apu on most airplanes doesn't really provide enough volume to cool a full airplane. Another option is for a ground a/c unit to be brought out to the airplane. That is just like a home unit or jet way unit.

United787 wrote:

Forgive me if this has been discussed, which I am sure it has, but the A-Net search engine still sucks.

My understanding is that cabin air is a mix of fresh air via the engines and recirculated cabin air through HEPA filters, correct? Any idea of approximate percentages for a 737NG when inflight?

My real question is that when you are on the ground, is there any fresh air being pulled into the cabin? Some potential scenarios:

1. At the gate. Is the airplane receiving power from the terminal; "plugged in" to the ground power unit or other OR is it using the APU? Is the airplane receiving any fresh air at this point? Does the APU provide fresh air to the cabin?

2. Taxing. The airplane is moving under it's engines power but not nearly at full throttle. How much fresh air is it receiving at this point? I assuming less then when inflight?

3. Other. If the airplane is indefinitly delayed, diverted or otherwise and just sitting on the tarmac, is it typically being powered by the APU for that time. Is the airplane receiving any fresh at this point?

Thank you.

1. Depends. Sometimes it’s gate power/gate air (big yellow hose). That’s fresh conditioned air going directly into the plane. Meanwhile, the recirculation fans are on and that portion of the air that’s recirculated goes through the filters. When the APU is on, and depending on the plane (Airbus APU bleed air isn’t supposed to be on while external air is on), APU bleed air comes in as well. Recircs still do their thing. And we try to get the external power and air disconnected when the APU is on. That’s a mix of fresh air and recirculated (filtered) air.

2. Taxiing can be done on one engine, one engine plus APU, or both engines. No matter the configuration, bleed air is being tapped from the engines/APU, and recircs are operating filtering the recirculated air. Is fresh air less than at cruise? Probably depends on the configuration, but shouldn’t be much less depending on the plane and configuration. If single engine and no APU, perhaps it’d be less. Also of note, when engines are started, bleeds are used to start the engines, so airflow is reduced to recirculation fans only for that 1-3 minutes per engine.

3. Same as above. I was delayed recently on the tarmac for 2.5 hours due to weather. No one was getting out so we were all just sitting waiting for weather to move through. I had one engine going, no APU (they kept moving us around and I had plenty of gas). Some airplanes shutdown both engines and were just on the APU. Either way, it’s tapped bleed air from either an engine or APU, plus recirculation fans. In any configuration, depending on the plane, air should, in theory, still be recycled every 2-5 minutes, and air that’s recirculated goes through the hepa filters.

I have no doubt air on airplanes is some of the cleanest and most recycled air for high density locations. The biggest issue with aircraft air is the “fresh outside air” comes from bleed air. Bleed air is not filtered. This bleed air often (even if undetected by the human nose) has small bits of oil that has bypassed seals/bearings and gets atomized/pyrolized. It often smells like dirty socks or magic markers depending on the oil type. Bleed air isn’t filtered until it goes through the recirculation fans. So it may be clean going into the bleeds, but can get contaminated by oil before entering the cabin. Google aerotoxic syndrome for a lot more info on that. But to prove this point that it’s not filtered, often times when you have a tailwind when starting an engine, or when sitting behind another airplane taxiing, exhaust smell enters the cabin. If the bleed air were filtered, that smell would get filtered.

United787 wrote:

Forgive me if this has been discussed, which I am sure it has, but the A-Net search engine still sucks.

My understanding is that cabin air is a mix of fresh air via the engines and recirculated cabin air through HEPA filters, correct? Any idea of approximate percentages for a 737NG when inflight?

My real question is that when you are on the ground, is there any fresh air being pulled into the cabin? Some potential scenarios:

1. At the gate. Is the airplane receiving power from the terminal; "plugged in" to the ground power unit or other OR is it using the APU? Is the airplane receiving any fresh air at this point? Does the APU provide fresh air to the cabin?

2. Taxing. The airplane is moving under it's engines power but not nearly at full throttle. How much fresh air is it receiving at this point? I assuming less then when inflight?

3. Other. If the airplane is indefinitly delayed, diverted or otherwise and just sitting on the tarmac, is it typically being powered by the APU for that time. Is the airplane receiving any fresh at this point?

Thank you.

This topic would be better suited for the Tech Ops forum, looks like someone reported it already. To answer your questions however...

The basic flow of air through the cabin on most aircraft is that air enters the cabin through two sources, a ground air connection, or the air conditioning packs. This air enters a mixing manifold where the temperature of the air is regulated, and mixed with recirculated air. The recirculated air is pulled from the cabin by large fans, blown through a HEPA filter, then it enters the manifold to mix with the fresh air from the packs or ground air unit.

1. There is a distinction between ground power and ground air. The air conditioning system on most aircraft relies on high pressure bleed air, provided from the engines, or APU. In simple terms, this high pressure air powers the air conditioning packs, and in turn, hot or cold air enters the cabin. The crew can select the source of bleed air, right or left engine, both engines, or the APU. Additionally a high pressure external source can also be attached. This is normally used when the APU is not working, as in additional to providing air conditioning, the high pressure bleed air from the APU is used to start the engines. Normally however, high pressure external air is used for starting the engines only, not running the air conditioning packs. Ground power only provides electrical power to the aircraft systems, and since the air conditioning packs are pneumatic, they don't run on ground power. I'm sure someone can correct me, but I believe the only exception is the 787, which has electrically powered air conditioning packs, and no bleed air system.

For your question, while parked on the gate, there are 2 sources of air into the cabin. The APU can be run, which provides bleed air to run the air conditioning packs, or low pressure ground air, which is essentially a large air conditioning unit placed somewhere in the gate area, or underneath the jetbridge. There are large hoses that run from the units and connect into a connection port under the aircraft. For narrowbodies there is one hose, but some of the larger aircraft use 2, or in the cast of the A380, four hoses. These are usually bright orange, or yellow and you can see them connected to the underside of the fuselage. The advantage of ground air is that by shutting the APU off, you can save fuel, and reduce the amount of noise on the ramp area.

Long story short, fresh air enters the cabin through the ground air connection, or by running the APU which powers the onboard air conditioning packs.

2. Not usually less, on some older aircraft there is a noticeable difference at idle vs full power, or if the APU is running to provide pressure. Usually noticeable on very hot days when the cabin just cant seem to cool down. But as far as the volume of air entering the cabin, I don't think it varies that much.

3. If you're delayed, it depends where you are on the airport. If you're sitting on a taxiway, the air will be provided by the engines or APU running the air packs. If you're at the gate, it might be provided by the ground air unit.

Below is a diagram for the 737-800 I believe. Fun fact, the pilots only receive fresh air, no recirculated air on the flight deck.

To answer your first question, at least for the Embraer 170/190, the mixture is 52% fresh, 48% recirculated air.

For more details on the aircraft air circulation system in use in most airliners and more please refer to a study that was done in 2002 by the National Research Council in the US. It is a little dated and some of the newer aircraft (787, etc.) use quite different systems for air circulation but does cover many of the aircraft in use in 2002. For more details than you ever wanted to know, and a downloadable pdf of the report go to:
https://www.nap.edu/catalog/10238/the-a ... s-and-crew
There are probably more recent studies/papers in the last couple of years because of Covid-19 if you search for them. I agree this should be moved to the Tech/Ops forum.

"Simulation of aerosol transmission on a Boeing 737 airplane with intervention measures for COVID-19 mitigation" Paper on Cabin Air Circulation simulation in the 737-700 and 737-800 for Covid-19 spread. For Details go to:
https://aip.scitation.org/doi/10.1063/5.0044720

Thank you everyone, all of this information is very helpful in my basic understanding of the systems.