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?
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.