Do Airconditioning Packs produce oxygen?
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which shouldn't be a problem at 35000'. if it is, you've got more than a smell in the conditioned air to worry about....
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OldFella/Guppy
Wish you old guys were still driving in my world. A C-1 leak will blow into the compressor. Most of the PIREPS I deal with are admittantly a result of APU fouled bleed air, once it gets into the ducts it gives reason for our average soft skinned drivers to big deal.
You gents are a rare breed in this world I live in, have had the pleasure to fly with a few of your era in the early part of my 15 year career.
Hell I had a jackass RTO yesterday for a pilot induced RTO as a result of an ATS run-away that did not exceed thrust limits.
There are more pilots out there these days are way more concerned with SOP's that they end up spending more time concerned with liability than commonsense.
You gents are a rare breed in this world I live in, have had the pleasure to fly with a few of your era in the early part of my 15 year career.
Hell I had a jackass RTO yesterday for a pilot induced RTO as a result of an ATS run-away that did not exceed thrust limits.
There are more pilots out there these days are way more concerned with SOP's that they end up spending more time concerned with liability than commonsense.
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recommended reading
Some people in this thread would be better informed if they read this (dont be put of by the Learmount word )Toxic cabin air is more poisonous than reckoned - Learmount
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A completely false statement as bearing leakage from the C1 and subsequent bearings in the compressor section, if leaking oil will introduce oil into the bleed secton used for trim air.
As for where contamination might be introduced, C1 in one engine isn't possible in another. Bearings, specific terminology, types, locations, and sources vary with the engine. However, introduction of oil or other contaminants into the airstream is a rare and unusual thing in bleed air used for cabin pressurization and air conditioning.
As for the highly scientific comments by one poster who "smelled" exhaust...this is really quite irrelevant, particularly at altitude where I'm quite sure he smelled no exhuast, oil, or other suggestions of contamination.
On the ground when operating around other aircraft, of course one is going to smell exhaust or other scents in the air...bleed air is air from outside the airplane being drawn in. If there's a forest fire nearby, you might smell that. If you're starting an engine with a tailwind, you may smell your own exhaust as it's re-ingested. This is exactly what you'd smell if you were standing on the ramp or taxiway...it's the air you'd be breathing anyway.
At altitude you're breathing air which often is far above most pollution and contaminants, and is among the cleanest of any air you'll ever breathe.
The cabin bleed air is indeed among the freshest a passenger will ever breathe.
Agreed though. Most airframe/engine combinations do not produce toxic or even non toxic but noticable fumes in the cabin.
The days when we used to go through a full case of 24qts Mobil JetII on one a/c turnround are long gone. Most of the big twins nowadays will happily go multisectors with no engine oil uplift required.
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I would like to read your opinions about the following:
Take an airliner at cruise speed and FL350 for instance.
What is the consequence for the engine in terms of bleed air consumption/power loss if the pilot decides to decrease the cabin temperature?
Will it take more bleed air for the ACM? Less bleed air (already hot)? Or will it remain unchanged?
Regards
Take an airliner at cruise speed and FL350 for instance.
What is the consequence for the engine in terms of bleed air consumption/power loss if the pilot decides to decrease the cabin temperature?
Will it take more bleed air for the ACM? Less bleed air (already hot)? Or will it remain unchanged?
Regards
Last edited by filalb; 15th May 2009 at 07:55.
At a rough guess I would say no consequences at all.
The aircraft will still need the same overall flow rate to maintain cabin pressure so all that will happen is the trim air valves will close a bit reducing the amount of hot air into the cabin and the pack flow control valves will compensate to maintain the cabin altitiude, I think. Could be wrong though, I often am.
The aircraft will still need the same overall flow rate to maintain cabin pressure so all that will happen is the trim air valves will close a bit reducing the amount of hot air into the cabin and the pack flow control valves will compensate to maintain the cabin altitiude, I think. Could be wrong though, I often am.
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What is the consequence for the engine in terms of bleed air consumption/power loss if the pilot decides to decrease the cabin temperature?
Will it take more bleed air for the ACM? Less bleed air (already hot)? Or will it remain unchanged?
Will it take more bleed air for the ACM? Less bleed air (already hot)? Or will it remain unchanged?
Small temperature changes can be effected by changing the ratio of warm "trim air" to a given zone in the airplane. This doesn't change any of the flow through the packs at all, but simply changes the ratio of cold pack air to the warm air being taken from the bleed air manifold. Total airflow doesn't change.
A larger change means that the bypass valve in the pack is opened or closed more, altering the amount of airflow which either goes through the pack, or around it. The pack doesn't use more air, but just uses it differently. The pack's job is to extract the proper amount of energy to release the air into the air conditioned ducts at the proper temperature.
In order for the pack to extract energy, cooling airflow is required through the pack heat exchangers, and this cooling air comes with a drag penalty. An increase in cooling drag means an adjustment required in thrust, which means slightly higher fuel consumption. Small changes, very little noticable amount. Bigger changes, a bigger ammount.
Some operators, particularly freight operators, will shut off a pack (or two) in flight in order to save on fuel. This reduces the amount of bleed air required, which makes the engines more efficient, which means a lower fuel burn.
Actually adjusting the temperature up or down by a few degrees makes little difference in the engine operation. The airflow into the bleed air manifold doesn't increase or decrease significantly at all with changes in cabin temperature, because it's all a matter of how the air is used to change the temperature...not how much air is used. It's about the ratio of warm to cold. The total amount doesn't really vary, just the amount of warm or cold...and the engines don't know the difference (it's decided downstream from them...think about it like this: the engines send X amount of airflow, and that's all...the environmental system decides what to do with it, and how to present it to the occupants of the aircraft). The only thing which makes a substantial difference in bleed demand so far as the environmental system goes, is how many packs are operating.
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Guppy
Actually adjusting the temperature up or down by a few degrees makes little difference in the engine operation. The airflow into the bleed air manifold doesn't increase or decrease significantly at all with changes in cabin temperature, because it's all a matter of how the air is used to change the temperature...not how much air is used. It's about the ratio of warm to cold. The total amount doesn't really vary, just the amount of warm or cold...and the engines don't know the difference (it's decided downstream from them...think about it like this: the engines send X amount of airflow, and that's all...the environmental system decides what to do with it, and how to present it to the occupants of the aircraft). The only thing which makes a substantial difference in bleed demand so far as the environmental system goes, is how many packs are operating.
At altitude the packs operate off of more efficient ram air, sure it takes some stress off of the bleed air needed to run the ACM's but given the OAT the trim Air "direct bleed" is dumping hot air in mass volumes into the aircraft to maintain desired temperature...
Thus..
A warmer demand required at altitude = more bleed flow = degraded engine performance/efficiency.
I may be corrected on other aircraft but am speaking of the DC/MD-10 and-11. I believe you may be confusing the turbine bypass valve in most ACM's that regulate ACM temp with trim air valves.
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There are more pilots out there these days are way more concerned with SOP's that they end up spending more time concerned with liability than commonsense.
A warmer demand required at altitude = more bleed flow = degraded engine performance/efficiency.
If the hot air flow rate increases to raise cabin temp then the pack flow rate will reduce accordingly, it will not matter if the cold air is coming from the ACM or from bypass air through the heat exchangers only.
It's been a while since I worked on the '10' but I would have thought it will be the same for all types.
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You may be correct on the old DC10/MD11 but modern aircraft have the pack flow rate set by the pack flow controller to a set schedule depending on various parameters - temp selection is not one of those parameters.
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At altitude the packs operate off of more efficient ram air, sure it takes some stress off of the bleed air needed to run the ACM's but given the OAT the trim Air "direct bleed" is dumping hot air in mass volumes into the aircraft to maintain desired temperature...
Thus..
A warmer demand required at altitude = more bleed flow = degraded engine performance/efficiency.
Thus..
A warmer demand required at altitude = more bleed flow = degraded engine performance/efficiency.
ALL the air that goes into the cabin comes from the engine compressors. This air is cooled down on its way to the cabin. In the cruise, this cooling is provided by the ram air going through the heat exchangers. On the ground the cooling is by the ACMs. The ram air flow does NOT enter the cabin.
The temp demand from the flight deck will change the ratio of bleed air passing through the heat exchangers, but the overall bleed demand from the engines is more or less fixed. (Some aircraft have a pack high flow selector which will increase it a bit)
