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Air Conditioning "packs"  
User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Posted (8 years 6 months 2 weeks 4 days 18 hours ago) and read 10803 times:

What are these packs? I was reading in an article about how to stay healthy in aircraft cabins. There was a section about hypoxia. This is what it says:

"Passengers in business and first class sections usually do not suffer from hypoxia as often as passengers in the economy section. "On an average flight, a passenger sitting in coach gets about seven cubic feet of fresh air per minute," says Michele Barry, M.D., director of the Department of International Travel and Tropical Medicine at Yale University School of Medicine. "In first class, it's about fifty cubic feet."

What makes matters worse for those in the economy section is that pilots commonly turn off one or more of the air conditioning "packs" as a fuel-saving measure, says Fairechild, which further lessens the air pressure."

Does this make any sense? Why would the air be thinner when cabin altitude is constant in all the cabin.

35 replies: All unread, showing first 25:
 
User currently offlineUA777222 From United States of America, joined Dec 2003, 3348 posts, RR: 11
Reply 1, posted (8 years 6 months 2 weeks 4 days 18 hours ago) and read 10809 times:

Kind of hard to understand but it might mean that the pilots turn off the amount of fresh air that is circulated through the cabin. I have never had this issue, actually quite the opposite. Always freezing my ass off across all cabins. I think just another excuse to drag on eco and the conditions provided. It almost tries to get you to think that the cabins are completely different atmospheres which, in many aspects they are, but at the end of the day it's just a diff. seat in the same tube.

Matt



"It wasn't raining when Noah built the ark."
User currently offline320tech From Turks and Caicos Islands, joined May 2004, 491 posts, RR: 5
Reply 2, posted (8 years 6 months 2 weeks 4 days 17 hours ago) and read 10796 times:

I'm sure the difference in flow rates has more to do with the amount of space allocated to a passenger. There are a few more seats jammed into the same amount of space in economy. There is no difference in how the air is piped into the cabin - it enters through ducts spread through the cabin. Essentially all air exits the fuselage through the outflow valve, at the aft end. So it's not like the aircraft is dumping "first class air" before it can get to the rabble in the back.

What makes matters worse for those in the economy section is that pilots commonly turn off one or more of the air conditioning "packs" as a fuel-saving measure, says Fairechild, which further lessens the air pressure."

An uninformed statement. The air pressure isn't reduced, the flow rate is. Air pressure remains constant, typically 8 psi diff or about 8,000 foot cabin altitude. I suspect that the difference in flow rate would be pretty hard to detect.

I've never heard of an airline passenger getting hypoxia under normal conditions. I'd like to see some evidence before I believe anything quoted in that article.



The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
User currently offlineKAUSpilot From United States of America, joined Jan 2002, 1963 posts, RR: 32
Reply 3, posted (8 years 6 months 2 weeks 4 days 17 hours ago) and read 10801 times:

Sounds like B.S. to me. PACK stands for Pressurization and Air Conditioning Kit. The design of these PACKs hasn't changed much over the past 30 years. Basically, each pack is a small turbine (compressor) which takes bleed air from the engines, cools it down, and feeds it into the cabin in order to maintain pressurization and temperature.

I can only speak for the aircraft I fly, but on the E145, we have two packs. If we turn off one pack, we are limited in flight to FL250 and below, which is an undersireable altitude for most flights due to fuel burn and pax comfort.

For us, Pack #1 dedicates 80% of its air to the cockpit and 20% to the cabin. Pack #2 is dedicated entirely to the pax cabin. Both packs are left on for the duration of the flight unless one is disabled for maintenance reasons.

[Edited 2006-06-06 07:38:53]

User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Reply 4, posted (8 years 6 months 2 weeks 4 days 17 hours ago) and read 10781 times:

Quoting KAUSpilot (Reply 3):
For us, Pack #1 is dedicates 80% of it's air to the cockpit and 20% to the cabin. Pack #2 is dedicated entirely to the pax cabin.

What does that mean in terms of air in the aircraft? Is that just an air conditioning thing?


User currently offlineDC-10Tech From United States of America, joined Jun 2001, 298 posts, RR: 2
Reply 5, posted (8 years 6 months 2 weeks 4 days 16 hours ago) and read 10770 times:

Quoting AirWillie6475 (Reply 4):
What does that mean in terms of air in the aircraft? Is that just an air conditioning thing?

What it means is that the temperature demand in those compartments is prioritized by different packs.

Most aircraft work this way. The #1 pack will adjust its temperature output to satisfy the demand of the cockpit temp setting, the #2 will adjust for the forward cabin, etc, etc.

Many passenger airliners have recirculation fans that scavenge air from the lower fuselage and redistribute it through the cabin to increase flow.

As far as turning off packs to save fuel, you'll see this more with freighters, where an aircraft with three packs may only operate on two (or even one if the aircraft is rated for it). In my experience, pax aircraft usually operate all available packs to keep the cabin temps in check.



Forums.AMTCentral.com
User currently offlineRendezvous From New Zealand, joined May 2001, 521 posts, RR: 0
Reply 6, posted (8 years 6 months 2 weeks 4 days 16 hours ago) and read 10765 times:

With the amount of regulation in the aviation industry, I'm pretty sure they have the amount of oxygen required for passengers covered nicely.

We can assume that the packs produce (more than) the required amount of fresh air to a full plane in all-economy configuration. Therefore, a plane with mixed configuration should theoretically have more fresh air per person than a single class configuration.


User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 25
Reply 7, posted (8 years 6 months 2 weeks 4 days 15 hours ago) and read 10750 times:

Legally the B747-400 must supply 10lbs (Need to double check the figure) of air per passenger per minute...

There are three packs installed, two can do the job, however you wont be able to be able to control the temperature as accurately...



Whoever said "laughter is the best medicine" never had Gonorrhea
User currently offlineKAUSpilot From United States of America, joined Jan 2002, 1963 posts, RR: 32
Reply 8, posted (8 years 6 months 2 weeks 4 days 15 hours ago) and read 10747 times:

Quoting AirWillie6475 (Reply 4):
What does that mean in terms of air in the aircraft? Is that just an air conditioning thing?

In terms of air in the aircraft, the percentages simply dicatate the distribution of conditioned air. For instance, if only pack 1 is operating, 80% of the cold air will be fed up front, 20% in back, resulting in a rather warm ride for the pax.

The cockpit is of course vented to the cabin, so there will be some flow-through, but the cockpit will be cooled more effeciently.

As far as pressurization (which the packs are also responsible for), as I stated before having one pack inoperative will limite the maximum operating altitude of the aircraft.


User currently offlineBuckFifty From Canada, joined Oct 2001, 1316 posts, RR: 19
Reply 9, posted (8 years 6 months 2 weeks 4 days 15 hours ago) and read 10745 times:

On the larger Airbuses, we don't have an altitude limitation when we shut one pack off (but Polar ops will not be feasible if one pack is INOP, due to depress fuel requirements). However, in our company SOP's, we do not turn off packs to save fuel. Depending on the number of passengers, however, we will tend to lower the flow rate coming from the packs. It does save fuel, even though only in small amounts.

To shut one off means that the other one has to be running almost at max capacity at altitude. This is undesirable, more for maintenance issues than operational aspects.


User currently offlineTristarsteve From Sweden, joined Nov 2005, 4068 posts, RR: 33
Reply 10, posted (8 years 6 months 2 weeks 4 days 12 hours ago) and read 10711 times:

Quoting KAUSpilot (Reply 3):
PACK stands for Pressurization and Air Conditioning Kit.

Well I never knew that! Thanks for info.
Unless a pack is off because it has failed, I have never known a crew turn it off in flight.


User currently offlinePilotaydin From Turkey, joined Sep 2004, 2539 posts, RR: 51
Reply 11, posted (8 years 6 months 2 weeks 4 days 9 hours ago) and read 10662 times:

oh god.....try flying the 737-400, the packs barely provide enough cool air, we can only operate one pack on the ground in the -400, and during summer when we max out the temp we get a wing body overheat light or pack light come on in the ramp....basically we boil lol

the NGs can use 2 packs on the ground and it's wonderful and cool on the ground  Smile



The only time there is too much fuel onboard, is when you're on fire!
User currently offlineOryx From Germany, joined Nov 2005, 126 posts, RR: 0
Reply 12, posted (8 years 6 months 2 weeks 4 days 9 hours ago) and read 10659 times:

Are there actually aircraft where the air is routed from the engines compressor into the cabin? I once had a look at the manuals for the B744. The engine bleed is only used to drive the pack's compressors and to heat the air. The cabin air itself is sucked from the ambient flow.

User currently offlineHAWK21M From India, joined Jan 2001, 31702 posts, RR: 56
Reply 13, posted (8 years 6 months 2 weeks 4 days 9 hours ago) and read 10652 times:

Normally the Riser Ducts pass on Conditioned Air thru the Overhead Distribution ducts Evenly thru the Pax cabin.First class or Economy.The Conditioned Air can be diluted with Recirculation air if needed.
But all Pax would recieve same Quality & Quantity of Conditioned Air.
Only difference could be the No of Pax in each class differs.

regds
MEL



Think of the brighter side!
User currently offlineAtlamt From United States of America, joined Sep 2001, 240 posts, RR: 3
Reply 14, posted (8 years 6 months 2 weeks 4 days 9 hours ago) and read 10652 times:

Quoting Oryx (Reply 12):
Are there actually aircraft where the air is routed from the engines compressor into the cabin? I once had a look at the manuals for the B744. The engine bleed is only used to drive the pack's compressors and to heat the air. The cabin air itself is sucked from the ambient flow.

Every commerical a/c I've worked on sends the bleed air through the packs and into the cabin. The air pulled from the compressor is clean outside air that has been compressed and is therefore hot. After going through the heat exchangers and air cycle machine it is ice cold. The temp control valve regulates the amount of hot bleed air that is mixed in to control the final pack output temp.



Fwd to MCO and Placard
User currently offlineTod From Denmark, joined Aug 2004, 1729 posts, RR: 3
Reply 15, posted (8 years 6 months 2 weeks 4 days 8 hours ago) and read 10643 times:

Quoting DC-10Tech (Reply 5):
Many passenger airliners have recirculation fans that scavenge air from the lower fuselage and redistribute it through the cabin to increase flow.

Some from below, some from above and some both.
Also, on at least most, if not all of the planes I've worked on, the flight deck gets it's air right from the mix manifold and does not get any of the recirc.

Quoting Kaddyuk (Reply 7):
Legally the B747-400 must supply 10lbs (Need to double check the figure) of air per passenger per minute...

The FAA minimum per 14CFR25.831 is .55 pounds of fresh (not including recirc) air per minute per pax. That works out to roughly 7.8 cfm. 10 lb/min/pax sounds sort of high as that would be about 140 cfm.

Tod


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 16, posted (8 years 6 months 2 weeks 4 days 7 hours ago) and read 10613 times:

Quoting AirWillie6475 (Thread starter):
Does this make any sense? Why would the air be thinner when cabin altitude is constant in all the cabin.

The cabin altitude is hardly 'constant' during a flight. It is maintained at a lower 'altitude' than that at which the plane is flying but it is still somewhat higher than most people are used to breathing.

Let me explain a few things just so we are talking the same language.

Atmospheric pressure: Nominally about 14.7 pounds per square inch at sea level, about 10.1 PSI at ten thousand feet and only about 3.5 PSI at 35000'. So if a typical airliner has the capability to maintain, say 8.6 PSI of 'differential' pressure then the following would be true.

We are going to make a flight from San Francisco (sea level) to Denver (let's call it 5400' I don't have my Jepps handy) and barometric pressure is 'standard' over the whole route. When we close the doors at SFO we trap 14.7 PSI in the cabin. Then we take off.

Jet engines take in more air than they need to produce their thrust, even on takeoff, so some of this is bled off at the compressor section. Some of the bleed air is just released into the fan discharge air but some is used for various utilities aboard the plane, including, possibly, flushing the toilets. A big bunch of this air gets precooled in the engine pylons, then goes to the 'packs.' The packs have additional heat exchangers and an air-cycle machine that drops this air to well below freezing. Then it is ducted toward the passenger spaces. Hotter air is added from upstream of the packs to maintain the temperature called for by the pilots.

In this ducting, there is a water separator and then the ductwork that distributes the air. This air is extremely dry now. The bigger share of the air goes to the cockpit for good reason. The pilots are sitting under very large, heated windows, one of them in almost direct sunlight and they are surrounded by hundreds of little lightbulbs and other heat-producing items. The air that enters here also cools the circuit breaker panels behind them, and is vented directly to the E&E compartment to cool the radios and computers.

Passenger cabin air may circulate through the sidewalls to insulate you from the minus-40 degree outside air, and be blown in your face by eyeball vents.

Some of the air will go from the passenger cabin to at least one of the bag pits where it will pressurize and heat that space. Some of the air goes to a 'RECIRC' intake and gets back into the air supply system. It is more economical to re-condition this air than to use raw bleed air.

All the air that does not get recirculated goes to the outflow valve(s) where the release of it is regulated by the cabin pressure controller.

Back to our flight, we shut in sea level pressure of 14.7 PSI. We are going to climb to 33000 feet today where the actual air pressure is about 3.8 PSI. You remember that our plane is capable of maintaining 8.6 PSID, so the 3.8 ambient, plus 8.6 differential equals a cabin internal pressure of 12.4 PSI. Well this is approximately the pressure you'd find at about 4500 feet above sea level. So that is the 'cabin altitude' for our flight. The pressure controller will close some initially so that we take off with slight positive pressure, then it will modulate to release some of the trapped internal pressure down to about 12.4 PSI. It will do this automatically during the approximate time it takes the airplane to climb to cruise altitude.

Of course when we descend to land at DEN we are going to have to land unpressurized so the system will allow the outflow valve to release some more pressure, say down to the vicinity of 12.0 PSI. It will be easy to do that because when the engines power back for the descent there is less bleed air available anyway.

The entire pressure vessel, cockpit, cabin, radio spaces, heated bag pits, will have pretty much the same pressure, but different flow rates. So the only real reason the air is not as good back in coach is because there are more bodies per cubic foot back there breathing it. Believe me, life being better in first class is not restricted to air quality.

We do not turn off a pack to save fuel. As has been said, most airliners have altitude restrictions with a pack OFF. 25000' is pretty common and it is partly due to regs based on losing that last pack - you have to be able to descend to, say, below ten thousand feet faster than your passengers go hypoxic on you.

Long answer, including questions you didn't even ask. So, please feel free to ask about any of this and I will focus on that question.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineTWAL1011727 From United States of America, joined Mar 2006, 637 posts, RR: 0
Reply 17, posted (8 years 6 months 2 weeks 4 days 6 hours ago) and read 10582 times:

Quoting SlamClick (Reply 16):
Let me explain a few things just so we are talking the same language.

Well said....no more to add to that explan..

KD MLB


User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 25
Reply 18, posted (8 years 6 months 2 weeks 4 days 6 hours ago) and read 10583 times:

Not in anyway critising your excellent reply slam, just adding some technical know how...

Quoting SlamClick (Reply 16):
. Hotter air is added from upstream of the packs to maintain the temperature called for by the pilots.

This is known as the Turbine Bypass and is controlled by a valve... (Called the turbine bypass valve)

After the air has been conditioned, dehumidified etc, it passes into the plenum which is a tank between the floor and the centre wing box where air is distributed...

Quoting SlamClick (Reply 16):
Passenger cabin air may circulate through the sidewalls to insulate you from the minus-40 degree outside air, and be blown in your face by eyeball vents.

On the B747-400 three riser ducts on either side of the aircraft duct air into the vents above and below the passenger cabin and into the gaspers...

Quoting SlamClick (Reply 16):
So the only real reason the air is not as good back in coach is because there are more bodies per cubic foot back there breathing it. Believe me, life being better in first class is not restricted to air quality

Common problems are Temperature Controllers, if the sensors get dusty they give the controllers incorrect temp readings which is a common cause of intense temperatures in specific areas in the cabin.

A Pack is a very simple machine in its operation, bleed air is compressed further by a compressor, this slows it down, and heats it up. Then you pass the air over a Heat Exchanger

Big version: Width: 600 Height: 600 File size: 182kb
B747-400 Air Conditioning System


That should indicate where items are located on the airframe...

The heat exchanger cools the compressed air down, so we now have low temp, high pressure, low velocity air. Passing some of this air over a turbine enables us to turn the compressor, the rest is bypassed and flows into the plenum.

Packs are set to produce air at a constant temperature. Using warm bleed air, and mixing this with cold conditioned air in the plenum we can control the temperature in the cabin.



Whoever said "laughter is the best medicine" never had Gonorrhea
User currently offlineKAUSpilot From United States of America, joined Jan 2002, 1963 posts, RR: 32
Reply 19, posted (8 years 6 months 2 weeks 4 days 6 hours ago) and read 10566 times:

Do PACK's ever get turned off intentionally druing the flight? Yes....

For the E145, a pack is automatically closed under the following conditions....

When the anti-ice system is turned on below FL 246, PACK 1 automatically shuts down in order to allow more bleed air to be routed through Piccolo tubes in the wings and horizontal stabilizer.

Anytime an Engine is started, the PACKs close until the engine start is complete.

Anytime a failure is detected in the pack (overpressure, overtemperature, duct leakage).


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 20, posted (8 years 6 months 2 weeks 4 days 6 hours ago) and read 10558 times:

Quoting KAUSpilot (Reply 19):
Do PACK's ever get turned off intentionally druing the flight? Yes....

You make this statement then give examples of times when a pack will be turned off automatically, which does not amplify or explain the statement.

Are you saying that there are times when you deliberately turn off a pack in flight? Does this include any NORMAL conditions or would this just be in a non-normal operation? Does your company have a policy on this?



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineTod From Denmark, joined Aug 2004, 1729 posts, RR: 3
Reply 21, posted (8 years 6 months 2 weeks 4 days 5 hours ago) and read 10535 times:

Quoting SlamClick (Reply 16):
Let me explain a few things just so we are talking the same language.

Nice explanations, thanks.

Just my $.02 to throw in:

Quoting SlamClick (Reply 16):
Some of the bleed air is just released into the fan discharge air but some is used for various utilities aboard the plane, including, possibly, flushing the toilets

With vacuum waste systems, during flight the pressure differential between the cabin and the outside create the vacuum. It is normal for the potable water system to be pressurized during flight with bleed air (and a compressor on the ground). Also, on 747 freighter variants, some bleed air is dumped in the forward cargo compartment to compensate for less want air coming from the e/e bay than on pax planes.

Quoting SlamClick (Reply 16):
In this ducting, there is a water separator and then the ductwork that distributes the air. This air is extremely dry now. The bigger share of the air goes to the cockpit for good reason.

On 744, on the way its way to the cockpit this dry air gets re-humidified for pilot comfort.

Quoting SlamClick (Reply 16):
The pilots are sitting under very large, heated windows, one of them in almost direct sunlight and they are surrounded by hundreds of little lightbulbs and other heat-producing items. The air that enters here also cools the circuit breaker panels behind them, and is vented directly to the E&E compartment to cool the radios and computers.

From there (at least on 747 models) the warm air flow under the forward cargo compartment to help with heating that area.

Quoting SlamClick (Reply 16):
Passenger cabin air may circulate through the sidewalls to insulate you from the minus-40 degree outside air,

Just to clarify, this airflow is on the cabin side of the sidewall panel. There are air baffles on the outboard side, just above the floor to prevent fire propagation.

Quoting Kaddyuk (Reply 18):
On the B747-400 three riser ducts on either side of the aircraft duct air into the vents above and below the passenger cabin and into the gaspers...

On all other Boeing planes, at least some conditioned air is include in the gasper system, but on the 744 the gasper is a complete stand-alone system.

Tod


User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 22, posted (8 years 6 months 2 weeks 4 days 5 hours ago) and read 10531 times:

Quoting Tod (Reply 21):
With vacuum waste systems, during flight the pressure differential between the cabin and the outside create the vacuum. It is normal for the potable water system to be pressurized during flight with bleed air

My mistake. That is exactly what I was thinking about when I wrote what I did. Actually I was thinking about a ground school about twenty years ago!

Bleed air may also be used to pressurize the hydraulic reservoirs as well, to reduce foaming and to deliver fluid to the engine-driven pumps at positive pressure.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineKAUSpilot From United States of America, joined Jan 2002, 1963 posts, RR: 32
Reply 23, posted (8 years 6 months 2 weeks 4 days 4 hours ago) and read 10508 times:

Quoting SlamClick (Reply 20):
You make this statement then give examples of times when a pack will be turned off automatically, which does not amplify or explain the statement.

Are you saying that there are times when you deliberately turn off a pack in flight? Does this include any NORMAL conditions or would this just be in a non-normal operation? Does your company have a policy on this?

Sorry if I misconstrued that in some way, but yes the pack shutdown in these scenarios is always done automatically by the FADEC.

There is no reason that I can think of to manually shut down a pack in flight other than an abnormal QRH procedure.


User currently offlineRedcordes From United States of America, joined Jan 2006, 245 posts, RR: 0
Reply 24, posted (8 years 6 months 2 weeks 4 days 4 hours ago) and read 10504 times:

The individual gases ( oxygen, nitogen, carbon dioxide) that make up the air behave independently. Meaning here, that if the percentage of oxygen were to drop in the economy section, it would immediately be equalized by the oxygen in any adjoining areas. I doubt that it's possible to have much of a difference in oxygen percentage between cabins unless they were somewhat sealed--certainly nowhere near enough for "hypoxia" to occur. Unless you're talking about a serious decompression problem, in which case, air-flow into the area would be important to consider.


"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
25 AirWillie6475 : How do you know what altitude corresponds to the PSI? What happens if the oxygen masks don't work, what is the TUC of a person at 25000 and below? At
26 SlamClick : How do I know? I use a standard atmosphere table. Available in print in good books for professional aviators. Available online somewhere, I'm sure. H
27 FDXMECH : The valve/s SlamClick refer to are the Trim Air Valve/s. Genarically speaking, the packs will control their temp to the coldest zone. Zones demanding
28 Post contains images David L : On the other hand, if the oxygen masks are working...
29 SlamClick : Never underestimate 'the habit.' Last time I saw one relative he crawled out of his oxygen tent and shuffled out to the porch to smoke a cigarette. G
30 Post contains images David L : Me? I'm not underestimating the habit as we speak.   Edit: The smiley has nothing to do with the rest of your post, obviously.[Edited 2006-06-07 00:
31 DC-10Tech : The air you are breathing in the cabin comes from the engines. Its bled off before any combustion takes place, so it is 'clean' air from the atmosphe
32 EMBQA : Nothing really. Every commercial aircraft I've been on is the same, or nearly the same way. The RH engine driven packs control the passenger cabin, t
33 Tod : On 767 and 747, if the left pack is off, the other packs will feed the flightdeck. The airflow via the mix manifold takes care of that. Tod
34 Kaddyuk : I call bullshit on this...
35 EMBQA : That's the same with most any aircraft. I just didn't want to get into all the Cross Feeding stuff.
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